[am11]

Fixes #135647

Adding or subtracting a compare result currently materializes it with
setcc/cset before the add/sub. Use the flags set by the compare
directly instead:

xarch:  x + (a <u b)   =>  cmp a, b; adc x, 0
        x - (a <u b)   =>  cmp a, b; sbb x, 0
        x + (a >=u b)  =>  cmp a, b; sbb x, -1
        x - (a >=u b)  =>  cmp a, b; adc x, -1
arm:    x + (a >=u b)  =>  cmp a, b; adc x, x, zr
        x - (a <u b)   =>  cmp a, b; sbc x, x, zr
arm64:  x + cond       =>  cmp a, b; cinc x, x, cond   (any other condition)

a == 0 and a != 0 are handled as a <u 1 and a >=u 1, and
a <=u c / a >u c as a <u c+1 / a >=u c+1. An added constant,
x +/- (cond + k), is folded into the carry op's immediate where the
target has one (xarch, arm32). This does not rely on bools being
normalized to 0/1, so for example num - (r ? 0 : 1) becomes
cmp r, 1; sbb num, 0 on x64.
GT_ADD_HI/GT_SUB_HI, previously only defined for long decomposition
on 32-bit targets, are now defined for all targets, like the other
flags consuming nodes (GT_JCC, GT_SETCC, GT_SELECTCC). The
transformation lives in common lowering (TryLowerAddSubOfCompare);
each target's TryGetCarryOp decides which add/subtract with carry, if
any, computes the result:

• xarch: adc adds the borrow and sbb subtracts it, any constant.
• arm32/arm64: adc adds the carry (the inverse of the borrow) and
  sbc subtracts the borrow; arm64 has no immediate form, so only
  without a constant. Remaining additions on arm64 use cinc
  (TryLowerAddOfCompareToCinc), which works for any condition.
• riscv64, loongarch64, wasm: no flags register, nothing changes.

Diffs

SuperPMI asmdiffs on the benchmarks collections:

Target	Contexts with diffs	Improvements / regressions	Bytes
windows-x64	135	135 / 0	-1,203
linux-arm64	60	55 / 0	-400
linux-arm	6	6 / 0	-36
linux-riscv64	0	-	0
linux-loongarch64	0	-	0
Example (x64), num - (r ? 0 : 1):

-       test     dl, dl
-       sete     al
-       movzx    rax, al
-       sub      ecx, eax
-       mov      eax, ecx
+       cmp      dl, 1
+       mov      eax, ecx
+       sbb      eax, 0

Note

This description was generated with AI assistance.

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[am11]

Wasm failure is unrelated (infra) issue.

cc @EgorBo

Diffs