[llvm] r278280 - Fix UB in APInt::ashr

[Jonathan Roelofs via llvm-commits]

Author: jroelofs
Date: Wed Aug 10 14:50:14 2016
New Revision: 278280

URL: http://llvm.org/viewvc/llvm-project?rev=278280&view=rev
Log:
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

Modified:
    llvm/trunk/lib/Support/APInt.cpp
    llvm/trunk/unittests/ADT/APIntTest.cpp

Modified: llvm/trunk/lib/Support/APInt.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/APInt.cpp?rev=278280&r1=278279&r2=278280&view=diff
==============================================================================
--- llvm/trunk/lib/Support/APInt.cpp (original)
+++ llvm/trunk/lib/Support/APInt.cpp Wed Aug 10 14:50:14 2016
@@ -1042,11 +1042,7 @@ APInt APInt::ashr(unsigned shiftAmt) con
   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.

Modified: llvm/trunk/unittests/ADT/APIntTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/ADT/APIntTest.cpp?rev=278280&r1=278279&r2=278280&view=diff
==============================================================================
--- llvm/trunk/unittests/ADT/APIntTest.cpp (original)
+++ llvm/trunk/unittests/ADT/APIntTest.cpp Wed Aug 10 14:50:14 2016
@@ -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());