[llvm-commits] bug fix for infinite loop in InstCombine

[Eli Friedman]

On Thu, Sep 15, 2011 at 4:45 PM, Guo, Xiaoyi <Xiaoyi.Guo at amd.com> wrote:
> Please review the attached fix for a bug in InstCombiner. The patch also includes a new test case which will cause the current InstCombiner to go into an infinite loop.
>
> Please help to commit if acceptable.
>
> The problem is with the following code in InstCombiner::visitAnd() in InstCombineAndOrXor.cpp:
>
> Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
>  bool Changed = SimplifyAssociativeOrCommutative(I);
>  ...
>    if (Op0->hasOneUse() &&
>        match(Op0, m_Xor(m_Value(A), m_Value(B)))) {
>      if (A == Op1) {                                // (A^B)&A -> A&(A^B)
>        I.swapOperands();     // Simplify below
>        std::swap(Op0, Op1);                            <==========
>      } else if (B == Op1) {                         // (A^B)&B -> B&(B^A)
>        cast<BinaryOperator>(Op0)->swapOperands();
>        I.swapOperands();     // Simplify below
>        std::swap(Op0, Op1);                            <==========
>      }
>    }
>
>    if (Op1->hasOneUse() &&
>        match(Op1, m_Xor(m_Value(A), m_Value(B)))) {
>      if (B == Op0) {                                // B&(A^B) -> B&(B^A)
>        cast<BinaryOperator>(Op1)->swapOperands();
>        std::swap(A, B);
>      }
>      // Notice that the patten (A&(~B)) is actually (A&(-1^B)), so if
>      // A is originally -1 (or a vector of -1 and undefs), then we enter
>      // an endless loop. By checking that A is non-constant we ensure that
>      // we will never get to the loop.
>      if (A == Op0 && !isa<Constant>(A)) // A&(A^B) -> A & ~B
>        return BinaryOperator::CreateAnd(A, Builder->CreateNot(B, "tmp"));
>    }
>
>
> With the given test case, op0 is a const and op1 is xor, so the two operands are be swapped in SimplifyAssociativeOrCommutative(). Then I's operands are swapped back at one of the statements marked "<===========" above. Then because A is a constant, a new instruction is not created. So we end up with Changed flag as true but the instruction is not changed and we go into an infinite loop in InstCombiner::DoOneIteration().
>
> My fix is to remove the swapOperands() calls, because I don't think they are necessary.

The general idea seems fine.  Your fix disables the transform in some
cases where both Op0 and Op1 are Xor operators, though.

-Eli