[llvm-dev] How to make LLVM go faster?

[Craig Topper via llvm-dev]

The X86 assembly printer is badly named. I think its a leftover from before
LLVM had an integrated assembler. It's where the assembly would have been
printed. Now it is where MachineInstrs are converted to MCInsts and either
printed or turned into binary.

~Craig


On Tue, Sep 11, 2018 at 6:49 PM Andrew Kelley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks, that was a really helpful suggestion. If you're curious- here are
> some of the high cost areas:
>
>
> ===-------------------------------------------------------------------------===
>                                  DWARF Emission
>
> ===-------------------------------------------------------------------------===
>   Total Execution Time: 2.0117 seconds (2.0185 wall clock)
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>    0.7977 ( 64.4%)   0.4517 ( 58.4%)   1.2494 ( 62.1%)   1.2557 ( 62.2%)
> Debug Info Emission
>    0.4383 ( 35.4%)   0.3221 ( 41.6%)   0.7603 ( 37.8%)   0.7608 ( 37.7%)
> DWARF Exception Writer
>    0.0019 (  0.2%)   0.0000 (  0.0%)   0.0019 (  0.1%)   0.0019 (  0.1%)
> DWARF Debug Writer
>    1.2379 (100.0%)   0.7738 (100.0%)   2.0117 (100.0%)   2.0185 (100.0%)
> Total
>
> ===-------------------------------------------------------------------------===
>                       ... Pass execution timing report ...
>
> ===-------------------------------------------------------------------------===
>   Total Execution Time: 10.3340 seconds (10.3289 wall clock)
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>    2.4747 ( 31.5%)   1.6998 ( 68.5%)   4.1744 ( 40.4%)   4.1745 ( 40.4%)
> X86 Assembly Printer
>    2.3159 ( 29.5%)   0.0103 (  0.4%)   2.3262 ( 22.5%)   2.3262 ( 22.5%)
> Coroutine Splitting
>    1.4348 ( 18.3%)   0.4092 ( 16.5%)   1.8440 ( 17.8%)   1.8428 ( 17.8%)
> X86 DAG->DAG Instruction Selection
>    0.1875 (  2.4%)   0.0384 (  1.5%)   0.2259 (  2.2%)   0.2252 (  2.2%)
> Fast Register Allocator
>
>
> I'm curious about the X86 Assembly Printer. What is it doing? The module
> in question has some inline assembly in it. So I would think that LLVM
> would parse it and then render it as machine code - at what point does it
> get printed?
>
> As for Coroutine Splitting - is this about as fast as it will get? If not
> then what's the point of LLVM having coroutines? Frontends would be better
> off implementing coroutines on top of structs, like Rust does.
>
>
> On Tue, Sep 11, 2018 at 9:01 PM Friedman, Eli <efriedma at codeaurora.org>
> wrote:
>
>> On 9/11/2018 5:48 PM, Andrew Kelley via llvm-dev wrote:
>>
>> Here is some timing information from running the Zig standard library
>> tests:
>>
>> $ ./zig test ../std/index.zig --enable-timing-info
>>                 Name       Start         End    Duration     Percent
>>           Initialize      0.0000      0.0010      0.0010      0.0001
>>    Semantic Analysis      0.0010      0.9968      0.9958      0.1192
>>      Code Generation      0.9968      1.4000      0.4032      0.0483
>>     LLVM Emit Output      1.4000      8.1759      6.7760      0.8112
>>   Build Dependencies      8.1759      8.3341      0.1581      0.0189
>>            LLVM Link      8.3341      8.3530      0.0189      0.0023
>>                Total      0.0000      8.3530      8.3530      1.0000
>>
>> 81% of the time was spent waiting for LLVM to turn a Module into an
>> object file. This is with optimizations off, FastISel, no module
>> verification, etc.
>>
>> How can I speed this up? Any tips or things to look into?
>>
>>
>> First step is probably setting TimePassesIsEnabled to true and looking at
>> the output.  It's hard to say where the time is going without any numbers.
>>
>> -Eli
>>
>> --
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>
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