[cfe-dev] [RFC] Clearing Clang AST before running backend optimizations/codegen to save memory

[Arthur Eubanks via cfe-dev]

Looking at -Rpass (and various things like warnings for inability to
vectorize when we specifically request it with #pragma clang loop
vectorize(enable)), it does end up using objects from the AST to
approximate the source location
(BackendConsumer::getBestLocationFromDebugLoc()) if it can't find the
source location from debug info. So this would affect users who don't build
with debug info. Without debug info or the AST, clang will print a
warning/remark without a source location. This is a tradeoff we'd have to
decide on.

There's a similar issue with backend warnings but those don't even pass
debug info to the diagnostic handler
(clang/test/Misc/backend-stack-frame-diagnostics.cpp). Perhaps it could be
extended to do that though.

I haven't looked too much into the -disable-free stuff, but the reason it
mitigates crashes is because if we clear AST objects we still have dangling
references to them that we later attempt to clean up unless we
-disable-free.

Updated https://reviews.llvm.org/D109781

On Fri, Sep 17, 2021 at 4:41 PM David Blaikie <dblaikie at gmail.com> wrote:

> I think it'd be unfortunate if certain features don't work in this mode,
> unless we understand why/are pretty sure that's a fairly fundamental
> limitation. For instance, at Google we've got memory limitations (hence the
> motivation for this work) and I think we created, or at least have some
> interest in -Rpass - if -Rpass couldn't be composed with this feature, then
> we'd make it harder to investigate performance issues (because -Rpass
> wouldn't be available) in larger compiles that need this memory savings to
> fit into the memory limits we have. I'd guess the issue is that -Rpass I
> think traffics in Clang source locations. So it's possible the source
> location infrastructure/data structures would have to be kept, even though
> the AST/semantic pieces could be torn down. (unless that source location
> stuff can refer into ASTs for differentiating template specializations, etc
> - that'd be the tipping point for me in "OK, it may be worth the benefit to
> make these incompatible, or reduce the quality of -Rpass diagnostics when
> using this memory saving technique" - wonder if it's only the -Rpass
> diagnostics, or other backend diagnostics that use that infrastructure)
>
> Which is to say I'd be /slightly/ averse to adding this feature as a Clang
> default or driver flag (& similarly averse to leaving it as a cc1
> off-by-default flag indefinitely) without a pretty good answer to those
> crashing/non-functioning tests.
>
> (lower priority, but fairly nice-to-have would be some answer to the
> cleaning up issues, -disable-free, etc - sort of weird that we'd have to
> /disable-free/ to enable freeing things earlier... that seems pretty
> suspicious)
>
> but if those issues can be resolved
>
> On Fri, Sep 17, 2021 at 2:30 PM Arthur Eubanks via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> We keep around the Clang AST when we do backend optimizations on the IR.
>> This causes the peak memory usage to be more than necessary since (I
>> believe) generally we don't need the Clang AST when running optimizations.
>> This gives us more room to work with things like caching analyses, at least
>> for frontend compilations.
>>
>> Measuring the effects of this when building LLVM's PassBuilder.cpp
>> (longest LLVM file to compile), I measured a drop of peak memory usage
>> (/usr/bin/time's max rss) from ~1.3-1.4G to ~1.0G.
>>
>> There are still a couple issues I haven't dug too deeply into yet, mostly
>> to do with cleaning things up when freeing memory, so right now it's only
>> enabled with -disable-free which works around those issues. Most clang
>> tests pass with this patch; there are a couple things that crash (e.g.
>> -Rpass, clang interpreter) where we can investigate further or just disable
>> this feature.
>>
>> Prototype: https://reviews.llvm.org/D109781
>> llvm-compile-time-tracker
>> <https://llvm-compile-time-tracker.com/compare.php?from=167ff5280d7fcad731810d5d2bf10561ed2adacc&to=b08fcae3a02d5ebe58afd8f8658d798b62ff8eb7&stat=max-rss> memory
>> metrics
>>
>> Any concerns with this?
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>
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