[llvm-dev] [MemorySSA] Potential CachingMemorySSAWalker bug

George Burgess IV via llvm-dev llvm-dev at lists.llvm.org
Tue May 3 11:53:38 PDT 2016

So, after looking at this, I think this is WAI, and the wonkiness is a
result of an arbitrary limit somewhere in BasicAA.

I say this because if you remove any unused GEP from your example, the
loads (MemoryUse(7), MemoryUse(7)) are both optimized to MemoryUse(1) when
we build MemorySSA. Add useless GEP back, and they turn back to
MemoryUse(7). This is valid, because BasicAA presumably sees that the
malloc'ed pointer doesn't escape, and it's marked with noalias. So, BasicAA
can assume that it isn't clobbered by @foo.

With respect to why the clobbers are *different*, it looks like the walker
first sees the code as

load i32, i32* %i27
load i32, i32* %i28

...Which some transform then changes to

load i32, i32* %i27
load i32, i32* %i27

...And then requerys the walker.

Because the MemoryLocation on the second load changes, when you call
getClobberingAccess on it, we won't find a cache entry (indeed, we end up
walking all defs up to @malloc in this query). Because this lookup is
happening after some useless GEPs have been removed, AA is more aggressive
here, and lets us optimize the clobber of the second load.

This doesn't happen for the first load because its cache is still valid
(its MemoryLocation is still %i27).

I can't think of a good way to address this (AA offering more aggressive
results for seemingly unrelated IR changes) in MSSA. Danny -- any ideas? :/

On Mon, May 2, 2016 at 12:22 PM, Geoff Berry <gberry at codeaurora.org> wrote:

> I've put my changes to EarlyCSE that trigger this case up on phab here:
> http://reviews.llvm.org/D19821.  These changes depend on
> http://reviews.llvm.org/D19664 so that will need to be applied first.
> With these changes applied, the original attached .ll file should trigger
> this bug when compiled with opt -early-cse -early-cse-use-memoryssa
> On 5/2/2016 2:34 PM, Daniel Berlin wrote:
> (IE the thing we cache has to be the same no matter which version is
> called. If it can't be for some reason, we can't cache results from both in
> the same cache. If it can be, i suspect there is a bug in making that work
> :P)
> I suspect the latter, and not the former.
> On Mon, May 2, 2016 at 11:32 AM, Daniel Berlin <dberlin at dberlin.org>
> wrote:
>> I suspect something is pulling the RHS of the memorydef and caching it
>> for calls it should not be used for.
>> In particular, i suspect we are about to discover we can't cache the
>> results from both versions of getClobberingMemoryAccess together, or that
>> the cache is not always getting consistently written.
>> On Mon, May 2, 2016 at 11:16 AM, George Burgess IV <
>> <george.burgess.iv at gmail.com>george.burgess.iv at gmail.com> wrote:
>>> Yeah, that sounds like a fun bug. I'll take a look later today and see
>>> what I can find out. :)
>>> On Fri, Apr 29, 2016 at 2:55 PM, Geoff Berry < <gberry at codeaurora.org>
>>> gberry at codeaurora.org> wrote:
>>>> Hi guys,
>>>> I think I have run into another CachingMemorySSAWalker cache bug.  It's
>>>> a bit tricky to reproduce, so I'd like to start by trying to show you what
>>>> is happening when running EarlyCSE with my local changes to use MemorySSA.
>>>> I've attached a debug log that shows that the value returned by
>>>> getClobberingMemoryAccess(Inst) after a call to removeMemoryAccess is
>>>> wrong.  The MemorySSA node in question is MemoryUse(7), and the corruption
>>>> happens after a call to remove MemoryUse(2), at which point its clobber
>>>> value changes to '1 = MemoryDef(liveOnEntry)'.  The interesting thing is
>>>> that is doesn't seem to be the first call to getClobberingMemoryAccess
>>>> after the removal that causes the corruption, but rather the second.
>>>> You'll notice that I added calls to getClobberingMemoryAccess when doing
>>>> MSSA.dump(), which is what I'm using to attempt to figure out when the
>>>> cache gets corrupted.
>>>> Hopefully this is enough information to debug the problem.  If not
>>>> perhaps we can look at getting my EarlyCSE changes checked in in a disabled
>>>> state so you can reproduce the problem directly.  I'm also happy to help
>>>> debug it farther.
>>>> --
>>>> Geoff Berry
>>>> Employee of Qualcomm Innovation Center, Inc.
>>>>  Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
> --
> Geoff Berry
> Employee of Qualcomm Innovation Center, Inc.
>  Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
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