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0fe549ac91
Our handling of select-of-constants is lumpy in IR (https://reviews.llvm.org/D24480), lumpy in DAGCombiner, and lumpy in X86ISelLowering. That's why we only had the 'sbb' codegen in 1 out of the 4 tests. This is a step towards smoothing that out. First, show that all of these IR forms are equivalent: http://rise4fun.com/Alive/mx Second, show that the 'sbb' version is faster/smaller. IACA output for SandyBridge (later Intel and AMD chips are similar based on Agner's tables): This is the "obvious" x86 codegen (what gcc appears to produce currently): | Num Of | Ports pressure in cycles | | | Uops | 0 - DV | 1 | 2 - D | 3 - D | 4 | 5 | | --------------------------------------------------------------------- | 1* | | | | | | | | xor eax, eax | 1 | 1.0 | | | | | | CP | test edi, edi | 1 | | | | | | 1.0 | CP | setnz al | 1 | | 1.0 | | | | | CP | neg eax This is the adc version: | 1* | | | | | | | | xor eax, eax | 1 | 1.0 | | | | | | CP | cmp edi, 0x1 | 2 | | 1.0 | | | | 1.0 | CP | adc eax, 0xffffffff And this is sbb: | 1 | 1.0 | | | | | | | neg edi | 2 | | 1.0 | | | | 1.0 | CP | sbb eax, eax If IACA is trustworthy, then sbb became a single uop in Broadwell, so this will be clearly better than the alternatives going forward. llvm-svn: 306040
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