[NVPTX] Fix sext(shl nsw x, topbit) miscompile. (#200924)
Consider the following IR.
i64 f(i32 %x) {
%s = shl nsw i32 %x, 31
%e = sext i32 %s to i64
ret %e
}
combineMulWide (renamed in this patch to combineSZExtToMulWide) rewrites
this into:
mul.wide.s32 %dst, %x, -2147483648
The LLVM IR is only meaningful for %x == 0 or -1; all other inputs
result in poison. Therefore to check whether this rewrite is correct, we
just need to ask if it generates the correct output when %x is 0 and
when %x is -1.
- When x == 0, IR and PTX both produce 0. OK.
- When x == -1:
- IR produces sext(INT32_MIN), whereas
- PTX produces sext(-1) * sext(INT32_MIN) = -sext(INT32_MIN)
Therefore the transformation is not correct.
This only happens when we're shifting by N-1 bits (where N is the narrow
integer width). In other cases, the multiplier is positive and it works
fine.
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