[llvm] [ValueTracking] Fix bit width handling in computeKnownBits() for GEPs (PR #125532)

[Yingwei Zheng via llvm-commits]

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@@ -1426,7 +1426,22 @@ static void computeKnownBitsFromOperator(const Operator *I,
     computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
     // Accumulate the constant indices in a separate variable
     // to minimize the number of calls to computeForAddSub.
-    APInt AccConstIndices(BitWidth, 0, /*IsSigned*/ true);
+    unsigned IndexWidth = Q.DL.getIndexTypeSizeInBits(I->getType());
+    APInt AccConstIndices(IndexWidth, 0);
+
+    auto AddIndex = [&](KnownBits IndexBits) {
+      if (IndexWidth == BitWidth) {
+        // Note that inbounds does *not* guarantee nsw for the addition, as only
+        // the offset is signed, while the base address is unsigned.
+        Known = KnownBits::add(Known, IndexBits);
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dtcxzyw wrote:

We cannot preserve `nsw` here because we changed the order of pointer addition (line 1485).

https://github.com/llvm/llvm-project/pull/125532