[llvm] r221318 - Analysis: Make isSafeToSpeculativelyExecute fire less for divides

Hal Finkel hfinkel at anl.gov
Wed Nov 5 18:39:49 PST 2014


----- Original Message -----
> From: "David Majnemer" <david.majnemer at gmail.com>
> To: "Philip Reames" <listmail at philipreames.com>
> Cc: "LLVM Commits" <llvm-commits at cs.uiuc.edu>
> Sent: Wednesday, November 5, 2014 8:28:24 PM
> Subject: Re: [llvm] r221318 - Analysis: Make isSafeToSpeculativelyExecute fire	less for divides
> 
> 
> 
> It's a correctness issue.
> 
> 
> In general one cannot know whether or not a divide instruction is
> safe to execute because isKnownNonZero returns true for poison.
> 
> 
> It is possible for the right hand side of the divide to always be
> poison in a valid program so long as the divide cannot ever be
> executed.
> 
> 
> For instructions which trap, we need a stronger predicate than
> 'isKnownNonZero': we need a hypothetical 'isKnownNeverToBeUndef'.

Wait, why? If the program produced undefined behavior, then it can trap in response to that. No?

 -Hal

> 
> 
> I doubt a correct implementation of isKnownNeverToBeUndef will ever
> fire because it must be incredibly conservative: function arguments
> might silently cary poison in them making them unsafe.
> 
> 
> On Wed, Nov 5, 2014 at 5:05 PM, Philip Reames <
> listmail at philipreames.com > wrote:
> 
> 
> 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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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory



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