Stream: git-wasmtime

Topic: wasmtime / PR #7818 x64: Break more data dependencies in ...

Wasmtime GitHub notifications bot (Jan 24 2024 at 23:36):

alexcrichton opened PR #7818 from alexcrichton:fix-false-dependencies to bytecodealliance:main:

This commit takes a stab at #7816 without diving a whole lot into it. I noticed that the loop started with vcvtss2sd which is along the same lines as previous false dependencies found earlier in PRs such as #7098. I had forgotten these instructions at the time and meant to go back and touch them up and #7731 has provided sufficient motivation to do so!

Locally this takes that test case from 1.6s to 0.4s for me.

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Wasmtime GitHub notifications bot (Jan 24 2024 at 23:36):

alexcrichton requested abrown for a review on PR #7818.

Wasmtime GitHub notifications bot (Jan 24 2024 at 23:36):

alexcrichton requested wasmtime-compiler-reviewers for a review on PR #7818.

Wasmtime GitHub notifications bot (Jan 24 2024 at 23:59):

alexcrichton updated PR #7818.

Wasmtime GitHub notifications bot (Jan 25 2024 at 00:22):

alexcrichton updated PR #7818.

Wasmtime GitHub notifications bot (Jan 25 2024 at 02:02):

github-actions[bot] commented on PR #7818:

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Wasmtime GitHub notifications bot (Jan 25 2024 at 23:41):

fitzgen submitted PR review:

Nice!! Out of curiosity, how did you end up root causing that perf bug to this false dependency?

Wasmtime GitHub notifications bot (Jan 26 2024 at 00:07):

fitzgen merged PR #7818.

Wasmtime GitHub notifications bot (Jan 26 2024 at 16:09):

alexcrichton commented on PR #7818:

Ah it was mostly from previous experience. I knew there were a set of instructions in the back of my mind which we still did the "fake the output register as the input" for AVX (e.g. the instructions modified here) and when I ran perf over the program the first very hot instruction in a loop was vcvtss2sd which I remembered was one of those. To test out I split the dependencies and then the performance improved so I assumed it was the cause.

Last updated: Dec 23 2024 at 12:05 UTC