[llvm] [InstCombine] Fold `(icmp pred (trunc nuw/nsw X), C)` -> `(icmp pred X, (zext/sext C))` (PR #87935)

[Nikita Popov via llvm-commits]

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@@ -1409,6 +1409,19 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
                                                      const APInt &C) {
   ICmpInst::Predicate Pred = Cmp.getPredicate();
   Value *X = Trunc->getOperand(0);
+  Type *SrcTy = X->getType();
+  unsigned DstBits = Trunc->getType()->getScalarSizeInBits(),
+           SrcBits = SrcTy->getScalarSizeInBits();
+
+  // Match (icmp pred (trunc nuw/nsw X), C)
+  // Which we can convert to (icmp pred X, (sext/zext C))
+  if (shouldChangeType(DstBits, SrcBits)) {
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nikic wrote:

This isn't really true for vectors. If the previous trunc + icmp sequence worked on legal types and you convert it into one that works on illegal types, that may be catastrophically worse (down to scalarization). There's a reason the special handling for vectors is there.

https://github.com/llvm/llvm-project/pull/87935