Ensure KnownBits passed when calculating from range md has right size

[LU-JOHN]

KnownBits passed to computeKnownBitsFromRangeMetadata must have the same bit width as the range metadata bit width. Otherwise the calculated results will be incorrect.

[llvmbot]

@llvm/pr-subscribers-llvm-selectiondag

@llvm/pr-subscribers-llvm-analysis

Author: None (LU-JOHN)

Changes

KnownBits passed to computeKnownBitsFromRangeMetadata must have the same bit width as the range metadata bit width. Otherwise the calculated results will be incorrect.

---

Full diff: https://github.com/llvm/llvm-project/pull/132985.diff

1 Files Affected:

• (modified) llvm/lib/Analysis/ValueTracking.cpp (+2)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 880781742fae0..4e3d5d8f12cbc 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -433,6 +433,8 @@ void llvm::computeKnownBitsFromRangeMetadata(const MDNode &Ranges,
     // The first CommonPrefixBits of all values in Range are equal.
     unsigned CommonPrefixBits =
         (Range.getUnsignedMax() ^ Range.getUnsignedMin()).countl_zero();
+    // BitWidth must equal the Ranges BitWidth for the correct number of high bits to be set.
+    assert(BitWidth == Lower->getBitWidth() );
     APInt Mask = APInt::getHighBitsSet(BitWidth, CommonPrefixBits);
     APInt UnsignedMax = Range.getUnsignedMax().zextOrTrunc(BitWidth);
     Known.One &= UnsignedMax & Mask;

[arsenm]

We do need something like this as it's the closest thing we have to a DAG verifier