tree dd0c2f7bcd070d3868a05f03278542b0038fd29e
parent 61e89737c5daf7b65341936f88f68efb680cdcd4
author Roman Lebedev <lebedev.ri@gmail.com> 1585808915 +0300
committer Roman Lebedev <lebedev.ri@gmail.com> 1585808915 +0300
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[llvm-exegesis] 'Min' repetition mode

Summary:
As noted in documentation, different repetition modes have different trade-offs:

> .. option:: -repetition-mode=[duplicate|loop]
>
>  Specify the repetition mode. `duplicate` will create a large, straight line
>  basic block with `num-repetitions` copies of the snippet. `loop` will wrap
>  the snippet in a loop which will be run `num-repetitions` times. The `loop`
>  mode tends to better hide the effects of the CPU frontend on architectures
>  that cache decoded instructions, but consumes a register for counting
>  iterations.

Indeed. Example:

>>! In D74156#1873657, @lebedev.ri wrote:
> At least for `CMOV`, i'm seeing wildly different results
> |           | Latency | RThroughput |
> | duplicate | 1       | 0.8         |
> | loop      | 2       | 0.6         |
> where latency=1 seems correct, and i'd expect the througput to be close to 1/2 (since there are two execution units).

This isn't great for analysis, at least for schedule model development.

As discussed in excruciating detail in

>>! In D74156#1924514, @gchatelet wrote:
>>>! In D74156#1920632, @lebedev.ri wrote:
>> ... did that explanation of the question i'm having made any sense?
>
> Thx for digging in the conversation !
> Ok it makes more sense now.
>
> I discussed it a bit with @courbet:
>  - We want the analysis tool to stay simple so we'd rather not make it knowledgeable of the repetition mode.
>  - We'd like to still be able to select either repetition mode to dig into special cases
>
> So we could add a third `min` repetition mode that would run both and take the minimum. It could be the default option.
> Would you have some time to look what it would take to add this third mode?

there appears to be an agreement that it is indeed sub-par,
and that we should provide an optional, measurement (not analysis!) -time
way to rectify the situation.

However, the solutions isn't entirely straight-forward.

We can just add an actual 'multiplexer' `MinSnippetRepetitor`, because
if we just concatenate snippets produced by `DuplicateSnippetRepetitor`
and `LoopSnippetRepetitor` and run+measure that, the measurement will
naturally be different from what we'd get by running+measuring
them separately and taking the min.
([[ https://www.wolframalpha.com/input/?i=%28x%2By%29%2F2+%21%3D+min%28x%2C+y%29 | `time(D+L)/2 != min(time(D), time(L))` ]])

Also, it seems best to me to have a single snippet instead of generating
a snippet per repetition mode, since the only difference here is that the
loop repetition mode reserves one register for loop counter.

As far as i can tell, we can either teach `BenchmarkRunner::runConfiguration()`
to produce a single report given multiple repetitors (as in the patch),
or do that one layer higher - don't modify `BenchmarkRunner::runConfiguration()`,
produce multiple reports, don't actually print each one, but aggregate them somehow
and only print the final one.

Initially i've gone ahead with the latter approach, but it didn't look like a natural fit;
the former (as in the diff) does seem like a better fit to me.

There's also a question of the test coverage. It sure currently does work here:
```
$ ./bin/llvm-exegesis --opcode-name=CMOV64rr --mode=inverse_throughput --repetition-mode=duplicate
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-8fb949.o
---
mode:            inverse_throughput
key:
  instructions:
    - 'CMOV64rr RAX RAX R11 i_0x0'
    - 'CMOV64rr RBP RBP R15 i_0x0'
    - 'CMOV64rr RBX RBX RBX i_0x0'
    - 'CMOV64rr RCX RCX RBX i_0x0'
    - 'CMOV64rr RDI RDI R10 i_0x0'
    - 'CMOV64rr RDX RDX RAX i_0x0'
    - 'CMOV64rr RSI RSI RAX i_0x0'
    - 'CMOV64rr R8 R8 R8 i_0x0'
    - 'CMOV64rr R9 R9 RDX i_0x0'
    - 'CMOV64rr R10 R10 RBX i_0x0'
    - 'CMOV64rr R11 R11 R14 i_0x0'
    - 'CMOV64rr R12 R12 R9 i_0x0'
    - 'CMOV64rr R13 R13 R12 i_0x0'
    - 'CMOV64rr R14 R14 R15 i_0x0'
    - 'CMOV64rr R15 R15 R13 i_0x0'
  config:          ''
  register_initial_values:
    - 'RAX=0x0'
    - 'R11=0x0'
    - 'EFLAGS=0x0'
    - 'RBP=0x0'
    - 'R15=0x0'
    - 'RBX=0x0'
    - 'RCX=0x0'
    - 'RDI=0x0'
    - 'R10=0x0'
    - 'RDX=0x0'
    - 'RSI=0x0'
    - 'R8=0x0'
    - 'R9=0x0'
    - 'R14=0x0'
    - 'R12=0x0'
    - 'R13=0x0'
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: inverse_throughput, value: 0.819, per_snippet_value: 12.285 }
error:           ''
info:            instruction has tied variables, using static renaming.
assembled_snippet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
...
$ ./bin/llvm-exegesis --opcode-name=CMOV64rr --mode=inverse_throughput --repetition-mode=loop
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-051eb3.o
---
mode:            inverse_throughput
key:
  instructions:
    - 'CMOV64rr RAX RAX R11 i_0x0'
    - 'CMOV64rr RBP RBP RSI i_0x0'
    - 'CMOV64rr RBX RBX R9 i_0x0'
    - 'CMOV64rr RCX RCX RSI i_0x0'
    - 'CMOV64rr RDI RDI RBP i_0x0'
    - 'CMOV64rr RDX RDX R9 i_0x0'
    - 'CMOV64rr RSI RSI RDI i_0x0'
    - 'CMOV64rr R9 R9 R12 i_0x0'
    - 'CMOV64rr R10 R10 R11 i_0x0'
    - 'CMOV64rr R11 R11 R9 i_0x0'
    - 'CMOV64rr R12 R12 RBP i_0x0'
    - 'CMOV64rr R13 R13 RSI i_0x0'
    - 'CMOV64rr R14 R14 R14 i_0x0'
    - 'CMOV64rr R15 R15 R10 i_0x0'
  config:          ''
  register_initial_values:
    - 'RAX=0x0'
    - 'R11=0x0'
    - 'EFLAGS=0x0'
    - 'RBP=0x0'
    - 'RSI=0x0'
    - 'RBX=0x0'
    - 'R9=0x0'
    - 'RCX=0x0'
    - 'RDI=0x0'
    - 'RDX=0x0'
    - 'R12=0x0'
    - 'R10=0x0'
    - 'R13=0x0'
    - 'R14=0x0'
    - 'R15=0x0'
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: inverse_throughput, value: 0.6083, per_snippet_value: 8.5162 }
error:           ''
info:            instruction has tied variables, using static renaming.
assembled_snippet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
...
$ ./bin/llvm-exegesis --opcode-name=CMOV64rr --mode=inverse_throughput --repetition-mode=min
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-c7a47d.o
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-2581f1.o
---
mode:            inverse_throughput
key:
  instructions:
    - 'CMOV64rr RAX RAX R11 i_0x0'
    - 'CMOV64rr RBP RBP R10 i_0x0'
    - 'CMOV64rr RBX RBX R10 i_0x0'
    - 'CMOV64rr RCX RCX RDX i_0x0'
    - 'CMOV64rr RDI RDI RAX i_0x0'
    - 'CMOV64rr RDX RDX R9 i_0x0'
    - 'CMOV64rr RSI RSI RAX i_0x0'
    - 'CMOV64rr R9 R9 RBX i_0x0'
    - 'CMOV64rr R10 R10 R12 i_0x0'
    - 'CMOV64rr R11 R11 RDI i_0x0'
    - 'CMOV64rr R12 R12 RDI i_0x0'
    - 'CMOV64rr R13 R13 RDI i_0x0'
    - 'CMOV64rr R14 R14 R9 i_0x0'
    - 'CMOV64rr R15 R15 RBP i_0x0'
  config:          ''
  register_initial_values:
    - 'RAX=0x0'
    - 'R11=0x0'
    - 'EFLAGS=0x0'
    - 'RBP=0x0'
    - 'R10=0x0'
    - 'RBX=0x0'
    - 'RCX=0x0'
    - 'RDX=0x0'
    - 'RDI=0x0'
    - 'R9=0x0'
    - 'RSI=0x0'
    - 'R12=0x0'
    - 'R13=0x0'
    - 'R14=0x0'
    - 'R15=0x0'
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: inverse_throughput, value: 0.6073, per_snippet_value: 8.5022 }
error:           ''
info:            instruction has tied variables, using static renaming.
assembled_snippet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
...
```
but i open to suggestions as to how test that.

I also have gone with the suggestion to default to this new mode.
This was irking me for some time, so i'm happy to finally see progress here.
Looking forward to feedback.

Reviewers: courbet, gchatelet

Reviewed By: courbet, gchatelet

Subscribers: mstojanovic, RKSimon, llvm-commits, courbet, gchatelet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76921