[PATCH] [globalsmodref-aa] Atomics *DO* touch memory...

[Sean Silva]

On Sat, Apr 25, 2015 at 2:21 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Sean Silva" <chisophugis at gmail.com>
> > To: llvm-commits at cs.uiuc.edu
> > Cc: "Hal Finkel" <hfinkel at anl.gov>, "Nick Lewycky" <nlewycky at google.com>,
> "Philip Reames" <listmail at philipreames.com>
> > Sent: Friday, April 24, 2015 10:56:58 PM
> > Subject: [PATCH] [globalsmodref-aa] Atomics *DO* touch memory...
> >
> >
> > GlobalsModRef was giving a spurious alias analysis result because it
> > failed to consider that atomics access memory. The attached patch
> > fixes that (PR23345).
> >
> >
> > I reduced this from a "compare and swap" loop that looked funny in
> > the machine code: the "compare and swap" had in fact been hoisted
> > out of the loop. I only ran into this when I tried `-flto -O2
> > -fno-inline`.
> >
> >
> > The structure of the test is pretty closely copied from one of the
> > others in the directory. Any better ideas for testing this (or ways
> > to make the testing more thorough) would be appreciated.
> >
> >
> > If somebody can doublecheck that these are the only missing cases,
> > that would be appreciated. This loop been touched since 2012 and I'm
> > relatively clueless about AA. Hopefully this patch also suggests a
> > couple other places to audit for similar bugs to those with a better
> > "big picture" of the IR analysis and transforms than myself.
>
> Hi Sean,
>
> The list is also missing VAArgInst, although maybe that's on purpose
> (although, in that case, a comment would be nice).
>
> While we're looking at this, other things I don't understand about this
> loop are 1) Why is is special casing alloc/free functions and 2) why is it
> treating all volatile accesses as read/write?
>

I know at least for volatile accesses it is because it might be a hardware
register that has some arbitrary effect on other memory. A simple example
is ARM's "bit-banding".



>
> It might be nice to refactor this code to call
>   ModRefResult getModRefInfo(const Instruction *I)
> from AliasAnalysis. That way we could reduce the number of places that
> replicate this kind of logic.
>


Hal, others: I'm really not an expert in AA or even really have that much
experience at the IR level. Would one of you be willing to comandeer this
patch from me? I'm afraid that I'm mostly going to be slowing things down
here.

-- Sean Silva


>
>  -Hal
>
> >
> >
> > -- Sean Silva
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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