[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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@@ -5705,6 +5705,22 @@ Instruction *InstCombinerImpl::foldICmpWithTrunc(ICmpInst &ICmp) {
   ICmpInst::Predicate Pred = ICmp.getPredicate();
   Value *Op0 = ICmp.getOperand(0), *Op1 = ICmp.getOperand(1);
 
+  // Match (icmp pred (trunc nuw/nsw X), C)
+  // Which we can convert to (icmp pred X, (sext/zext C))
+  // TODO: Maybe this makes sense as a general canonicalization?
+  if (match(Op1, m_ImmConstant())) {
+    if (auto *TI = dyn_cast<TruncInst>(Op0)) {
+      Value *ExtOp0 = TI->getOperand(0);
+      Value *ExtOp1 = nullptr;
+      if (!ICmp.isSigned() && TI->hasNoUnsignedWrap())
+        ExtOp1 = Builder.CreateZExt(Op1, ExtOp0->getType());
+      else if (!ICmp.isUnsigned() && TI->hasNoSignedWrap())
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nikic wrote:

This is actually also valid for `isUnsigned()` predicates: https://alive2.llvm.org/ce/z/MiDiYZ

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