[PATCH] D99507: [amdgpu] Add a pass to avoid jump into blocks with 0 exec mask.

[Michael Liao via Phabricator via llvm-commits]

hliao added a comment.

This's the companion fix for D96980 <https://reviews.llvm.org/D96980>. That explain the myth why the original agnostic SGPR spill/reload is proposed to solve the issue SGRP spill/reload may be executed when exec mask goes to zero.
We did try to skip executing code when exec mask goes to zero by branch on EXECZ (to the target block) or EXECNZ (to the fallthrough block.) We may run instructions with zero exec mask. But, that's usually not an issue as we immediately restore the exec mask on the targeted block. Code following that exec mask restoration won't be executed in 0 mask. However, if that mask restoration needs to reload a spilled exec mask, we will run the SGPR reload with 0 mask, where `v_readfristlane` has undefined behavior when exec mask is zero.
This patch tries to mitigate that case by not evaluating exec mask that early or clearing the exec mask when the branch target has mask restoration following SGRP reload. Instead of checking EXECZ or EXECNZ, the exec mask evaluation is duplicated with a temporary SGRP as the destination (without updating exec mask directly), checking SCC0 is equivalent to EXECZ. Exec mask is only evaluated when the result won't be zero. For instance,

  $exec = MASK_EVALUATION()
  s_cbranch_execz TARGET
  ...

TARGET:

  $mask = SGPR_RELOAD(SLOT)
  $exec = OR $exec, $exec, $mask

is translated into

  $tmp = MASK_EVALUATION())
  s_cbranch_scc0 TARGET
  $exec = MASK_EVALUATION()
  ...

TARGET:

  $mask = SGPR_RELOAD(SLOT)
  $exec = OR $exec, $exec, $mask

Note that such transformation is only applied when that mask restoration needs SGPR reloading. As the mask restoration is the merge point of CFG, the predecessor block should have its exec mask always subset of the mask to be restored. It's safe to use the exec mask before that exec mask evaluation.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D99507/new/

https://reviews.llvm.org/D99507