Previously, power-of-2 div/rem operations wider than
MaxLegalDivRemBitWidth were excluded from IR expansion and left for
backend peephole optimizations. Some backends can fail to process such
instructions in case we switch off DAGCombiner.
Now ExpandIRInsts expands them into shift/mask sequences:
- udiv X, 2^C -> lshr X, C
- urem X, 2^C -> and X, (2^C - 1)
- sdiv X, 2^C -> bias adjustment + ashr X, C
- srem X, 2^C -> X - (((X + Bias) >> C) << C)
Special cases handled:
- Division/remainder by 1 or -1 (identity, negation, or zero)
- Exact division (sdiv exact skips bias, produces ashr exact)
- Negative power-of-2 divisors (result is negated)
- INT_MIN divisor (correct via countr_zero on bit pattern)
Proofs: https://alive2.llvm.org/ce/z/Y-iWm-
Assisted-by: Cursor // Claude Opus 4.6
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