[llvm] 92b7089 - [ValueTracking] Don't set nsw flag for inbounds addition

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2020-11-13T17:58:21+01:00
New Revision: 92b708902e1db30a976e168111be5bbe81ee8191

URL: https://github.com/llvm/llvm-project/commit/92b708902e1db30a976e168111be5bbe81ee8191
DIFF: https://github.com/llvm/llvm-project/commit/92b708902e1db30a976e168111be5bbe81ee8191.diff

LOG: [ValueTracking] Don't set nsw flag for inbounds addition

When computing the known bits for a GEP, don't set the nsw flag
when adding an offset to an address. The nsw flag only applies to
pure offset additions (see also D90708).

The nsw flag is only used in a very minor way by the code, to the
point that I was not able to come up with a test case where it
makes a difference.

Differential Revision: https://reviews.llvm.org/D90637

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 0a18718fd9b5..e6d3727a2390 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1289,9 +1289,6 @@ static void computeKnownBitsFromOperator(const Operator *I,
     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())
@@ -1356,17 +1353,17 @@ static void computeKnownBitsFromOperator(const Operator *I,
       // 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;
   }