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

Tue Sep 21 12:38:00 PDT 2021

On Tue, Sep 21, 2021 at 6:30 AM <paul.robinson at sony.com> wrote:

> Drive-by thought, debug-info-for-profiling retains source info, maybe that
> could be unconditionally on and Rpass could use it?
>

I think -fdebug-info-for-profiling tweaks the DWARF output to include some
more info (like mangled names for inlined functions even in gmlt, start of
function line numbers so sample profiles can be robust to certain amounts
of code change, etc)

But yeah - we do have the https://reviews.llvm.org/D4234 "LocTrackingOnly"
mode which looks like it could be used for Rpass diagnostics, for instance.
(& removing use of the AST from the LLVM diagnostic system might help make
it more consistent behavior even when doing LTO or other separations
between AST parsing and IR transformations)

> --paulr
>
>
>
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> *On Behalf Of *David
> Blaikie via cfe-dev
> *Sent:* Monday, September 20, 2021 8:19 PM
> *To:* Arthur Eubanks <aeubanks at google.com>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] [RFC] Clearing Clang AST before running backend
> optimizations/codegen to save memory
>
>
>
> On Mon, Sep 20, 2021 at 3:04 PM Arthur Eubanks <aeubanks at google.com>
> wrote:
>
> 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.
>
>
>
> Ah, the AST lookup is only to retrieve the location of a function by name
> - if we made a mapping/record of all those locations (shouldn't take up
> much space, I'd think) then we could use that instead and not need the AST
> for that callback?
>
>
>
> 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.
>
>
>
> So disabling free is keeping more things alive - perhaps then the RAM
> savings aren't as much as they could be if freeing was enabled? But yeah,
> more to look into.
>
>
>
>
>
> Updated https://reviews.llvm.org/D109781
> <https://urldefense.com/v3/__https:/reviews.llvm.org/D109781__;!!JmoZiZGBv3RvKRSx!u5xov9_zHGWKWsFWg-FlBBUrKZWcBw4KyfXq4eY9dQRAIJUNxoTfDwD40IYGep3Mdg$>
>
>
>
> 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
> <https://urldefense.com/v3/__https:/reviews.llvm.org/D109781__;!!JmoZiZGBv3RvKRSx!u5xov9_zHGWKWsFWg-FlBBUrKZWcBw4KyfXq4eY9dQRAIJUNxoTfDwD40IYGep3Mdg$>
>
> llvm-compile-time-tracker
> <https://urldefense.com/v3/__https:/llvm-compile-time-tracker.com/compare.php?from=167ff5280d7fcad731810d5d2bf10561ed2adacc&to=b08fcae3a02d5ebe58afd8f8658d798b62ff8eb7&stat=max-rss__;!!JmoZiZGBv3RvKRSx!u5xov9_zHGWKWsFWg-FlBBUrKZWcBw4KyfXq4eY9dQRAIJUNxoTfDwD40IbggPFwHQ$> memory
> metrics
>
>
>
> Any concerns with this?
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev__;!!JmoZiZGBv3RvKRSx!u5xov9_zHGWKWsFWg-FlBBUrKZWcBw4KyfXq4eY9dQRAIJUNxoTfDwD40IZRLUF5mg$>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20210921/0142a704/attachment.html>