JitArm64: Fix special cases of cmp

This fixes a regression from 592ba31. When `a` was a constant 0 and `b`
was a non-constant 0x80000000, the 32-bit negation operation would
overflow, causing an incorrect result. The sign extension needs to happen
before the negation to avoid overflow.

Note that I can't merge the SXTW and NEG into one instruction.
NEG is an alias for SUB with the first operand being set to ZR,
but "SUB (extended register)" treats register 31 as SP instead of ZR.

I've also changed the order for the case where `a` is a constant
0xFFFFFFFF. I don't think the order actually affects correctness here,
but let's use the same order for all the cases since it makes the code
easier to reason about.

0 files changed, 0 insertions, 0 deletions