[llvm] f7dd258 - [ADT] Fix a bug in PackedVector::setValue for signed types (#159239)

[via llvm-commits]

Author: Kazu Hirata
Date: 2025-09-28T10:27:05-07:00
New Revision: f7dd258635af4d85bd8f25729c7f56aadb4c5913

URL: https://github.com/llvm/llvm-project/commit/f7dd258635af4d85bd8f25729c7f56aadb4c5913
DIFF: https://github.com/llvm/llvm-project/commit/f7dd258635af4d85bd8f25729c7f56aadb4c5913.diff

LOG: [ADT] Fix a bug in PackedVector::setValue for signed types (#159239)

Without this patch, we forget to update the sign bit.  When we assign:

  Vec[0] = -1;

the sign bit is correctly set to 1.  Overwriting the same element:

  Vec[0] = 1;

does not update the sign bit, leaving the 4-bit as 0b1001, which reads
-2 according to PackedVector's encoding.  (It does not use two's
complement.)

This patch fixes the bug by clearing the sign bit when we are
assigning a non-negative value.

Added: 
    

Modified: 
    llvm/include/llvm/ADT/PackedVector.h
    llvm/unittests/ADT/PackedVectorTest.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/ADT/PackedVector.h b/llvm/include/llvm/ADT/PackedVector.h
index 1146cc4bd6d23..4e31d3f098d44 100644
--- a/llvm/include/llvm/ADT/PackedVector.h
+++ b/llvm/include/llvm/ADT/PackedVector.h
@@ -58,6 +58,8 @@ class PackedVectorBase<T, BitNum, BitVectorTy, true> {
     if (val < 0) {
       val = ~val;
       Bits.set((Idx * BitNum) + BitNum - 1);
+    } else {
+      Bits.reset((Idx * BitNum) + BitNum - 1);
     }
     assert((val >> (BitNum-1)) == 0 && "value is too big");
     for (unsigned i = 0; i != BitNum-1; ++i)

diff  --git a/llvm/unittests/ADT/PackedVectorTest.cpp b/llvm/unittests/ADT/PackedVectorTest.cpp
index 30fc7c0b6d07f..df2cbf0e7f0f8 100644
--- a/llvm/unittests/ADT/PackedVectorTest.cpp
+++ b/llvm/unittests/ADT/PackedVectorTest.cpp
@@ -71,6 +71,14 @@ TEST(PackedVectorTest, RawBitsSize) {
   EXPECT_EQ(12u, Vec.raw_bits().size());
 }
 
+TEST(PackedVectorTest, SignedValueOverwrite) {
+  PackedVector<signed, 4> Vec(1);
+  Vec[0] = -1;
+  EXPECT_EQ(-1, Vec[0]);
+  Vec[0] = 1;
+  EXPECT_EQ(1, Vec[0]);
+}
+
 #ifdef EXPECT_DEBUG_DEATH
 
 TEST(PackedVectorTest, UnsignedValues) {