[X86] Improve (select carry C1+1 C1)
Without this patch:
return X < 4 ? 3 : 2;
return X < 9 ? 7 : 6;
are compiled as:
31 c0 xor %eax,%eax
83 ff 04 cmp $0x4,%edi
0f 93 c0 setae %al
83 f0 03 xor $0x3,%eax
31 c0 xor %eax,%eax
83 ff 09 cmp $0x9,%edi
0f 92 c0 setb %al
83 c8 06 or $0x6,%eax
respectively. With this patch, we generate:
31 c0 xor %eax,%eax
83 ff 04 cmp $0x4,%edi
83 d0 02 adc $0x2,%eax
31 c0 xor %eax,%eax
83 ff 04 cmp $0x4,%edi
83 d0 02 adc $0x2,%eax
respectively, saving 3 bytes while reducing the tree height.
This patch recognizes the equivalence of OR and ADD
(if bits do not overlap) and delegates to combineAddOrSubToADCOrSBB
for further processing. The same applies to the equivalence of XOR
and SUB.
Differential Revision: https://reviews.llvm.org/D143838