[llvm-dev] -speculative-execution moving load before store

[Daniel Berlin via llvm-dev]

(staring at this pass, i'm not sure it's supposed to work in all cases on
all processors.
If it is, it ... uh, needs help :P)


On Wed, May 10, 2017 at 12:00 PM, Amara Emerson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> This looks like it's due to r301950, which has today been reverted in
> r302640. Can you try with the latest trunk?
>
> Regardless, there seems to be an underlying bug exposed.
>
> On 10 May 2017 at 07:37, Mikael Holmén via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hi,
> >
> > A few days ago I stumbled upon a problem where SpeculativeExecution
> > changed the order of a load and a store to the same address.
> >
> > I wrote
> >
> >  https://bugs.llvm.org//show_bug.cgi?id=32964
> >
> > about it but no response there so far.
> >
> > In the input we have
> >
> >   store i8 0, i8* @i
> >   %.pre = load i8, i8* @i
> >
> > and then in the output the load is moved so it's before the store which
> > clearly makes it load the wrong value.
> >
> > isSafeToSpeculativelyExecute happily says "true" for the load, and there
> is
> > a comment on isSafeToSpeculativelyExecute saying
> >
> >   /// This method can return true for instructions that read memory;
> >   /// for such instructions, moving them may change the resulting value.
> >
> > And that is indeed the case in PR32964.
> >
> > How is this supposed to work?
> >
> > I started seeing this after the commit
> >
> >  SpeculativeExecution: Stop using whitelist for costs
> >
> > Before that, e.g. a load got the cost UINT_MAX which I suppose disabled
> > hoisting, but since the commit TTI.getUserCost(I) is used for all
> > instructions.
> >
> > Regards,
> > Mikael
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