[all-commits] [llvm/llvm-project] fb5683: [Pipelines] Restore old DAE position in LTO pipeline

[Nikita Popov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fb5683449e97bd8c2d107128dcf08d6f93789315
      https://github.com/llvm/llvm-project/commit/fb5683449e97bd8c2d107128dcf08d6f93789315
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2023-03-14 (Tue, 14 Mar 2023)

  Changed paths:
    M llvm/lib/Passes/PassBuilderPipelines.cpp
    M llvm/test/Other/new-pm-lto-defaults.ll
    M llvm/test/Transforms/PhaseOrdering/dae-dce.ll

  Log Message:
  -----------
  [Pipelines] Restore old DAE position in LTO pipeline

This is a partial revert of D128830, restoring the previous
position of DeadArgElim in the fat LTO pipeline. The motivation
for this is a major code size regression observed in Rust and
illustrated in the PhaseOrdering test.

This is a conservative fix restoring the previous pipeline order.
The real problem is that the LTO pipeline is conceptually broken:
It doesn't have a CGSCC function simplification pipeline. The
inliner is just being run by itself. This wouldn't be a problem
if fat LTO used a standard design where ArgPromotion and DAE are
only run after functions have already been simplified by the
CGSCC inliner pipeline.

Differential Revision: https://reviews.llvm.org/D146051