diff options
Diffstat (limited to 'lib/Target/X86/README.txt')
| -rw-r--r-- | lib/Target/X86/README.txt | 45 |
1 files changed, 40 insertions, 5 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 3796aac57cb5..4464878ce217 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -1858,8 +1858,23 @@ Ideal output: setne %al ret -We could do this transformation in instcombine, but it's only clearly -beneficial on platforms with a test instruction. +This should definitely be done in instcombine, canonicalizing the range +condition into a != condition. We get this IR: + +define i32 @a(i32 %x) nounwind readnone { +entry: + %0 = and i32 %x, 127 ; <i32> [#uses=1] + %1 = icmp ugt i32 %0, 31 ; <i1> [#uses=1] + %2 = zext i1 %1 to i32 ; <i32> [#uses=1] + ret i32 %2 +} + +Instcombine prefers to strength reduce relational comparisons to equality +comparisons when possible, this should be another case of that. This could +be handled pretty easily in InstCombiner::visitICmpInstWithInstAndIntCst, but it +looks like InstCombiner::visitICmpInstWithInstAndIntCst should really already +be redesigned to use ComputeMaskedBits and friends. + //===---------------------------------------------------------------------===// Testcase: @@ -1880,20 +1895,40 @@ Ideal output: Testcase: int x(int a) { return (a & 0x80) ? 0x100 : 0; } +int y(int a) { return (a & 0x80) *2; } -Current output: +Current: testl $128, 4(%esp) setne %al movzbl %al, %eax shll $8, %eax ret -Ideal output: +Better: movl 4(%esp), %eax addl %eax, %eax andl $256, %eax ret -We generally want to fold shifted tests of a single bit into a shift+and on x86. +This is another general instcombine transformation that is profitable on all +targets. In LLVM IR, these functions look like this: + +define i32 @x(i32 %a) nounwind readnone { +entry: + %0 = and i32 %a, 128 + %1 = icmp eq i32 %0, 0 + %iftmp.0.0 = select i1 %1, i32 0, i32 256 + ret i32 %iftmp.0.0 +} + +define i32 @y(i32 %a) nounwind readnone { +entry: + %0 = shl i32 %a, 1 + %1 = and i32 %0, 256 + ret i32 %1 +} + +Replacing an icmp+select with a shift should always be considered profitable in +instcombine. //===---------------------------------------------------------------------===// |