[llvm] r221318 - Analysis: Make isSafeToSpeculativelyExecute fire less for divides
Philip Reames
listmail at philipreames.com
Wed Nov 5 17:05:50 PST 2014
David,
Is this a correctness issue or a heuristic? The description doesn't say.
I find this change potentially concerning since it will heavily
influence LICM behaviour.
Philip
On 11/04/2014 03:49 PM, David Majnemer wrote:
> Author: majnemer
> Date: Tue Nov 4 17:49:08 2014
> New Revision: 221318
>
> URL: http://llvm.org/viewvc/llvm-project?rev=221318&view=rev
> Log:
> Analysis: Make isSafeToSpeculativelyExecute fire less for divides
>
> Divides and remainder operations do not behave like other operations
> when they are given poison: they turn into undefined behavior.
>
> It's really hard to know if the operands going into a div are or are not
> poison. Because of this, we should only choose to speculate if there
> are constant operands which we can easily reason about.
>
> This fixes PR21412.
>
> Modified:
> llvm/trunk/lib/Analysis/ValueTracking.cpp
> llvm/trunk/test/Transforms/LICM/speculate.ll
>
> Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=221318&r1=221317&r2=221318&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
> +++ llvm/trunk/lib/Analysis/ValueTracking.cpp Tue Nov 4 17:49:08 2014
> @@ -2549,23 +2549,31 @@ bool llvm::isSafeToSpeculativelyExecute(
> default:
> return true;
> case Instruction::UDiv:
> - case Instruction::URem:
> - // x / y is undefined if y == 0, but calculations like x / 3 are safe.
> - return isKnownNonZero(Inst->getOperand(1), TD);
> + case Instruction::URem: {
> + // x / y is undefined if y == 0.
> + const APInt *V;
> + if (match(Inst->getOperand(1), m_APInt(V)))
> + return *V != 0;
> + return false;
> + }
> case Instruction::SDiv:
> case Instruction::SRem: {
> - Value *Op = Inst->getOperand(1);
> - // x / y is undefined if y == 0
> - if (!isKnownNonZero(Op, TD))
> - return false;
> - // x / y might be undefined if y == -1
> - unsigned BitWidth = getBitWidth(Op->getType(), TD);
> - if (BitWidth == 0)
> - return false;
> - APInt KnownZero(BitWidth, 0);
> - APInt KnownOne(BitWidth, 0);
> - computeKnownBits(Op, KnownZero, KnownOne, TD);
> - return !!KnownZero;
> + // x / y is undefined if y == 0 or x == INT_MIN and y == -1
> + const APInt *X, *Y;
> + if (match(Inst->getOperand(1), m_APInt(Y))) {
> + if (*Y != 0) {
> + if (*Y == -1) {
> + // The numerator can't be MinSignedValue if the denominator is -1.
> + if (match(Inst->getOperand(0), m_APInt(X)))
> + return !Y->isMinSignedValue();
> + // The numerator *might* be MinSignedValue.
> + return false;
> + }
> + // The denominator is not 0 or -1, it's safe to proceed.
> + return true;
> + }
> + }
> + return false;
> }
> case Instruction::Load: {
> const LoadInst *LI = cast<LoadInst>(Inst);
>
> Modified: llvm/trunk/test/Transforms/LICM/speculate.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LICM/speculate.ll?rev=221318&r1=221317&r2=221318&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/LICM/speculate.ll (original)
> +++ llvm/trunk/test/Transforms/LICM/speculate.ll Tue Nov 4 17:49:08 2014
> @@ -3,12 +3,11 @@
> ; UDiv is safe to speculate if the denominator is known non-zero.
>
> ; CHECK-LABEL: @safe_udiv(
> -; CHECK: %div = udiv i64 %x, %or
> +; CHECK: %div = udiv i64 %x, 2
> ; CHECK-NEXT: br label %for.body
>
> define void @safe_udiv(i64 %x, i64 %m, i64 %n, i32* %p, i64* %q) nounwind {
> entry:
> - %or = or i64 %m, 1
> br label %for.body
>
> for.body: ; preds = %entry, %for.inc
> @@ -19,7 +18,7 @@ for.body:
> br i1 %tobool, label %for.inc, label %if.then
>
> if.then: ; preds = %for.body
> - %div = udiv i64 %x, %or
> + %div = udiv i64 %x, 2
> %arrayidx1 = getelementptr inbounds i64* %q, i64 %i.02
> store i64 %div, i64* %arrayidx1, align 8
> br label %for.inc
> @@ -69,13 +68,12 @@ for.end:
> ; known to have at least one zero bit.
>
> ; CHECK-LABEL: @safe_sdiv(
> -; CHECK: %div = sdiv i64 %x, %or
> +; CHECK: %div = sdiv i64 %x, 2
> ; CHECK-NEXT: br label %for.body
>
> define void @safe_sdiv(i64 %x, i64 %m, i64 %n, i32* %p, i64* %q) nounwind {
> entry:
> %and = and i64 %m, -3
> - %or = or i64 %and, 1
> br label %for.body
>
> for.body: ; preds = %entry, %for.inc
> @@ -86,7 +84,7 @@ for.body:
> br i1 %tobool, label %for.inc, label %if.then
>
> if.then: ; preds = %for.body
> - %div = sdiv i64 %x, %or
> + %div = sdiv i64 %x, 2
> %arrayidx1 = getelementptr inbounds i64* %q, i64 %i.02
> store i64 %div, i64* %arrayidx1, align 8
> br label %for.inc
>
>
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