[llvm] [InstCombine] Check nowrap flags when folding comparison of GEPs with the same base pointer (PR #121892)

[Nikita Popov via llvm-commits]

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@@ -833,8 +833,11 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
       if (NumDifferences == 0)   // SAME GEP?
         return replaceInstUsesWith(I, // No comparison is needed here.
           ConstantInt::get(I.getType(), ICmpInst::isTrueWhenEqual(Cond)));
-
-      else if (NumDifferences == 1 && CanFold(NW)) {
+      // If two GEPs only differ by an index/the base pointer, compare them.
+      // Note that nowrap flags are always needed when comparing two indices.
+      else if (NumDifferences == 1 &&
+               (DiffOperand == 0 ? CanFold(NW)
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nikic wrote:

I don't think DiffOperand == 0 can happen here? The above loop only goes over the index operands. The case of base pointer difference is handled by different code further up.

https://github.com/llvm/llvm-project/pull/121892