[llvm] r272703 - [ValueTracking] Calls to @llvm.assume always return

[Sanjoy Das via llvm-commits]

On Tue, Jun 14, 2016 at 1:48 PM, David Majnemer
<david.majnemer at gmail.com> wrote:
> Hmm, what if we have:
>
> volatile store i32 0, i32* %p
> call void @llvm.assume(i1 false)
> ...
>
> Can't this sort of logic let us sink the store past the assume? What if the
> assume gets replaced with unreachable?

I think that's fine.  assume(X) does not mean "the control flow can
move past the assume only if X is true", it means "due to some
external invariant I know that X is true".  So in the above example,
we know that the volatile store itself is UB since it postdominates
assume(false).

-- Sanjoy

>
>
> On Tue, Jun 14, 2016 at 1:33 PM, Sanjoy Das via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>>
>> Forgot to mention in the commit message: this regression was
>> introduced in http://reviews.llvm.org/rL272489
>>
>> On Tue, Jun 14, 2016 at 1:23 PM, Sanjoy Das via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>> > Author: sanjoy
>> > Date: Tue Jun 14 15:23:16 2016
>> > New Revision: 272703
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=272703&view=rev
>> > Log:
>> > [ValueTracking] Calls to @llvm.assume always return
>> >
>> > This change teaches llvm::isGuaranteedToTransferExecutionToSuccessor
>> > that calls to @llvm.assume always terminate.  Most other relevant
>> > intrinsics should be covered by the "CS.onlyReadsMemory() ||
>> > CS.onlyAccessesArgMemory()" bit but we were missing @llvm.assumes
>> > because we state that it clobbers memory.
>> >
>> > Added an LICM test case, but this change is not specific to LICM.
>> >
>> > Modified:
>> >     llvm/trunk/lib/Analysis/ValueTracking.cpp
>> >     llvm/trunk/test/Transforms/LICM/assume.ll
>> >
>> > Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
>> > URL:
>> > http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=272703&r1=272702&r2=272703&view=diff
>> >
>> > ==============================================================================
>> > --- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
>> > +++ llvm/trunk/lib/Analysis/ValueTracking.cpp Tue Jun 14 15:23:16 2016
>> > @@ -3478,7 +3478,8 @@ bool llvm::isGuaranteedToTransferExecuti
>> >      // but it's consistent with other passes. See http://llvm.org/PR965
>> > .
>> >      // FIXME: This isn't aggressive enough; a call which only writes to
>> > a
>> >      // global is guaranteed to return.
>> > -    return CS.onlyReadsMemory() || CS.onlyAccessesArgMemory();
>> > +    return CS.onlyReadsMemory() || CS.onlyAccessesArgMemory() ||
>> > +           match(I, m_Intrinsic<Intrinsic::assume>());
>> >    }
>> >
>> >    // Other instructions return normally.
>> >
>> > Modified: llvm/trunk/test/Transforms/LICM/assume.ll
>> > URL:
>> > http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LICM/assume.ll?rev=272703&r1=272702&r2=272703&view=diff
>> >
>> > ==============================================================================
>> > --- llvm/trunk/test/Transforms/LICM/assume.ll (original)
>> > +++ llvm/trunk/test/Transforms/LICM/assume.ll Tue Jun 14 15:23:16 2016
>> > @@ -1,6 +1,7 @@
>> >  ; RUN: opt -licm -basicaa < %s -S | FileCheck %s
>> >
>> > -define void @f(i1 %p) nounwind ssp {
>> > +define void @f_0(i1 %p) nounwind ssp {
>> > +; CHECK-LABEL: @f_0(
>> >  entry:
>> >    br label %for.body
>> >
>> > @@ -31,4 +32,20 @@ for.end104:
>> >    ret void
>> >  }
>> >
>> > +define void @f_1(i1 %cond, i32* %ptr) {
>> > +; CHECK-LABEL: @f_1(
>> > +; CHECK: %val = load i32, i32* %ptr
>> > +; CHECK-NEXT:  br label %loop
>> > +
>> > +entry:
>> > +  br label %loop
>> > +
>> > +loop:
>> > +  %x = phi i32 [ 0, %entry ], [ %x.inc, %loop ]
>> > +  call void @llvm.assume(i1 %cond)
>> > +  %val = load i32, i32* %ptr
>> > +  %x.inc = add i32 %x, %val
>> > +  br label %loop
>> > +}
>> > +
>> >  declare void @llvm.assume(i1)
>> >
>> >
>> > _______________________________________________
>> > llvm-commits mailing list
>> > llvm-commits at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>>
>> --
>> Sanjoy Das
>> http://playingwithpointers.com
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
>



-- 
Sanjoy Das
http://playingwithpointers.com