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

[Hal Finkel via cfe-dev]

----- Original Message -----
> From: "Mehdi Amini via cfe-dev" <cfe-dev at lists.llvm.org>
> To: "Rafael Espíndola" <rafael.espindola at gmail.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "cfe-dev" <cfe-dev at lists.llvm.org>
> Sent: Tuesday, March 8, 2016 11:40:47 AM
> Subject: Re: [cfe-dev] [llvm-dev] llvm and clang are getting slower
> 
> 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>

My two largest pet peeves in this area are:

 1. We often use functions from ValueTracking (to get known bits, the number of sign bits, etc.) as through they're low cost. They're not really low cost. The problem is that they *should* be. These functions do bottom-up walks, and could cache their results. Instead, they do a limited walk and recompute everything each time. This is expensive, and a significant amount of our InstCombine time goes to ValueTracking, and that shouldn't be the case. The more we add to InstCombine (and related passes), and the more we run InstCombine, the worse this gets. On the other hand, fixing this will help both compile time and code quality.

  Furthermore, BasicAA has the same problem.

 2. We have "cleanup" passes in the pipeline, such as those that run after loop unrolling and/or vectorization, that run regardless of whether the preceding pass actually did anything. We've been adding more of these, and they catch important use cases, but we need a better infrastructure for this (either with the new pass manager or otherwise).

Also, I'm very hopeful that as our new MemorySSA and GVN improvements materialize, we'll see large compile-time improvements from that work. We spend a huge amount of time in GVN computing memory-dependency information (the dwarfs the time spent by GVN doing actual value numbering work by an order of magnitude or more).

 -Hal

> 
> --
> Mehdi
> 
> 
> 
> 
> 
> 
> > On Mar 8, 2016, at 8:13 AM, Rafael Espíndola 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>_______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory