[all-commits] [llvm/llvm-project] f0b365: [LoopUnroll] Clamp PartialThreshold for large Loop...

[Nikita Popov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f0b3654701bde1cf7821d60698b42383edaff9f3
      https://github.com/llvm/llvm-project/commit/f0b3654701bde1cf7821d60698b42383edaff9f3
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2024-05-16 (Thu, 16 May 2024)

  Changed paths:
    M llvm/include/llvm/CodeGen/BasicTTIImpl.h
    M llvm/test/Transforms/LoopUnroll/X86/znver3.ll

  Log Message:
  -----------
  [LoopUnroll] Clamp PartialThreshold for large LoopMicroOpBufferSize (#67657)

The znver3/znver4 scheduler models are outliers, specifying very large
LoopMicroOpBufferSizes at 512, while typical values for other subtargets
are on the order of ~50. Even if this information is
micro-architecturally correct (*), this does not mean that we want to
runtime unroll all loops to a size that completely fills the loop
buffer. Unless this is the single hot loop in the entire application,
the massive code size increase will bust the micro-op and instruction
caches.

Protect against this by clamping to the default PartialThreshold of 150,
which is the same as the default full-unroll threshold and half the
aggressive full-unroll threshold. Allowing more partial unrolling than
full unrolling certainly does not make sense.

(*) I strongly doubt that this is actually correct -- I believe this may
derive from an incorrect reading of Agner Fog's micro-architecture
guide. The number 4096 that was originally used here is the size of the
general micro-op cache, not that of a loop buffer. A separate loop
buffer is not listed for the Zen microarchitecture. Comparing this to
the listing for Skylake, it has a 1536 micro-op buffer, but only a 64
micro-op loopback buffer, with a note that it's rarely fully utilized.
Our scheduling model specifies LoopMicroOpBufferSize of 50 in that case.



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