| commit | 54e52aa5ebe68de122a3fe6064e0abef97f6b8e0 | [log] [tgz] |
|---|---|---|
| author | Simon Pilgrim <llvm-dev@redking.me.uk> | Thu May 16 14:44:00 2024 +0100 |
| committer | GitHub <noreply@github.com> | Thu May 16 14:44:00 2024 +0100 |
| tree | 40998ea2100ddefaff8d959bbb89e17d840f914c | |
| parent | 4a5dffc67499640c71357d8f3f49edc97af5482f [diff] |
[X86] Reduce znver3/4 LoopMicroOpBufferSize to practical loop unrolling values (#91340) The znver3/4 scheduler models have previously associated the LoopMicroOpBufferSize with the maximum size of their op caches, and when this led to quadratic complexity issues this were reduced to a value of 512 uops, based mainly on compilation time and not its effectiveness on runtime performance. From a runtime performance POV, a large LoopMicroOpBufferSize leads to a higher number of loop unrolls, meaning the cpu has to rely on the frontend decode rate (4 ins/cy max) for much longer to fill the op cache before looping begins and we make use of the faster op cache rate (8/9 ops/cy). This patch proposes we instead cap the size of the LoopMicroOpBufferSize based off the maximum rate from the op cache (znver3 = 8op/cy, znver4 = 9op/cy) and the branch misprediction penalty from the opcache (~12cy) as a estimate of the useful number of ops we can unroll a loop by before mispredictions are likely to cause stalls. This isn't a perfect metric, but does try to be closer to the spirit of how we use LoopMicroOpBufferSize in the compiler vs the size of a similar naming buffer in the cpu.
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