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A couple of small missed optimizations.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112411 91177308-0d34-0410-b5e6-96231b3b80d8
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Eli Friedman 2010-08-29 05:07:40 +00:00
parent a6a36f59b4
commit 5033f64694
1 changed files with 52 additions and 0 deletions
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52
lib/Target/X86/README.txt
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@ -1863,3 +1863,55 @@ The code produced by gcc is 3 bytes shorter. This sort of construct often
shows up with bitfields.
//===---------------------------------------------------------------------===//
Take the following C code:
int f(int a, int b) { return (unsigned char)a == (unsigned char)b; }
We generate the following IR with clang:
define i32 @f(i32 %a, i32 %b) nounwind readnone {
entry:
%tmp = xor i32 %b, %a ; <i32> [#uses=1]
%tmp6 = and i32 %tmp, 255 ; <i32> [#uses=1]
%cmp = icmp eq i32 %tmp6, 0 ; <i1> [#uses=1]
%conv5 = zext i1 %cmp to i32 ; <i32> [#uses=1]
ret i32 %conv5
}
And the following x86 code:
xorl %esi, %edi
testb $-1, %dil
sete %al
movzbl %al, %eax
ret
A cmpb instead of the xorl+testb would be one instruction shorter.
//===---------------------------------------------------------------------===//
Given the following C code:
int f(int a, int b) { return (signed char)a == (signed char)b; }
We generate the following IR with clang:
define i32 @f(i32 %a, i32 %b) nounwind readnone {
entry:
%sext = shl i32 %a, 24 ; <i32> [#uses=1]
%conv1 = ashr i32 %sext, 24 ; <i32> [#uses=1]
%sext6 = shl i32 %b, 24 ; <i32> [#uses=1]
%conv4 = ashr i32 %sext6, 24 ; <i32> [#uses=1]
%cmp = icmp eq i32 %conv1, %conv4 ; <i1> [#uses=1]
%conv5 = zext i1 %cmp to i32 ; <i32> [#uses=1]
ret i32 %conv5
}
And the following x86 code:
movsbl %sil, %eax
movsbl %dil, %ecx
cmpl %eax, %ecx
sete %al
movzbl %al, %eax
ret
It should be possible to eliminate the sign extensions.
//===---------------------------------------------------------------------===//