Fix UB in APInt::ashr

i64 -1, whose sign bit is the 0th one, can't be left shifted without invoking UB.

https://reviews.llvm.org/D23362

llvm-svn: 278280

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 8c921ce..31ffffc 100644 (file)
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -1042,11 +1042,7 @@ APInt APInt::ashr(unsigned shiftAmt) const {
   if (isSingleWord()) {
     if (shiftAmt == BitWidth)
       return APInt(BitWidth, 0); // undefined
-    else {
-      unsigned SignBit = APINT_BITS_PER_WORD - BitWidth;
-      return APInt(BitWidth,
-        (((int64_t(VAL) << SignBit) >> SignBit) >> shiftAmt));
-    }
+    return APInt(BitWidth, SignExtend64(VAL, BitWidth) >> shiftAmt);
   }
 
   // If all the bits were shifted out, the result is, technically, undefined.

diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index 3a7c2d7..cbffdc0 100644 (file)
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -32,6 +32,11 @@ TEST(APIntTest, ShiftLeftByZero) {
   EXPECT_FALSE(Shl[1]);
 }
 
+TEST(APIntTest, i64_ArithmeticRightShiftNegative) {
+  const APInt neg_one(64, static_cast<uint64_t>(-1), true);
+  EXPECT_EQ(neg_one, neg_one.ashr(7));
+}
+
 TEST(APIntTest, i128_NegativeCount) {
   APInt Minus3(128, static_cast<uint64_t>(-3), true);
   EXPECT_EQ(126u, Minus3.countLeadingOnes());