[llvm-dev] llvm (the middle-end) is getting slower, December edition
Michael Gottesman via llvm-dev
llvm-dev at lists.llvm.org
Sat Dec 17 17:19:25 PST 2016
> On Dec 17, 2016, at 1:35 PM, Davide Italiano via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> First of all, sorry for the long mail.
> Inspired by the excellent analysis Rui did for lld, I decided to do
> the same for llvm.
> I'm personally very interested in build-time for LTO configuration,
> with particular attention to the time spent in the optimizer.
> Rafael did something similar back in March, so this can be considered
> as an update. This tries to include a more accurate high-level
> analysis of where llvm is spending CPU cycles.
> Here I present 2 cases: clang building itself with `-flto` (Full), and
> clang building an internal codebase which I'm going to refer as
> `game7`.
> It's a mid-sized program (it's actually a game), more or less of the
> size of clang, which we use internally as benchmark to track
> compile-time/runtime improvements/regression.
> I picked two random revisions of llvm: trunk (December 16th 2016) and
> trunk (June 2nd 2016), so, roughly, 6 months period.
> My setup is a Mac Pro running Linux (NixOS).
> These are the numbers I collected (including the output of -mllvm -time-passes).
> For clang:
>
> June 2nd:
> real 22m9.278s
> user 21m30.410s
> sys 0m38.834s
> Total Execution Time: 1270.4795 seconds (1269.1330 wall clock)
> 289.8102 ( 23.5%) 18.8891 ( 53.7%) 308.6993 ( 24.3%) 308.6906 (
> 24.3%) X86 DAG->DAG Instruction Selection
> 97.2730 ( 7.9%) 0.7656 ( 2.2%) 98.0386 ( 7.7%) 98.0010 (
> 7.7%) Global Value Numbering
> 62.4091 ( 5.1%) 0.4779 ( 1.4%) 62.8870 ( 4.9%) 62.8665 (
> 5.0%) Function Integration/Inlining
> 58.6923 ( 4.8%) 0.4767 ( 1.4%) 59.1690 ( 4.7%) 59.1323 (
> 4.7%) Combine redundant instructions
> 53.9602 ( 4.4%) 0.6163 ( 1.8%) 54.5765 ( 4.3%) 54.5409 (
> 4.3%) Combine redundant instructions
> 51.0470 ( 4.1%) 0.5703 ( 1.6%) 51.6173 ( 4.1%) 51.5425 (
> 4.1%) Loop Strength Reduction
> 47.4067 ( 3.8%) 1.3040 ( 3.7%) 48.7106 ( 3.8%) 48.7034 (
> 3.8%) Greedy Register Allocator
> 36.7463 ( 3.0%) 0.8133 ( 2.3%) 37.5597 ( 3.0%) 37.4612 (
> 3.0%) Induction Variable Simplification
> 37.0125 ( 3.0%) 0.2699 ( 0.8%) 37.2824 ( 2.9%) 37.2478 (
> 2.9%) Combine redundant instructions
> 34.2071 ( 2.8%) 0.2737 ( 0.8%) 34.4808 ( 2.7%) 34.4487 (
> 2.7%) Combine redundant instructions
> 25.6627 ( 2.1%) 0.3215 ( 0.9%) 25.9842 ( 2.0%) 25.9509 (
> 2.0%) Combine redundant instructions
>
> Dec 16th:
> real 27m34.922s
> user 26m53.489s
> sys 0m41.533s
>
> 287.5683 ( 18.5%) 19.7048 ( 52.3%) 307.2731 ( 19.3%) 307.2648 (
> 19.3%) X86 DAG->DAG Instruction Selection
> 197.9211 ( 12.7%) 0.5104 ( 1.4%) 198.4314 ( 12.5%) 198.4091 (
> 12.5%) Function Integration/Inlining
> 106.9669 ( 6.9%) 0.8316 ( 2.2%) 107.7984 ( 6.8%) 107.7633 (
> 6.8%) Global Value Numbering
> 89.7571 ( 5.8%) 0.4840 ( 1.3%) 90.2411 ( 5.7%) 90.2067 (
> 5.7%) Combine redundant instructions
> 79.0456 ( 5.1%) 0.7534 ( 2.0%) 79.7990 ( 5.0%) 79.7630 (
> 5.0%) Combine redundant instructions
> 55.6393 ( 3.6%) 0.3116 ( 0.8%) 55.9509 ( 3.5%) 55.9187 (
> 3.5%) Combine redundant instructions
> 51.8663 ( 3.3%) 1.4090 ( 3.7%) 53.2754 ( 3.3%) 53.2684 (
> 3.3%) Greedy Register Allocator
> 52.5721 ( 3.4%) 0.3021 ( 0.8%) 52.8743 ( 3.3%) 52.8416 (
> 3.3%) Combine redundant instructions
> 49.0593 ( 3.2%) 0.6101 ( 1.6%) 49.6694 ( 3.1%) 49.5904 (
> 3.1%) Loop Strength Reduction
> 41.2602 ( 2.7%) 0.9608 ( 2.5%) 42.2209 ( 2.7%) 42.1122 (
> 2.6%) Induction Variable Simplification
> 38.1438 ( 2.5%) 0.3486 ( 0.9%) 38.4923 ( 2.4%) 38.4603 (
> 2.4%) Combine redundant instructions
>
> so, llvm is around 20% slower than it used to be.
>
> For our internal codebase the situation seems slightly worse:
>
> `game7`
>
> June 2nd:
>
> Total Execution Time: 464.3920 seconds (463.8986 wall clock)
>
> 88.0204 ( 20.3%) 6.0310 ( 20.0%) 94.0514 ( 20.3%) 94.0473 (
> 20.3%) X86 DAG->DAG Instruction Selection
> 27.4382 ( 6.3%) 16.2437 ( 53.9%) 43.6819 ( 9.4%) 43.6823 (
> 9.4%) X86 Assembly / Object Emitter
> 34.9581 ( 8.1%) 0.5274 ( 1.8%) 35.4855 ( 7.6%) 35.4679 (
> 7.6%) Function Integration/Inlining
> 27.8556 ( 6.4%) 0.3419 ( 1.1%) 28.1975 ( 6.1%) 28.1824 (
> 6.1%) Global Value Numbering
> 22.1479 ( 5.1%) 0.2258 ( 0.7%) 22.3737 ( 4.8%) 22.3593 (
> 4.8%) Combine redundant instructions
> 19.2346 ( 4.4%) 0.3639 ( 1.2%) 19.5985 ( 4.2%) 19.5870 (
> 4.2%) Post RA top-down list latency scheduler
> 15.8085 ( 3.6%) 0.2675 ( 0.9%) 16.0760 ( 3.5%) 16.0614 (
> 3.5%) Combine redundant instructions
>
> Dec 16th:
>
> Total Execution Time: 861.0898 seconds (860.5808 wall clock)
>
> 135.7207 ( 15.7%) 0.2484 ( 0.8%) 135.9692 ( 15.2%) 135.9531 (
> 15.2%) Combine redundant instructions
> 103.6609 ( 12.0%) 0.4566 ( 1.4%) 104.1175 ( 11.7%) 104.1014 (
> 11.7%) Combine redundant instructions
> 97.1083 ( 11.3%) 6.9183 ( 21.8%) 104.0266 ( 11.6%) 104.0181 (
> 11.6%) X86 DAG->DAG Instruction Selection
> 72.6125 ( 8.4%) 0.1701 ( 0.5%) 72.7826 ( 8.1%) 72.7678 (
> 8.1%) Combine redundant instructions
> 69.2144 ( 8.0%) 0.6060 ( 1.9%) 69.8204 ( 7.8%) 69.8007 (
> 7.8%) Function Integration/Inlining
> 60.7837 ( 7.1%) 0.3783 ( 1.2%) 61.1620 ( 6.8%) 61.1455 (
> 6.8%) Global Value Numbering
> 56.5650 ( 6.6%) 0.1980 ( 0.6%) 56.7630 ( 6.4%) 56.7476 (
> 6.4%) Combine redundant instructions
>
> so, using LTO, lld takes 2x to build what it used to take (and all the
> extra time seems spent in the optimizer).
>
> As an (extra) experiment, I decided to take the unoptimized output of
> game7 (via lld -save-temps) and pass to -opt -O2. That shows another
> significant regression (with different characteristics).
>
> June 2nd:
> time opt -O2
> real 6m23.016s
> user 6m20.900s
> sys 0m2.113s
>
> 35.9071 ( 10.0%) 0.7996 ( 10.9%) 36.7066 ( 10.0%) 36.6900 ( 10.1%)
> Function Integration/Inlining
> 33.4045 ( 9.3%) 0.4053 ( 5.5%) 33.8098 ( 9.3%) 33.7919 ( 9.3%)
> Global Value Numbering
> 27.1053 ( 7.6%) 0.5940 ( 8.1%) 27.6993 ( 7.6%) 27.6995 ( 7.6%)
> Bitcode Writer
> 25.6492 ( 7.2%) 0.2491 ( 3.4%) 25.8984 ( 7.1%) 25.8805 ( 7.1%)
> Combine redundant instructions
> 19.2686 ( 5.4%) 0.2956 ( 4.0%) 19.5642 ( 5.4%) 19.5471 ( 5.4%)
> Combine redundant instructions
> 18.6697 ( 5.2%) 0.2625 ( 3.6%) 18.9323 ( 5.2%) 18.9148 ( 5.2%)
> Combine redundant instructions
> 16.1294 ( 4.5%) 0.2320 ( 3.2%) 16.3614 ( 4.5%) 16.3434 ( 4.5%)
> Combine redundant instructions
> 13.5476 ( 3.8%) 0.3945 ( 5.4%) 13.9421 ( 3.8%) 13.9295 ( 3.8%)
> Combine redundant instructions
> 13.1746 ( 3.7%) 0.1767 ( 2.4%) 13.3512 ( 3.7%) 13.3405 ( 3.7%)
> Combine redundant instructions
>
> Dec 16th:
>
> real 20m10.734s
> user 20m8.523s
> sys 0m2.197s
>
> 208.8113 ( 17.6%) 0.1703 ( 1.9%) 208.9815 ( 17.5%) 208.9698 (
> 17.5%) Value Propagation
> 179.6863 ( 15.2%) 0.1215 ( 1.3%) 179.8077 ( 15.1%) 179.7974 (
> 15.1%) Value Propagation
> 92.0158 ( 7.8%) 0.2674 ( 3.0%) 92.2832 ( 7.7%) 92.2613 (
> 7.7%) Combine redundant instructions
> 72.3330 ( 6.1%) 0.6026 ( 6.7%) 72.9356 ( 6.1%) 72.9210 (
> 6.1%) Combine redundant instructions
> 72.2505 ( 6.1%) 0.2167 ( 2.4%) 72.4672 ( 6.1%) 72.4539 (
> 6.1%) Combine redundant instructions
> 66.6765 ( 5.6%) 0.3482 ( 3.9%) 67.0247 ( 5.6%) 67.0040 (
> 5.6%) Combine redundant instructions
> 65.5029 ( 5.5%) 0.4092 ( 4.5%) 65.9121 ( 5.5%) 65.8913 (
> 5.5%) Combine redundant instructions
> 61.8355 ( 5.2%) 0.8150 ( 9.0%) 62.6505 ( 5.2%) 62.6315 (
> 5.2%) Function Integration/Inlining
> 54.9184 ( 4.6%) 0.3359 ( 3.7%) 55.2543 ( 4.6%) 55.2332 (
> 4.6%) Combine redundant instructions
> 50.2597 ( 4.2%) 0.2187 ( 2.4%) 50.4784 ( 4.2%) 50.4654 (
> 4.2%) Combine redundant instructions
> 47.2597 ( 4.0%) 0.3719 ( 4.1%) 47.6316 ( 4.0%) 47.6105 (
> 4.0%) Global Value Numbering
>
> I don't have an infrastructure to measure the runtime performance
> benefits/regression of clang, but I have for `game7`.
> I wasn't able to notice any fundamental speedup (at least, not
> something that justifies a 2x build-time).
>
> tl;dr:
> There are quite a few things to notice:
> 1) GVN used to be the top pass in the middle-end, in some cases, and
> pretty much always in the top-3. This is not the case anymore, but
> it's still a pass where we spend a lot of time. This is being worked
> on by Daniel Berlin and me) https://reviews.llvm.org/D26224 so there's
> some hope that will be sorted out (or at least there's a plan for it).
> 2) For clang, we spend 35% more time inside instcombine, and for game7
> instcombine seems to largely dominate the amount of time we spend
> optimizing IR. I tried to bisect (which is not easy considering the
> test takes a long time to run), but I wasn't able to identify a single
> point in time responsible for the regression. It seems to be an
> additive effect. My wild (or not so wild) guess is that every day
> somebody adds a matcher of two because that improves their testcase,
> and at some point all this things add up. I'll try to do some
> additional profiling but I guess large part of our time is spent
> solving bitwise-domain dataflow problems (ComputeKnownBits et
> similia). Once GVN will be in a more stable state, I plan to
> experiment with caching results.
We have seen a similar thing when compiling the swift standard library. We have talked about making a small simple instcombine pass that doesn't iterate to a fixed point (IIRC). IIRC Andy/Mark looked at this (so my memory might be wrong).
> 3) Something peculiar is that we spend 2x time in the inliner. I'm not
> familiar with the inliner, IIRC there were some changes to threshold
> recently, so any help here will be appreciated (both in reproducing
> the results and with analysis).
> 4) For the last testcase (opt -O2 on large unoptimized chunk of
> bitcode) llvm spends 33% of its time in CVP, and very likely in LVI. I
> think it's not as lazy as it claims to be (or at least, the way we use
> it). This doesn't show up in a full LTO run because we don't run CVP
> as part of the default LTO pipeline, but the case shows how LVI can be
> a bottleneck for large TUs (or at least how -O2 compile time degraded
> on large TUs). I haven't thought about the problem very carefully, but
> there seems to be some progress on this front (
> https://llvm.org/bugs/show_bug.cgi?id=10584). I can't share the
> original bitcode file but I can probably do some profiling on it as
> well.
>
> As next steps I'll try to get a more detailed analysis of the
> problems. In particular, try to do what Rui did for lld but with more
> coarse granularity (every week) to have a chart of the compile time
> trend for these cases over the last 6 months, and post here.
>
> I think (I know) some people are aware of the problems I outline in
> this e-mail. But apparently not everybody. We're in a situation where
> compile time is increasing without real control. I'm happy that Apple
> is doing a serious effort to track build-time, so hopefully things
> will improve. There are, although, some cases (like adding matchers in
> instcombine or knobs) where the compile time regression is hard to
> track until it's too late. LLVM as a project tries not to stop people
> trying to get things done and that's great, but from time to time it's
> good to take a step back and re-evaluate approaches.
> The purpose of this e-mail was to outline where we regressed, for
> those interested.
>
> Thanks for your time, and of course, feedback welcome!
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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