[PATCH] D90637: [ValueTracking] Inbounds goes not imply nsw

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: qcolombet, aqjune, spatel, efriedma.
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The more precise known bits analysis for GEPs introduced in D86364 <https://reviews.llvm.org/D86364> assumes that inbounds implies nsw for the additions. This is not the case, as the base pointer is an unsigned value.

I was not able to come up with a test case where this actually makes a difference, because KnownBits::computeMul() is too imprecise (doing something like adding a non-negative base and a non-negative offset fails because multiplication by 1 loses the non-negativity information.)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D90637

Files:
  llvm/lib/Analysis/ValueTracking.cpp


Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -1372,9 +1372,6 @@
     APInt AccConstIndices(BitWidth, 0, /*IsSigned*/ true);
 
     gep_type_iterator GTI = gep_type_begin(I);
-    // If the inbounds keyword is not present, the offsets are added to the
-    // base address with silently-wrapping two’s complement arithmetic.
-    bool IsInBounds = cast<GEPOperator>(I)->isInBounds();
     for (unsigned i = 1, e = I->getNumOperands(); i != e; ++i, ++GTI) {
       // TrailZ can only become smaller, short-circuit if we hit zero.
       if (Known.isUnknown())
@@ -1439,17 +1436,17 @@
       // to the width of the pointer.
       IndexBits = IndexBits.sextOrTrunc(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::computeForAddSub(
-          /*Add=*/true,
-          /*NSW=*/IsInBounds, Known, IndexBits);
+          /*Add=*/true, /*NSW=*/false, Known, IndexBits);
     }
     if (!Known.isUnknown() && !AccConstIndices.isNullValue()) {
       KnownBits Index(BitWidth);
       Index.Zero = ~AccConstIndices;
       Index.One = AccConstIndices;
       Known = KnownBits::computeForAddSub(
-          /*Add=*/true,
-          /*NSW=*/IsInBounds, Known, Index);
+          /*Add=*/true, /*NSW=*/false, Known, Index);
     }
     break;
   }


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