llvm dev - Aug 2016 - LLVM 3.9 RC2's SCCP pass removing calls to external functions?!

I am probably stating the obvious, but if the function is side-effect free
(onlyReadsMemory) it is valid to remove it.

But I am guessing that does not belong to this pass.

On Wed, Aug 24, 2016 at 8:38 AM, Sanjoy Das via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Btw, looks like the bug is in runSCCP in
>
>     for (BasicBlock::iterator BI = BB.begin(), E = BB.end(); BI != E;) {
>       Instruction *Inst = &*BI++;
>       if (Inst->getType()->isVoidTy() ||
isa<TerminatorInst>(Inst))
>         continue;
>
>       if (tryToReplaceInstWithConstant(Solver, Inst,
>                                        true /* shouldEraseFromParent */)) {
>         // Hey, we just changed something!
>         MadeChanges = true;
>         ++NumInstRemoved;
>       }
>
> It should not be passing in true for shouldEraseFromParent.  I think
> the right fix here is to not have the shouldEraseFromParent parameter
> at all, but in tryToReplaceInstWithConstant to do:
>
>   // replace Inst with constant
>   llvm::RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI);
>
> Let me know if you want to take a crack at this, otherwise I'll get to
> it this week.
>
> -- Sanjoy
>
>
> On Wed, Aug 24, 2016 at 12:12 AM, Sanjoy Das
> <sanjoy at playingwithpointers.com> wrote:
> > Hi Félix,
> >
> > Sanjoy Das wrote:
> >> Félix Cloutier via llvm-dev wrote:
> >>  > Assuming that this is a bug, what are the next steps?
> >>
> >> Looks like you already have a very small test case -- have you
tried
> >> sticking it in a debugger to see why SCCP thinks removing the call
is
> >> okay?
> >>
> >> Alternatively, file a bug at llvm.org/bugs and someone will get to
it.
> >
> > The third thing you can do is run a bisection to find the problematic
> > commit, since you said this was a regression.
> >
> > -- Sanjoy
>
>
>
> --
> Sanjoy Das
> http://playingwithpointers.com
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


-- 
*Alexandre Isoard*
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On 24 August 2016 at 08:48, Alexandre Isoard via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I am probably stating the obvious, but if the function is side-effect free
> (onlyReadsMemory) it is valid to remove it.
>
> But I am guessing that does not belong to this pass.
I think it does.

This is a very aggressive pass that does all sorts of inter-procedural
cleanups, and unused side-effect free function calls is just the
thing.

It looks to me that the compiler just got a bit more picky and your
original assumption was wrong to begin with.

Maybe your project needs to tell the middle-end (via function
attributes) that the function does have side effects?

cheers,
--renato

On Wed, Aug 24, 2016 at 10:11 AM, Renato Golin <renato.golin at
linaro.org>
wrote:
> On 24 August 2016 at 08:48, Alexandre Isoard via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I am probably stating the obvious, but if the function is side-effect
> free
> > (onlyReadsMemory) it is valid to remove it.
> >
> > But I am guessing that does not belong to this pass.
>
> I think it does.
>
> This is a very aggressive pass that does all sorts of inter-procedural
> cleanups, and unused side-effect free function calls is just the
> thing.
I see. Then I agree.
>
> It looks to me that the compiler just got a bit more picky and your
> original assumption was wrong to begin with.
>
However, he also stated that in the case the functions returns void it is
not removed, which suggest there is something fishier going on.
>
> Maybe your project needs to tell the middle-end (via function
> attributes) that the function does have side effects?
>
> cheers,
> --renato
>


-- 
*Alexandre Isoard*
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