[cfe-dev] [RFC] Adding a different mode of "where clang spends time" reporting (timeline/flamegraph style)

Roman Lebedev via cfe-dev cfe-dev at lists.llvm.org
Sun Jan 13 04:46:02 PST 2019

Non-feedback passing-by remark:
while a command-line switch to enable such timing traces is good, it
should be noted that llvm xray exists,
and it can nicely produce this "Chrome Tracing format output", so i
almost wonder if simply building clang
with that instrumentation would be better overall, if not for the ease
of adoption.

On Sun, Jan 13, 2019 at 3:31 PM Aras Pranckevicius via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Hello!
> TL;DR: I have made a Clang/LLVM code change that adds "-ftime-trace" option, that produces Chrome Tracing format output. Would like comments on whether that is a good idea or not, or perhaps someone else is already doing this. My current (WIP) patch in github PR format is here https://github.com/aras-p/llvm-project-20170507/pull/2 -- with images and trace output files attached.
> Longer version:
> Current implementation of "-ftime-report" has several issues, particularly when I'm just a "user" of the compiler:
> - it outputs a lot of information (almost half of it is duplicated since clang 7.0),
> - a lot of that information is things that only compiler developers would know about,
> - has quite large overhead, I've seen it make compile times take 1.5x longer,
> - has very little information about "frontend" part (preprocessing, parsing, instantiation, C++ modules),
> - the things it reports are only "summaries", i.e. "how much time it took to do work X in total". e.g. it can tell that "inlining all functions took X seconds", but in case there was just one super slow function to inline among
> thousands, it will not tell which one was the slow one.
> I have written a blog post about this (as well as lack of "good" time reporting tools in Visual Studio and gcc) recently, http://aras-p.info/blog/2019/01/12/Investigating-compile-times-and-Clang-ftime-report/
> At work (Unity game engine), with codebase of similar size to whole of Clang/LLVM (2-3 million lines of code), we had really good experience in adding timeline/flamegraph visualizations to various parts of our "build system". This can tell us which .cpp files were slowest to compile in the whole build, but I also wanted similar tooling for things "inside" single .cpp file compilation.
> Thus this attempt at adding a new time trace profiling mode.
> I have current changes on github here, https://github.com/aras-p/llvm-project-20170507/pull/2 -- can do a proper "patch" thing via Phabricator if needed.
> My current code change does not quite match Clang/LLVM code standards and probably needs some work, but the general approach seems to work. Already found one case of Clang being very slow at parsing some weird recursive macro thingamabob that we had; was causing about 5-8 seconds just to include one header file. I probably would have never found it without this type of visualization. Here it is very clear that among all the things, parsing just that one header file takes almost all the time: https://user-images.githubusercontent.com/348087/51038295-76efb780-15bb-11e9-926f-a6be1ffd03f1.png
> Regards,
> --
> Aras Pranckevičius

> work: http://unity3d.com
> home: http://aras-p.info
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