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Fix UB in APInt::ashr

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

https://reviews.llvm.org/D23362


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278280 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jonathan Roelofs
2016-08-10 19:50:14 +00:00
parent 7616e5d399
commit 9715a2d267
2 changed files with 6 additions and 5 deletions
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lib/Support/APInt.cpp
+1 -5
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@@ -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.