[llvm-exegesis][llvm-mca] Real vs theoretical code snippet analysis

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[llvm-exegesis][llvm-mca] Real vs theoretical code snippet analysis #37785

Open RKSimon opened 6 years ago

RKSimon commented 6 years ago


Bugzilla Link	38437
Version	trunk
OS	Windows NT
Depends On	llvm/llvm-project#37044 llvm/llvm-project#37396
CC	@adibiagio,@legrosbuffle,@topperc,@gchatelet,@gregbedwell,@LebedevRI

Extended Description

For a given code snippet, it would be very useful for llvm-exegesis to compare the actual performance vs llvm-mca's prediction.

This could be driven by code snippets from real code or possibly from a fuzzer

gregbedwell commented 6 years ago

This is a good idea.

From a design perspective I think it's better to have a script that runs llvm-mca and llvm-exegesis separately so that the two tools don't depend on each other.

I think that makes sense. Internally we're using a script that compares 'real-world' and semi-randomized instruction sequences to compare llvm-mca's estimated IPC against the measured IPC using Linux perf and a simple asm harness to run the snippet in a loop. We're getting some nice results from that; potentially we should consider upstreaming that to llvm/utils or similar. Right now it's quite btver2 specific though.

We could certainly consider expanding that to incorporate llvm-exegesis results.

gchatelet commented 6 years ago

This is a good idea.

From a design perspective I think it's better to have a script that runs llvm-mca and llvm-exegesis separately so that the two tools don't depend on each other.

For llvm-exegesis the inputs would be:

the snippet
the state of the registers and FPU on entry

I'm unsure how precise we can be on the number of cycles/uOps.

for one execution the numbers will be noisy (they will include setup and counter on/off time)
for many executions it will be more precise but we need to remove the setup and loop instructions.

llvmbot commented 1 month ago

@llvm/issue-subscribers-tools-llvm-exegesis

Author: Simon Pilgrim (RKSimon)

| | | | --- | --- | | Bugzilla Link | [38437](https://llvm.org/bz38437) | | Version | trunk | | OS | Windows NT | | Depends On | llvm/llvm-project#37044 llvm/llvm-project#37396 | | CC | @adibiagio,@legrosbuffle,@topperc,@gchatelet,@gregbedwell,@LebedevRI | ## Extended Description For a given code snippet, it would be very useful for llvm-exegesis to compare the actual performance vs llvm-mca's prediction. This could be driven by code snippets from real code or possibly from a fuzzer