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

Amara Emerson via llvm-dev llvm-dev at lists.llvm.org
Wed May 10 12:00:59 PDT 2017


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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