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author | Chris Lattner <sabre@nondot.org> | 2008-12-15 08:32:28 +0000 |
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committer | Chris Lattner <sabre@nondot.org> | 2008-12-15 08:32:28 +0000 |
commit | 582048d5b8e832649ff725a6fe124efb50e86b58 (patch) | |
tree | 5a5793ade6a5165e13bf914123b5f980ea9abf97 /lib | |
parent | 8f416f3afd3c0ebae2ae897cfdaf5009e99f19d5 (diff) | |
download | external_llvm-582048d5b8e832649ff725a6fe124efb50e86b58.zip external_llvm-582048d5b8e832649ff725a6fe124efb50e86b58.tar.gz external_llvm-582048d5b8e832649ff725a6fe124efb50e86b58.tar.bz2 |
add some more notes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61033 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib')
-rw-r--r-- | lib/Target/README.txt | 55 |
1 files changed, 53 insertions, 2 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index 381d0c3..2b7e9ed 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -1354,12 +1354,58 @@ GCC PR33344 is a similar case. There are many load PRE testcases in testsuite/gcc.dg/tree-ssa/loadpre* in the GCC testsuite. There are many pre testcases as ssa-pre-*.c +//===---------------------------------------------------------------------===// + +There are some interesting cases in testsuite/gcc.dg/tree-ssa/pred-comm* in the +GCC testsuite. For example, predcom-1.c is: + + for (i = 2; i < 1000; i++) + fib[i] = (fib[i-1] + fib[i - 2]) & 0xffff; + +which compiles into: + +bb1: ; preds = %bb1, %bb1.thread + %indvar = phi i32 [ 0, %bb1.thread ], [ %0, %bb1 ] + %i.0.reg2mem.0 = add i32 %indvar, 2 + %0 = add i32 %indvar, 1 ; <i32> [#uses=3] + %1 = getelementptr [1000 x i32]* @fib, i32 0, i32 %0 + %2 = load i32* %1, align 4 ; <i32> [#uses=1] + %3 = getelementptr [1000 x i32]* @fib, i32 0, i32 %indvar + %4 = load i32* %3, align 4 ; <i32> [#uses=1] + %5 = add i32 %4, %2 ; <i32> [#uses=1] + %6 = and i32 %5, 65535 ; <i32> [#uses=1] + %7 = getelementptr [1000 x i32]* @fib, i32 0, i32 %i.0.reg2mem.0 + store i32 %6, i32* %7, align 4 + %exitcond = icmp eq i32 %0, 998 ; <i1> [#uses=1] + br i1 %exitcond, label %return, label %bb1 + +This is basically: + LOAD fib[i+1] + LOAD fib[i] + STORE fib[i+2] + +instead of handling this as a loop or other xform, all we'd need to do is teach +load PRE to phi translate the %0 add (i+1) into the predecessor as (i'+1+1) = +(i'+2) (where i' is the previous iteration of i). This would find the store +which feeds it. + +predcom-2.c is apparently the same as predcom-1.c +predcom-3.c is very similar but needs loads feeding each other instead of +store->load. +predcom-4.c seems the same as the rest. + + +//===---------------------------------------------------------------------===// + Other simple load PRE cases: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35287 [LPRE crit edge splitting] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34677 (licm does this, LPRE crit edge) llvm-gcc t2.c -S -o - -O0 -emit-llvm | llvm-as | opt -mem2reg -simplifycfg -gvn | llvm-dis +//===---------------------------------------------------------------------===// + +Type based alias analysis: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14705 //===---------------------------------------------------------------------===// @@ -1380,7 +1426,7 @@ int foo(int C, int *P, float X) { One example (that requires crazy phi translation) is: -http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16799 +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16799 [BITCAST PHI TRANS] //===---------------------------------------------------------------------===// @@ -1388,9 +1434,14 @@ A/B get pinned to the stack because we turn an if/then into a select instead of PRE'ing the load/store. This may be fixable in instcombine: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37892 + + Interesting missed case because of control flow flattening (should be 2 loads): http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26629 - +With: llvm-gcc t2.c -S -o - -O0 -emit-llvm | llvm-as | + opt -mem2reg -gvn -instcombine | llvm-dis +we miss it because we need 1) GEP PHI TRAN, 2) CRIT EDGE 3) MULTIPLE DIFFERENT +VALS PRODUCED BY ONE BLOCK OVER DIFFERENT PATHS //===---------------------------------------------------------------------===// |