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| -rw-r--r-- | lib/Target/X86/README.txt | 52 |
1 files changed, 52 insertions, 0 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 6034a09..69c6d33 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -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. + +//===---------------------------------------------------------------------===// |