1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>LLVM Coding Standards</title>
</head>
<body>
<h1>
LLVM Coding Standards
</h1>
<ol>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#mechanicalissues">Mechanical Source Issues</a>
<ol>
<li><a href="#sourceformating">Source Code Formatting</a>
<ol>
<li><a href="#scf_commenting">Commenting</a></li>
<li><a href="#scf_commentformat">Comment Formatting</a></li>
<li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
<li><a href="#scf_codewidth">Source Code Width</a></li>
<li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
<li><a href="#scf_indentation">Indent Code Consistently</a></li>
</ol></li>
<li><a href="#compilerissues">Compiler Issues</a>
<ol>
<li><a href="#ci_warningerrors">Treat Compiler Warnings Like
Errors</a></li>
<li><a href="#ci_portable_code">Write Portable Code</a></li>
<li><a href="#ci_rtti_exceptions">Do not use RTTI or Exceptions</a></li>
<li><a href="#ci_class_struct">Use of <tt>class</tt>/<tt>struct</tt> Keywords</a></li>
</ol></li>
</ol></li>
<li><a href="#styleissues">Style Issues</a>
<ol>
<li><a href="#macro">The High-Level Issues</a>
<ol>
<li><a href="#hl_module">A Public Header File <b>is</b> a
Module</a></li>
<li><a href="#hl_dontinclude"><tt>#include</tt> as Little as Possible</a></li>
<li><a href="#hl_privateheaders">Keep "internal" Headers
Private</a></li>
<li><a href="#hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify
Code</a></li>
<li><a href="#hl_else_after_return">Don't use <tt>else</tt> after a
<tt>return</tt></a></li>
<li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
Functions</a></li>
</ol></li>
<li><a href="#micro">The Low-Level Issues</a>
<ol>
<li><a href="#ll_naming">Name Types, Functions, Variables, and Enumerators Properly</a></li>
<li><a href="#ll_assert">Assert Liberally</a></li>
<li><a href="#ll_ns_std">Do not use '<tt>using namespace std</tt>'</a></li>
<li><a href="#ll_virtual_anch">Provide a virtual method anchor for
classes in headers</a></li>
<li><a href="#ll_end">Don't evaluate <tt>end()</tt> every time through a
loop</a></li>
<li><a href="#ll_iostream"><tt>#include <iostream></tt> is
<em>forbidden</em></a></li>
<li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a></li>
<li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
</ol></li>
<li><a href="#nano">Microscopic Details</a>
<ol>
<li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
<li><a href="#micro_preincrement">Prefer Preincrement</a></li>
<li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
<li><a href="#micro_anonns">Anonymous Namespaces</a></li>
</ol></li>
</ol></li>
<li><a href="#seealso">See Also</a></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="introduction">Introduction</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>This document attempts to describe a few coding standards that are being used
in the LLVM source tree. Although no coding standards should be regarded as
absolute requirements to be followed in all instances, coding standards can be
useful.</p>
<p>This document intentionally does not prescribe fixed standards for religious
issues such as brace placement and space usage. For issues like this, follow
the golden rule:</p>
<blockquote>
<p><b><a name="goldenrule">If you are adding a significant body of source to a
project, feel free to use whatever style you are most comfortable with. If you
are extending, enhancing, or bug fixing already implemented code, use the style
that is already being used so that the source is uniform and easy to
follow.</a></b></p>
</blockquote>
<p>The ultimate goal of these guidelines is the increase readability and
maintainability of our common source base. If you have suggestions for topics to
be included, please mail them to <a
href="mailto:sabre@nondot.org">Chris</a>.</p>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="mechanicalissues">Mechanical Source Issues</a>
</h2>
<!-- *********************************************************************** -->
<div>
<!-- ======================================================================= -->
<h3>
<a name="sourceformating">Source Code Formatting</a>
</h3>
<div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="scf_commenting">Commenting</a>
</h4>
<div>
<p>Comments are one critical part of readability and maintainability. Everyone
knows they should comment, so should you. When writing comments, write them as
English prose, which means they should use proper capitalization, punctuation,
etc. Although we all should probably
comment our code more than we do, there are a few very critical places that
documentation is very useful:</p>
<h5>File Headers</h5>
<div>
<p>Every source file should have a header on it that describes the basic
purpose of the file. If a file does not have a header, it should not be
checked into Subversion. Most source trees will probably have a standard
file header format. The standard format for the LLVM source tree looks like
this:</p>
<div class="doc_code">
<pre>
//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the declaration of the Instruction class, which is the
// base class for all of the VM instructions.
//
//===----------------------------------------------------------------------===//
</pre>
</div>
<p>A few things to note about this particular format: The "<tt>-*- C++
-*-</tt>" string on the first line is there to tell Emacs that the source file
is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default).
Note that this tag is not necessary in <tt>.cpp</tt> files. The name of the file is also
on the first line, along with a very short description of the purpose of the
file. This is important when printing out code and flipping though lots of
pages.</p>
<p>The next section in the file is a concise note that defines the license
that the file is released under. This makes it perfectly clear what terms the
source code can be distributed under and should not be modified in any way.</p>
<p>The main body of the description does not have to be very long in most cases.
Here it's only two lines. If an algorithm is being implemented or something
tricky is going on, a reference to the paper where it is published should be
included, as well as any notes or "gotchas" in the code to watch out for.</p>
</div>
<h5>Class overviews</h5>
<p>Classes are one fundamental part of a good object oriented design. As such,
a class definition should have a comment block that explains what the class is
used for... if it's not obvious. If it's so completely obvious your grandma
could figure it out, it's probably safe to leave it out. Naming classes
something sane goes a long ways towards avoiding writing documentation.</p>
<h5>Method information</h5>
<div>
<p>Methods defined in a class (as well as any global functions) should also be
documented properly. A quick note about what it does and a description of the
borderline behaviour is all that is necessary here (unless something
particularly tricky or insidious is going on). The hope is that people can
figure out how to use your interfaces without reading the code itself... that is
the goal metric.</p>
<p>Good things to talk about here are what happens when something unexpected
happens: does the method return null? Abort? Format your hard disk?</p>
</div>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="scf_commentformat">Comment Formatting</a>
</h4>
<div>
<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
require less typing, don't have nesting problems, etc. There are a few cases
when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
<ol>
<li>When writing C code: Obviously if you are writing C code, use C style
comments.</li>
<li>When writing a header file that may be <tt>#include</tt>d by a C source
file.</li>
<li>When writing a source file that is used by a tool that only accepts C
style comments.</li>
</ol>
<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
These nest properly and are better behaved in general than C style comments.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="scf_includes"><tt>#include</tt> Style</a>
</h4>
<div>
<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
include guards if working on a header file), the <a
href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
file should be listed. We prefer these <tt>#include</tt>s to be listed in this
order:</p>
<ol>
<li><a href="#mmheader">Main Module Header</a></li>
<li><a href="#hl_privateheaders">Local/Private Headers</a></li>
<li><tt>llvm/*</tt></li>
<li><tt>llvm/Analysis/*</tt></li>
<li><tt>llvm/Assembly/*</tt></li>
<li><tt>llvm/Bitcode/*</tt></li>
<li><tt>llvm/CodeGen/*</tt></li>
<li>...</li>
<li><tt>Support/*</tt></li>
<li><tt>Config/*</tt></li>
<li>System <tt>#includes</tt></li>
</ol>
<p>and each category should be sorted by name.</p>
<p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files
which implement an interface defined by a <tt>.h</tt> file. This <tt>#include</tt>
should always be included <b>first</b> regardless of where it lives on the file
system. By including a header file first in the <tt>.cpp</tt> files that implement the
interfaces, we ensure that the header does not have any hidden dependencies
which are not explicitly #included in the header, but should be. It is also a
form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it
implements are defined.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="scf_codewidth">Source Code Width</a>
</h4>
<div>
<p>Write your code to fit within 80 columns of text. This helps those of us who
like to print out code and look at your code in an xterm without resizing
it.</p>
<p>The longer answer is that there must be some limit to the width of the code
in order to reasonably allow developers to have multiple files side-by-side in
windows on a modest display. If you are going to pick a width limit, it is
somewhat arbitrary but you might as well pick something standard. Going with
90 columns (for example) instead of 80 columns wouldn't add any significant
value and would be detrimental to printing out code. Also many other projects
have standardized on 80 columns, so some people have already configured their
editors for it (vs something else, like 90 columns).</p>
<p>This is one of many contentious issues in coding standards, but it is not up
for debate.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
</h4>
<div>
<p>In all cases, prefer spaces to tabs in source files. People have different
preferred indentation levels, and different styles of indentation that they
like; this is fine. What isn't fine is that different editors/viewers expand
tabs out to different tab stops. This can cause your code to look completely
unreadable, and it is not worth dealing with.</p>
<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
style of existing code if you are modifying and extending it. If you like four
spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
with two spaces of indentation. Also, do not reindent a whole source file: it
makes for incredible diffs that are absolutely worthless.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="scf_indentation">Indent Code Consistently</a>
</h4>
<div>
<p>Okay, in your first year of programming you were told that indentation is
important. If you didn't believe and internalize this then, now is the time.
Just do it.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="compilerissues">Compiler Issues</a>
</h3>
<div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
</h4>
<div>
<p>If your code has compiler warnings in it, something is wrong — you
aren't casting values correctly, your have "questionable" constructs in your
code, or you are doing something legitimately wrong. Compiler warnings can
cover up legitimate errors in output and make dealing with a translation unit
difficult.</p>
<p>It is not possible to prevent all warnings from all compilers, nor is it
desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
a good thorough set of warnings, and stick to it. At least in the case of
<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
syntax of the code slightly. For example, a warning that annoys me occurs when
I write code like this:</p>
<div class="doc_code">
<pre>
if (V = getValue()) {
...
}
</pre>
</div>
<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
operator, and that I probably mistyped it. In most cases, I haven't, and I
really don't want the spurious errors. To fix this particular problem, I
rewrite the code like this:</p>
<div class="doc_code">
<pre>
if ((V = getValue())) {
...
}
</pre>
</div>
<p>which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
be fixed by massaging the code appropriately.</p>
<p>These are the <tt>gcc</tt> warnings that I prefer to enable:</p>
<div class="doc_code">
<pre>
-Wall -Winline -W -Wwrite-strings -Wno-unused
</pre>
</div>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ci_portable_code">Write Portable Code</a>
</h4>
<div>
<p>In almost all cases, it is possible and within reason to write completely
portable code. If there are cases where it isn't possible to write portable
code, isolate it behind a well defined (and well documented) interface.</p>
<p>In practice, this means that you shouldn't assume much about the host
compiler, and Visual Studio tends to be the lowest common denominator.
If advanced features are used, they should only be an implementation detail of
a library which has a simple exposed API, and preferably be buried in
libSystem.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ci_rtti_exceptions">Do not use RTTI or Exceptions</a>
</h4>
<div>
<p>In an effort to reduce code and executable size, LLVM does not use RTTI
(e.g. <tt>dynamic_cast<></tt>) or exceptions. These two language features
violate the general C++ principle of <i>"you only pay for what you use"</i>,
causing executable bloat even if exceptions are never used in the code base, or
if RTTI is never used for a class. Because of this, we turn them off globally
in the code.</p>
<p>That said, LLVM does make extensive use of a hand-rolled form of RTTI that
use templates like <a href="ProgrammersManual.html#isa"><tt>isa<></tt>,
<tt>cast<></tt>, and <tt>dyn_cast<></tt></a>. This form of RTTI is
opt-in and can be added to any class. It is also substantially more efficient
than <tt>dynamic_cast<></tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
</h4>
<div>
<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
interchangeably. The only difference is when they are used to declare a class:
<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
all members public by default.</p>
<p>Unfortunately, not all compilers follow the rules and some will generate
different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
declare the symbol. This can lead to problems at link time.</p>
<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
<b>all</b> members are public and the type is a C++
<a href="http://en.wikipedia.org/wiki/Plain_old_data_structure">POD</a> type, in
which case <tt>struct</tt> is allowed.</p>
</div>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="styleissues">Style Issues</a>
</h2>
<!-- *********************************************************************** -->
<div>
<!-- ======================================================================= -->
<h3>
<a name="macro">The High-Level Issues</a>
</h3>
<!-- ======================================================================= -->
<div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="hl_module">A Public Header File <b>is</b> a Module</a>
</h4>
<div>
<p>C++ doesn't do too well in the modularity department. There is no real
encapsulation or data hiding (unless you use expensive protocol classes), but it
is what we have to work with. When you write a public header file (in the LLVM
source tree, they live in the top level "<tt>include</tt>" directory), you are
defining a module of functionality.</p>
<p>Ideally, modules should be completely independent of each other, and their
header files should only <tt>#include</tt> the absolute minimum number of
headers possible. A module is not just a class, a function, or a
namespace: <a href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's
a collection of these</a> that defines an interface. This interface may be
several functions, classes, or data structures, but the important issue is how
they work together.</p>
<p>In general, a module should be implemented by one or more <tt>.cpp</tt>
files. Each of these <tt>.cpp</tt> files should include the header that defines
their interface first. This ensures that all of the dependences of the module
header have been properly added to the module header itself, and are not
implicit. System headers should be included after user headers for a
translation unit.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
</h4>
<div>
<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
have to, especially in header files.</p>
<p>But wait! Sometimes you need to have the definition of a class to use it, or
to inherit from it. In these cases go ahead and <tt>#include</tt> that header
file. Be aware however that there are many cases where you don't need to have
the full definition of a class. If you are using a pointer or reference to a
class, you don't need the header file. If you are simply returning a class
instance from a prototyped function or method, you don't need it. In fact, for
most cases, you simply don't need the definition of a class. And not
<tt>#include</tt>'ing speeds up compilation.</p>
<p>It is easy to try to go too overboard on this recommendation, however. You
<b>must</b> include all of the header files that you are using — you can
include them either directly or indirectly (through another header file). To
make sure that you don't accidentally forget to include a header file in your
module header, make sure to include your module header <b>first</b> in the
implementation file (as mentioned above). This way there won't be any hidden
dependencies that you'll find out about later.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="hl_privateheaders">Keep "Internal" Headers Private</a>
</h4>
<div>
<p>Many modules have a complex implementation that causes them to use more than
one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
internal communication interface (helper classes, extra functions, etc) in the
public module header file. Don't do this!</p>
<p>If you really need to do something like this, put a private header file in
the same directory as the source files, and include it locally. This ensures
that your private interface remains private and undisturbed by outsiders.</p>
<p>Note however, that it's okay to put extra implementation methods in a public
class itself. Just make them private (or protected) and all is well.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify Code</a>
</h4>
<div>
<p>When reading code, keep in mind how much state and how many previous
decisions have to be remembered by the reader to understand a block of code.
Aim to reduce indentation where possible when it doesn't make it more difficult
to understand the code. One great way to do this is by making use of early
exits and the <tt>continue</tt> keyword in long loops. As an example of using
an early exit from a function, consider this "bad" code:</p>
<div class="doc_code">
<pre>
Value *DoSomething(Instruction *I) {
if (!isa<TerminatorInst>(I) &&
I->hasOneUse() && SomeOtherThing(I)) {
... some long code ....
}
return 0;
}
</pre>
</div>
<p>This code has several problems if the body of the '<tt>if</tt>' is large.
When you're looking at the top of the function, it isn't immediately clear that
this <em>only</em> does interesting things with non-terminator instructions, and
only applies to things with the other predicates. Second, it is relatively
difficult to describe (in comments) why these predicates are important because
the <tt>if</tt> statement makes it difficult to lay out the comments. Third,
when you're deep within the body of the code, it is indented an extra level.
Finally, when reading the top of the function, it isn't clear what the result is
if the predicate isn't true; you have to read to the end of the function to know
that it returns null.</p>
<p>It is much preferred to format the code like this:</p>
<div class="doc_code">
<pre>
Value *DoSomething(Instruction *I) {
// Terminators never need 'something' done to them because ...
if (isa<TerminatorInst>(I))
return 0;
// We conservatively avoid transforming instructions with multiple uses
// because goats like cheese.
if (!I->hasOneUse())
return 0;
// This is really just here for example.
if (!SomeOtherThing(I))
return 0;
... some long code ....
}
</pre>
</div>
<p>This fixes these problems. A similar problem frequently happens in <tt>for</tt>
loops. A silly example is something like this:</p>
<div class="doc_code">
<pre>
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
Value *LHS = BO->getOperand(0);
Value *RHS = BO->getOperand(1);
if (LHS != RHS) {
...
}
}
}
</pre>
</div>
<p>When you have very, very small loops, this sort of structure is fine. But if
it exceeds more than 10-15 lines, it becomes difficult for people to read and
understand at a glance. The problem with this sort of code is that it gets very
nested very quickly. Meaning that the reader of the code has to keep a lot of
context in their brain to remember what is going immediately on in the loop,
because they don't know if/when the <tt>if</tt> conditions will have elses etc.
It is strongly preferred to structure the loop like this:</p>
<div class="doc_code">
<pre>
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
if (!BO) continue;
Value *LHS = BO->getOperand(0);
Value *RHS = BO->getOperand(1);
if (LHS == RHS) continue;
...
}
</pre>
</div>
<p>This has all the benefits of using early exits for functions: it reduces
nesting of the loop, it makes it easier to describe why the conditions are true,
and it makes it obvious to the reader that there is no <tt>else</tt> coming up
that they have to push context into their brain for. If a loop is large, this
can be a big understandability win.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="hl_else_after_return">Don't use <tt>else</tt> after a <tt>return</tt></a>
</h4>
<div>
<p>For similar reasons above (reduction of indentation and easier reading),
please do not use '<tt>else</tt>' or '<tt>else if</tt>' after something that
interrupts control flow — like <tt>return</tt>, <tt>break</tt>,
<tt>continue</tt>, <tt>goto</tt>, etc. For example, this is <em>bad</em>:</p>
<div class="doc_code">
<pre>
case 'J': {
if (Signed) {
Type = Context.getsigjmp_bufType();
if (Type.isNull()) {
Error = ASTContext::GE_Missing_sigjmp_buf;
return QualType();
<b>} else {
break;
}</b>
} else {
Type = Context.getjmp_bufType();
if (Type.isNull()) {
Error = ASTContext::GE_Missing_jmp_buf;
return QualType();
<b>} else {
break;
}</b>
}
}
}
</pre>
</div>
<p>It is better to write it like this:</p>
<div class="doc_code">
<pre>
case 'J':
if (Signed) {
Type = Context.getsigjmp_bufType();
if (Type.isNull()) {
Error = ASTContext::GE_Missing_sigjmp_buf;
return QualType();
}
} else {
Type = Context.getjmp_bufType();
if (Type.isNull()) {
Error = ASTContext::GE_Missing_jmp_buf;
return QualType();
}
}
<b>break;</b>
</pre>
</div>
<p>Or better yet (in this case) as:</p>
<div class="doc_code">
<pre>
case 'J':
if (Signed)
Type = Context.getsigjmp_bufType();
else
Type = Context.getjmp_bufType();
if (Type.isNull()) {
Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
ASTContext::GE_Missing_jmp_buf;
return QualType();
}
<b>break;</b>
</pre>
</div>
<p>The idea is to reduce indentation and the amount of code you have to keep
track of when reading the code.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
</h4>
<div>
<p>It is very common to write small loops that just compute a boolean value.
There are a number of ways that people commonly write these, but an example of
this sort of thing is:</p>
<div class="doc_code">
<pre>
<b>bool FoundFoo = false;</b>
for (unsigned i = 0, e = BarList.size(); i != e; ++i)
if (BarList[i]->isFoo()) {
<b>FoundFoo = true;</b>
break;
}
<b>if (FoundFoo) {</b>
...
}
</pre>
</div>
<p>This sort of code is awkward to write, and is almost always a bad sign.
Instead of this sort of loop, we strongly prefer to use a predicate function
(which may be <a href="#micro_anonns">static</a>) that uses
<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
the code to be structured like this:</p>
<div class="doc_code">
<pre>
/// ListContainsFoo - Return true if the specified list has an element that is
/// a foo.
static bool ListContainsFoo(const std::vector<Bar*> &List) {
for (unsigned i = 0, e = List.size(); i != e; ++i)
if (List[i]->isFoo())
return true;
return false;
}
...
<b>if (ListContainsFoo(BarList)) {</b>
...
}
</pre>
</div>
<p>There are many reasons for doing this: it reduces indentation and factors out
code which can often be shared by other code that checks for the same predicate.
More importantly, it <em>forces you to pick a name</em> for the function, and
forces you to write a comment for it. In this silly example, this doesn't add
much value. However, if the condition is complex, this can make it a lot easier
for the reader to understand the code that queries for this predicate. Instead
of being faced with the in-line details of how we check to see if the BarList
contains a foo, we can trust the function name and continue reading with better
locality.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="micro">The Low-Level Issues</a>
</h3>
<!-- ======================================================================= -->
<div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_naming">
Name Types, Functions, Variables, and Enumerators Properly
</a>
</h4>
<div>
<p>Poorly-chosen names can mislead the reader and cause bugs. We cannot stress
enough how important it is to use <em>descriptive</em> names. Pick names that
match the semantics and role of the underlying entities, within reason. Avoid
abbreviations unless they are well known. After picking a good name, make sure
to use consistent capitalization for the name, as inconsistency requires clients
to either memorize the APIs or to look it up to find the exact spelling.</p>
<p>In general, names should be in camel case (e.g. <tt>TextFileReader</tt>
and <tt>isLValue()</tt>). Different kinds of declarations have different
rules:</p>
<ul>
<li><p><b>Type names</b> (including classes, structs, enums, typedefs, etc)
should be nouns and start with an upper-case letter (e.g.
<tt>TextFileReader</tt>).</p></li>
<li><p><b>Variable names</b> should be nouns (as they represent state). The
name should be camel case, and start with an upper case letter (e.g.
<tt>Leader</tt> or <tt>Boats</tt>).</p></li>
<li><p><b>Function names</b> should be verb phrases (as they represent
actions), and command-like function should be imperative. The name should
be camel case, and start with a lower case letter (e.g. <tt>openFile()</tt>
or <tt>isFoo()</tt>).</p></li>
<li><p><b>Enum declarations</b> (e.g. <tt>enum Foo {...}</tt>) are types, so
they should follow the naming conventions for types. A common use for enums
is as a discriminator for a union, or an indicator of a subclass. When an
enum is used for something like this, it should have a <tt>Kind</tt> suffix
(e.g. <tt>ValueKind</tt>).</p></li>
<li><p><b>Enumerators</b> (e.g. <tt>enum { Foo, Bar }</tt>) and <b>public member
variables</b> should start with an upper-case letter, just like types.
Unless the enumerators are defined in their own small namespace or inside a
class, enumerators should have a prefix corresponding to the enum
declaration name. For example, <tt>enum ValueKind { ... };</tt> may contain
enumerators like <tt>VK_Argument</tt>, <tt>VK_BasicBlock</tt>, etc.
Enumerators that are just convenience constants are exempt from the
requirement for a prefix. For instance:</p>
<div class="doc_code">
<pre>
enum {
MaxSize = 42,
Density = 12
};
</pre>
</div>
</li>
</ul>
<p>As an exception, classes that mimic STL classes can have member names in
STL's style of lower-case words separated by underscores (e.g. <tt>begin()</tt>,
<tt>push_back()</tt>, and <tt>empty()</tt>).</p>
<p>Here are some examples of good and bad names:</p>
<div class="doc_code">
<pre>
class VehicleMaker {
...
Factory<Tire> F; // Bad -- abbreviation and non-descriptive.
Factory<Tire> Factory; // Better.
Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one
// kind of factories.
};
Vehicle MakeVehicle(VehicleType Type) {
VehicleMaker M; // Might be OK if having a short life-span.
Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information.
Light headlight = M.makeLight("head"); // Good -- descriptive.
...
}
</pre>
</div>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_assert">Assert Liberally</a>
</h4>
<div>
<p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your
preconditions and assumptions, you never know when a bug (not necessarily even
yours) might be caught early by an assertion, which reduces debugging time
dramatically. The "<tt><cassert></tt>" header file is probably already
included by the header files you are using, so it doesn't cost anything to use
it.</p>
<p>To further assist with debugging, make sure to put some kind of error message
in the assertion statement, which is printed if the assertion is tripped. This
helps the poor debugger make sense of why an assertion is being made and
enforced, and hopefully what to do about it. Here is one complete example:</p>
<div class="doc_code">
<pre>
inline Value *getOperand(unsigned i) {
assert(i < Operands.size() && "getOperand() out of range!");
return Operands[i];
}
</pre>
</div>
<p>Here are more examples:</p>
<div class="doc_code">
<pre>
assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
assert(idx < getNumSuccessors() && "Successor # out of range!");
assert(V1.getType() == V2.getType() && "Constant types must be identical!");
assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
</pre>
</div>
<p>You get the idea.</p>
<p>Please be aware that, when adding assert statements, not all compilers are aware of
the semantics of the assert. In some places, asserts are used to indicate a piece of
code that should not be reached. These are typically of the form:</p>
<div class="doc_code">
<pre>
assert(0 && "Some helpful error message");
</pre>
</div>
<p>When used in a function that returns a value, they should be followed with a return
statement and a comment indicating that this line is never reached. This will prevent
a compiler which is unable to deduce that the assert statement never returns from
generating a warning.</p>
<div class="doc_code">
<pre>
assert(0 && "Some helpful error message");
// Not reached
return 0;
</pre>
</div>
<p>Another issue is that values used only by assertions will produce an "unused
value" warning when assertions are disabled. For example, this code will
warn:</p>
<div class="doc_code">
<pre>
unsigned Size = V.size();
assert(Size > 42 && "Vector smaller than it should be");
bool NewToSet = Myset.insert(Value);
assert(NewToSet && "The value shouldn't be in the set yet");
</pre>
</div>
<p>These are two interesting different cases. In the first case, the call to
V.size() is only useful for the assert, and we don't want it executed when
assertions are disabled. Code like this should move the call into the assert
itself. In the second case, the side effects of the call must happen whether
the assert is enabled or not. In this case, the value should be cast to void to
disable the warning. To be specific, it is preferred to write the code like
this:</p>
<div class="doc_code">
<pre>
assert(V.size() > 42 && "Vector smaller than it should be");
bool NewToSet = Myset.insert(Value); (void)NewToSet;
assert(NewToSet && "The value shouldn't be in the set yet");
</pre>
</div>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_ns_std">Do Not Use '<tt>using namespace std</tt>'</a>
</h4>
<div>
<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
namespace with an "<tt>std::</tt>" prefix, rather than rely on
"<tt>using namespace std;</tt>".</p>
<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
the namespace of any source file that <tt>#include</tt>s the header. This is
clearly a bad thing.</p>
<p>In implementation files (e.g. <tt>.cpp</tt> files), the rule is more of a stylistic
rule, but is still important. Basically, using explicit namespace prefixes
makes the code <b>clearer</b>, because it is immediately obvious what facilities
are being used and where they are coming from. And <b>more portable</b>, because
namespace clashes cannot occur between LLVM code and other namespaces. The
portability rule is important because different standard library implementations
expose different symbols (potentially ones they shouldn't), and future revisions
to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
<p>The exception to the general rule (i.e. it's not an exception for
the <tt>std</tt> namespace) is for implementation files. For example, all of
the code in the LLVM project implements code that lives in the 'llvm' namespace.
As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a
'<tt>using namespace llvm;</tt>' directive at the top, after the
<tt>#include</tt>s. This reduces indentation in the body of the file for source
editors that indent based on braces, and keeps the conceptual context cleaner.
The general form of this rule is that any <tt>.cpp</tt> file that implements
code in any namespace may use that namespace (and its parents'), but should not
use any others.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_virtual_anch">
Provide a Virtual Method Anchor for Classes in Headers
</a>
</h4>
<div>
<p>If a class is defined in a header file and has a v-table (either it has
virtual methods or it derives from classes with virtual methods), it must
always have at least one out-of-line virtual method in the class. Without
this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
increasing link times.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a>
</h4>
<div>
<p>Because C++ doesn't have a standard "<tt>foreach</tt>" loop (though it can be
emulated with macros and may be coming in C++'0x) we end up writing a lot of
loops that manually iterate from begin to end on a variety of containers or
through other data structures. One common mistake is to write a loop in this
style:</p>
<div class="doc_code">
<pre>
BasicBlock *BB = ...
for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
... use I ...
</pre>
</div>
<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
every time through the loop. Instead of writing the loop like this, we strongly
prefer loops to be written so that they evaluate it once before the loop starts.
A convenient way to do this is like so:</p>
<div class="doc_code">
<pre>
BasicBlock *BB = ...
for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
... use I ...
</pre>
</div>
<p>The observant may quickly point out that these two loops may have different
semantics: if the container (a basic block in this case) is being mutated, then
"<tt>BB->end()</tt>" may change its value every time through the loop and the
second loop may not in fact be correct. If you actually do depend on this
behavior, please write the loop in the first form and add a comment indicating
that you did it intentionally.</p>
<p>Why do we prefer the second form (when correct)? Writing the loop in the
first form has two problems. First it may be less efficient than evaluating it
at the start of the loop. In this case, the cost is probably minor — a
few extra loads every time through the loop. However, if the base expression is
more complex, then the cost can rise quickly. I've seen loops where the end
expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
lookups really aren't cheap. By writing it in the second form consistently, you
eliminate the issue entirely and don't even have to think about it.</p>
<p>The second (even bigger) issue is that writing the loop in the first form
hints to the reader that the loop is mutating the container (a fact that a
comment would handily confirm!). If you write the loop in the second form, it
is immediately obvious without even looking at the body of the loop that the
container isn't being modified, which makes it easier to read the code and
understand what it does.</p>
<p>While the second form of the loop is a few extra keystrokes, we do strongly
prefer it.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_iostream"><tt>#include <iostream></tt> is Forbidden</a>
</h4>
<div>
<p>The use of <tt>#include <iostream></tt> in library files is
hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
support clients using LLVM libraries as part of larger systems. In particular,
we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
the static constructors are run whenever an application starts up that uses the
dynamic library. There are two problems with this:</p>
<ol>
<li>The time to run the static c'tors impacts startup time of applications
— a critical time for GUI apps.</li>
<li>The static c'tors cause the app to pull many extra pages of memory off the
disk: both the code for the static c'tors in each <tt>.o</tt> file and the
small amount of data that gets touched. In addition, touched/dirty pages
put more pressure on the VM system on low-memory machines.</li>
</ol>
<p>Note that using the other stream headers (<tt><sstream></tt> for
example) is not problematic in this regard —
just <tt><iostream></tt>. However, <tt>raw_ostream</tt> provides various
APIs that are better performing for almost every use than <tt>std::ostream</tt>
style APIs. <b>Therefore new code should always
use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or
the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
</h4>
<div>
<p>LLVM includes a lightweight, simple, and efficient stream implementation
in <tt>llvm/Support/raw_ostream.h</tt>, which provides all of the common
features of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt>
instead of <tt>ostream</tt>.</p>
<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
be forward declared as <tt>class raw_ostream</tt>. Public headers should
generally not include the <tt>raw_ostream</tt> header, but use forward
declarations and constant references to <tt>raw_ostream</tt> instances.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
</h4>
<div>
<p>The <tt>std::endl</tt> modifier, when used with <tt>iostreams</tt> outputs a
newline to the output stream specified. In addition to doing this, however, it
also flushes the output stream. In other words, these are equivalent:</p>
<div class="doc_code">
<pre>
std::cout << std::endl;
std::cout << '\n' << std::flush;
</pre>
</div>
<p>Most of the time, you probably have no reason to flush the output stream, so
it's better to use a literal <tt>'\n'</tt>.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="nano">Microscopic Details</a>
</h3>
<!-- ======================================================================= -->
<div>
<p>This section describes preferred low-level formatting guidelines along with
reasoning on why we prefer them.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="micro_spaceparen">Spaces Before Parentheses</a>
</h4>
<div>
<p>We prefer to put a space before an open parenthesis only in control flow
statements, but not in normal function call expressions and function-like
macros. For example, this is good:</p>
<div class="doc_code">
<pre>
<b>if (</b>x) ...
<b>for (</b>i = 0; i != 100; ++i) ...
<b>while (</b>llvm_rocks) ...
<b>somefunc(</b>42);
<b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me");
a = <b>foo(</b>42, 92) + <b>bar(</b>x);
</pre>
</div>
<p>and this is bad:</p>
<div class="doc_code">
<pre>
<b>if(</b>x) ...
<b>for(</b>i = 0; i != 100; ++i) ...
<b>while(</b>llvm_rocks) ...
<b>somefunc (</b>42);
<b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me");
a = <b>foo (</b>42, 92) + <b>bar (</b>x);
</pre>
</div>
<p>The reason for doing this is not completely arbitrary. This style makes
control flow operators stand out more, and makes expressions flow better. The
function call operator binds very tightly as a postfix operator. Putting a
space after a function name (as in the last example) makes it appear that the
code might bind the arguments of the left-hand-side of a binary operator with
the argument list of a function and the name of the right side. More
specifically, it is easy to misread the "a" example as:</p>
<div class="doc_code">
<pre>
a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
</pre>
</div>
<p>when skimming through the code. By avoiding a space in a function, we avoid
this misinterpretation.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="micro_preincrement">Prefer Preincrement</a>
</h4>
<div>
<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
preincrementation whenever possible.</p>
<p>The semantics of postincrement include making a copy of the value being
incremented, returning it, and then preincrementing the "work value". For
primitive types, this isn't a big deal... but for iterators, it can be a huge
issue (for example, some iterators contains stack and set objects in them...
copying an iterator could invoke the copy ctor's of these as well). In general,
get in the habit of always using preincrement, and you won't have a problem.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="micro_namespaceindent">Namespace Indentation</a>
</h4>
<div>
<p>
In general, we strive to reduce indentation wherever possible. This is useful
because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
wrapping horribly, but also because it makes it easier to understand the code.
Namespaces are a funny thing: they are often large, and we often desire to put
lots of stuff into them (so they can be large). Other times they are tiny,
because they just hold an enum or something similar. In order to balance this,
we use different approaches for small versus large namespaces.
</p>
<p>
If a namespace definition is small and <em>easily</em> fits on a screen (say,
less than 35 lines of code), then you should indent its body. Here's an
example:
</p>
<div class="doc_code">
<pre>
namespace llvm {
namespace X86 {
/// RelocationType - An enum for the x86 relocation codes. Note that
/// the terminology here doesn't follow x86 convention - word means
/// 32-bit and dword means 64-bit.
enum RelocationType {
/// reloc_pcrel_word - PC relative relocation, add the relocated value to
/// the value already in memory, after we adjust it for where the PC is.
reloc_pcrel_word = 0,
/// reloc_picrel_word - PIC base relative relocation, add the relocated
/// value to the value already in memory, after we adjust it for where the
/// PIC base is.
reloc_picrel_word = 1,
/// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
/// add the relocated value to the value already in memory.
reloc_absolute_word = 2,
reloc_absolute_dword = 3
};
}
}
</pre>
</div>
<p>Since the body is small, indenting adds value because it makes it very clear
where the namespace starts and ends, and it is easy to take the whole thing in
in one "gulp" when reading the code. If the blob of code in the namespace is
larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not
indent the code, and add a comment indicating what namespace is being closed.
For example:</p>
<div class="doc_code">
<pre>
namespace llvm {
namespace knowledge {
/// Grokable - This class represents things that Smith can have an intimate
/// understanding of and contains the data associated with it.
class Grokable {
...
public:
explicit Grokable() { ... }
virtual ~Grokable() = 0;
...
};
} // end namespace knowledge
} // end namespace llvm
</pre>
</div>
<p>Because the class is large, we don't expect that the reader can easily
understand the entire concept in a glance, and the end of the file (where the
namespaces end) may be a long ways away from the place they open. As such,
indenting the contents of the namespace doesn't add any value, and detracts from
the readability of the class. In these cases it is best to <em>not</em> indent
the contents of the namespace.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="micro_anonns">Anonymous Namespaces</a>
</h4>
<div>
<p>After talking about namespaces in general, you may be wondering about
anonymous namespaces in particular.
Anonymous namespaces are a great language feature that tells the C++ compiler
that the contents of the namespace are only visible within the current
translation unit, allowing more aggressive optimization and eliminating the
possibility of symbol name collisions. Anonymous namespaces are to C++ as
"static" is to C functions and global variables. While "static" is available
in C++, anonymous namespaces are more general: they can make entire classes
private to a file.</p>
<p>The problem with anonymous namespaces is that they naturally want to
encourage indentation of their body, and they reduce locality of reference: if
you see a random function definition in a C++ file, it is easy to see if it is
marked static, but seeing if it is in an anonymous namespace requires scanning
a big chunk of the file.</p>
<p>Because of this, we have a simple guideline: make anonymous namespaces as
small as possible, and only use them for class declarations. For example, this
is good:</p>
<div class="doc_code">
<pre>
<b>namespace {</b>
class StringSort {
...
public:
StringSort(...)
bool operator<(const char *RHS) const;
};
<b>} // end anonymous namespace</b>
static void Helper() {
...
}
bool StringSort::operator<(const char *RHS) const {
...
}
</pre>
</div>
<p>This is bad:</p>
<div class="doc_code">
<pre>
<b>namespace {</b>
class StringSort {
...
public:
StringSort(...)
bool operator<(const char *RHS) const;
};
void Helper() {
...
}
bool StringSort::operator<(const char *RHS) const {
...
}
<b>} // end anonymous namespace</b>
</pre>
</div>
<p>This is bad specifically because if you're looking at "Helper" in the middle
of a large C++ file, that you have no immediate way to tell if it is local to
the file. When it is marked static explicitly, this is immediately obvious.
Also, there is no reason to enclose the definition of "operator<" in the
namespace just because it was declared there.
</p>
</div>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="seealso">See Also</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>A lot of these comments and recommendations have been culled for other
sources. Two particularly important books for our work are:</p>
<ol>
<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
C++</a> by Scott Meyers. Also
interesting and useful are "More Effective C++" and "Effective STL" by the same
author.</li>
<li>Large-Scale C++ Software Design by John Lakos</li>
</ol>
<p>If you get some free time, and you haven't read them: do so, you might learn
something.</p>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
</body>
</html>
|
