[cfe-dev] [llvm-dev] llvm and clang are getting slower

Mehdi Amini via cfe-dev cfe-dev at lists.llvm.org
Tue Mar 8 09:40:47 PST 2016


Hi Rafael,

CC: cfe-dev

Thanks for sharing. We also noticed this internally, and I know that Bruno and Chris are working on some infrastructure and tooling to help tracking closely compile time regressions.

We had this conversation internally about the tradeoff between compile-time and runtime performance, and I planned to bring-up the topic on the list in the coming months, this looks like a good occasion to plant the seed. Apparently in the past (years/decade ago?) the project was very conservative on adding any optimizations that would impact compile time, however there is no explicit policy (that I know of) to address this tradeoff.
The closest I could find would be what Chandler wrote in: http://reviews.llvm.org/D12826 ; for instance for O2 he stated that "if an optimization increases compile time by 5% or increases code size by 5% for a particular benchmark, that benchmark should also be one which sees a 5% runtime improvement".

My hope is that with better tooling for tracking compile time in the future, we'll reach a state where we'll be able to consider "breaking" the compile-time regression test as important as breaking any test: i.e. the offending commit should be reverted unless it has been shown to significantly (hand wavy...) improve the runtime performance.

<troll>
With the current trend, the Polly developers don't have to worry about improving their compile time, we'll catch up with them ;)
</troll>

-- 
Mehdi






> On Mar 8, 2016, at 8:13 AM, Rafael EspĂ­ndola via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> I have just benchmarked building trunk llvm and clang in Debug,
> Release and LTO modes (see the attached scrip for the cmake lines).
> 
> The compilers used were clang 3.5, 3.6, 3.7, 3.8 and trunk. In all
> cases I used the system libgcc and libstdc++.
> 
> For release builds there is a monotonic increase in each version. From
> 163 minutes with 3.5 to 212 minutes with trunk. For comparison, gcc
> 5.3.2 takes 205 minutes.
> 
> Debug and LTO show an improvement in 3.7, but have regressed again in 3.8.
> 
> Cheers,
> Rafael
> <run.sh><LTO.time><Debug.time><Release.time>_______________________________________________
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