Craig Topper fa630e7594 [RISCV][AMDGPU][TargetLowering] Special case overflow expansion for (uaddo X, 1). ...

If we expand (uaddo X, 1) we previously expanded the overflow calculation
as (X + 1) <u X. This potentially increases the live range of X and
can prevent X+1 from reusing the register that previously held X.

Since we're adding 1, overflow only occurs if X was UINT_MAX in which
case (X+1) would be 0. So this patch adds a special case to expand
the overflow calculation to (X+1) == 0.

This seems to help with uaddo intrinsics that get introduced by
CodeGenPrepare after LSR. Alternatively, we could block the uaddo
transform in CodeGenPrepare for this case.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D122933

2022-04-01 13:14:10 -07:00

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