[llvm] r219634 - InstCombine: Don't miscompile (x lshr C1) udiv C2

[Timur Iskhodzhanov]

This has regressed on our internal build, I'll send you the details off the
list.

2014-10-14 1:48 GMT+04:00 David Majnemer <david.majnemer at gmail.com>:

> Author: majnemer
> Date: Mon Oct 13 16:48:30 2014
> New Revision: 219634
>
> URL: http://llvm.org/viewvc/llvm-project?rev=219634&view=rev
> Log:
> InstCombine: Don't miscompile (x lshr C1) udiv C2
>
> We have a transform that changes:
>   (x lshr C1) udiv C2
> into:
>   x udiv (C2 << C1)
>
> However, it is unsafe to do so if C2 << C1 discards any of C2's bits.
>
> This fixes PR21255.
>
> Modified:
>     llvm/trunk/include/llvm/ADT/APInt.h
>     llvm/trunk/lib/Support/APInt.cpp
>     llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
>     llvm/trunk/test/Transforms/InstCombine/div.ll
>
> Modified: llvm/trunk/include/llvm/ADT/APInt.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/APInt.h?rev=219634&r1=219633&r2=219634&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/ADT/APInt.h (original)
> +++ llvm/trunk/include/llvm/ADT/APInt.h Mon Oct 13 16:48:30 2014
> @@ -945,7 +945,8 @@ public:
>    APInt sdiv_ov(const APInt &RHS, bool &Overflow) const;
>    APInt smul_ov(const APInt &RHS, bool &Overflow) const;
>    APInt umul_ov(const APInt &RHS, bool &Overflow) const;
> -  APInt sshl_ov(unsigned Amt, bool &Overflow) const;
> +  APInt sshl_ov(const APInt &Amt, bool &Overflow) const;
> +  APInt ushl_ov(const APInt &Amt, bool &Overflow) const;
>
>    /// \brief Array-indexing support.
>    ///
>
> Modified: llvm/trunk/lib/Support/APInt.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/APInt.cpp?rev=219634&r1=219633&r2=219634&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Support/APInt.cpp (original)
> +++ llvm/trunk/lib/Support/APInt.cpp Mon Oct 13 16:48:30 2014
> @@ -2064,19 +2064,29 @@ APInt APInt::umul_ov(const APInt &RHS, b
>    return Res;
>  }
>
> -APInt APInt::sshl_ov(unsigned ShAmt, bool &Overflow) const {
> -  Overflow = ShAmt >= getBitWidth();
> +APInt APInt::sshl_ov(const APInt &ShAmt, bool &Overflow) const {
> +  Overflow = ShAmt.uge(getBitWidth());
>    if (Overflow)
> -    ShAmt = getBitWidth()-1;
> +    return APInt(BitWidth, 0);
>
>    if (isNonNegative()) // Don't allow sign change.
> -    Overflow = ShAmt >= countLeadingZeros();
> +    Overflow = ShAmt.uge(countLeadingZeros());
>    else
> -    Overflow = ShAmt >= countLeadingOnes();
> +    Overflow = ShAmt.uge(countLeadingOnes());
>
>    return *this << ShAmt;
>  }
>
> +APInt APInt::ushl_ov(const APInt &ShAmt, bool &Overflow) const {
> +  Overflow = ShAmt.uge(getBitWidth());
> +  if (Overflow)
> +    return APInt(BitWidth, 0);
> +
> +  Overflow = ShAmt.ugt(countLeadingZeros());
> +
> +  return *this << ShAmt;
> +}
> +
>
>
>
>
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp?rev=219634&r1=219633&r2=219634&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
> (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp Mon Oct
> 13 16:48:30 2014
> @@ -965,11 +965,17 @@ Instruction *InstCombiner::visitUDiv(Bin
>      return Common;
>
>    // (x lshr C1) udiv C2 --> x udiv (C2 << C1)
> -  if (Constant *C2 = dyn_cast<Constant>(Op1)) {
> +  {
>      Value *X;
> -    Constant *C1;
> -    if (match(Op0, m_LShr(m_Value(X), m_Constant(C1))))
> -      return BinaryOperator::CreateUDiv(X, ConstantExpr::getShl(C2, C1));
> +    const APInt *C1, *C2;
> +    if (match(Op0, m_LShr(m_Value(X), m_APInt(C1))) &&
> +        match(Op1, m_APInt(C2))) {
> +      bool Overflow;
> +      APInt C2ShlC1 = C2->ushl_ov(*C1, Overflow);
> +      if (!Overflow)
> +        return BinaryOperator::CreateUDiv(
> +            X, ConstantInt::get(X->getType(), C2ShlC1));
> +    }
>    }
>
>    // (zext A) udiv (zext B) --> zext (A udiv B)
>
> Modified: llvm/trunk/test/Transforms/InstCombine/div.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/div.ll?rev=219634&r1=219633&r2=219634&view=diff
>
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/div.ll (original)
> +++ llvm/trunk/test/Transforms/InstCombine/div.ll Mon Oct 13 16:48:30 2014
> @@ -132,11 +132,11 @@ define i32 @test15(i32 %a, i32 %b) nounw
>  }
>
>  define <2 x i64> @test16(<2 x i64> %x) nounwind {
> -  %shr = lshr <2 x i64> %x, <i64 3, i64 5>
> -  %div = udiv <2 x i64> %shr, <i64 4, i64 6>
> +  %shr = lshr <2 x i64> %x, <i64 5, i64 5>
> +  %div = udiv <2 x i64> %shr, <i64 6, i64 6>
>    ret <2 x i64> %div
>  ; CHECK-LABEL: @test16(
> -; CHECK-NEXT: udiv <2 x i64> %x, <i64 32, i64 192>
> +; CHECK-NEXT: udiv <2 x i64> %x, <i64 192, i64 192>
>  ; CHECK-NEXT: ret <2 x i64>
>  }
>
> @@ -264,3 +264,13 @@ define i32 @test30(i32 %a) {
>  ; CHECK-LABEL: @test30(
>  ; CHECK-NEXT: ret i32 %a
>  }
> +
> +define <2 x i32> @test31(<2 x i32> %x) nounwind {
> +  %shr = lshr <2 x i32> %x, <i32 31, i32 31>
> +  %div = udiv <2 x i32> %shr, <i32 2147483647, i32 2147483647>
> +  ret <2 x i32> %div
> +; CHECK-LABEL: @test31(
> +; CHECK-NEXT: %[[shr:.*]] = lshr <2 x i32> %x, <i32 31, i32 31>
> +; CHECK-NEXT: udiv <2 x i32> %[[shr]], <i32 2147483647, i32 2147483647>
> +; CHECK-NEXT: ret <2 x i32>
> +}
>
>
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