A few missed optimizations; the last one could have a significant impact on

code with lots of bitfields.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95809 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 66 insertions, 0 deletions

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index aa7bb3d..3c6138b 100644
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -1868,3 +1868,69 @@ carried over to machine instructions. Asm printer (or JIT) can use this
 information to add the "lock" prefix.
 
 //===---------------------------------------------------------------------===//
+
+_Bool bar(int *x) { return *x & 1; }
+
+define zeroext i1 @bar(i32* nocapture %x) nounwind readonly {
+entry:
+  %tmp1 = load i32* %x                            ; <i32> [#uses=1]
+  %and = and i32 %tmp1, 1                         ; <i32> [#uses=1]
+  %tobool = icmp ne i32 %and, 0                   ; <i1> [#uses=1]
+  ret i1 %tobool
+}
+
+bar:                                                        # @bar
+# BB#0:                                                     # %entry
+	movl	4(%esp), %eax
+	movb	(%eax), %al
+	andb	$1, %al
+	movzbl	%al, %eax
+	ret
+
+Missed optimization: should be movl+andl.
+
+//===---------------------------------------------------------------------===//
+
+Consider the following two functions compiled with clang:
+_Bool foo(int *x) { return !(*x & 4); }
+unsigned bar(int *x) { return !(*x & 4); }
+
+foo:
+	movl	4(%esp), %eax
+	testb	$4, (%eax)
+	sete	%al
+	movzbl	%al, %eax
+	ret
+
+bar:
+	movl	4(%esp), %eax
+	movl	(%eax), %eax
+	shrl	$2, %eax
+	andl	$1, %eax
+	xorl	$1, %eax
+	ret
+
+The second function generates more code even though the two functions are
+are functionally identical.
+
+//===---------------------------------------------------------------------===//
+
+Take the following C code:
+int x(int y) { return (y & 63) << 14; }
+
+Code produced by gcc:
+	andl	$63, %edi
+	sall	$14, %edi
+	movl	%edi, %eax
+	ret
+
+Code produced by clang:
+	shll	$14, %edi
+	movl	%edi, %eax
+	andl	$1032192, %eax
+	ret
+
+The code produced by gcc is 3 bytes shorter.  This sort of construct often
+shows up with bitfields.
+
+//===---------------------------------------------------------------------===//