[clang] [llvm] [Instcombine] use zext's nneg flag for icmp folding (PR #70845)

[Nikita Popov via llvm-commits]

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@@ -5587,11 +5587,20 @@ Instruction *InstCombinerImpl::foldICmpWithZextOrSext(ICmpInst &ICmp) {
         return new ICmpInst(ICmp.getPredicate(), Builder.CreateOr(X, Y),
                             Constant::getNullValue(X->getType()));
 
+      // Treat "zext nneg" as "sext"
+      auto *NonNegInst0 = dyn_cast<PossiblyNonNegInst>(ICmp.getOperand(0));
+      auto *NonNegInst1 = dyn_cast<PossiblyNonNegInst>(ICmp.getOperand(1));
+
+      bool IsNonNeg0 = NonNegInst0 && NonNegInst0->hasNonNeg();
+      bool IsNonNeg1 = NonNegInst1 && NonNegInst1->hasNonNeg();
+
       // If we have mismatched casts, treat the zext of a non-negative source as
       // a sext to simulate matching casts. Otherwise, we are done.
       // TODO: Can we handle some predicates (equality) without non-negative?
-      if ((IsZext0 && isKnownNonNegative(X, DL, 0, &AC, &ICmp, &DT)) ||
-          (IsZext1 && isKnownNonNegative(Y, DL, 0, &AC, &ICmp, &DT)))
+      if ((IsZext0 &&
+           (IsNonNeg0 || isKnownNonNegative(X, DL, 0, &AC, &ICmp, &DT))) ||
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nikic wrote:

After rebasing over https://github.com/llvm/llvm-project/commit/5918f62301788b53e7d3a23f3203c483e9d4d791 it's possible to drop the isKnownNonNegative() call here entirely. We should only check for zext nneg.

https://github.com/llvm/llvm-project/pull/70845