[PATCH] D58450: [DebugInfo][MachineCSE] Don't try to copy-propagate debuginfo for every COPY seen

[Jeremy Morse via Phabricator via llvm-commits]

jmorse created this revision.
jmorse added reviewers: aprantl, bjope, vsk.
Herald added subscribers: llvm-commits, jdoerfert.
Herald added a project: LLVM.

This patch alters the debug-info salvaging behaviour of MachineCSE, and adds a test for its behaviour (only one opaque test is currently sensitive to MachineCSEs DI salvaging).

In the loop in MachineCSE::PerformTrivialCopyPropagation (see patch), for an arbitrary input instruction, MachineCSE tries to copy-propagate vreg reading operands to expose more CSE opportunities. This is great -- but the current call to changeDebugValuesDefReg then unconditionally rewrites the DBG_VALUE users of the COPY that was propagated. This means that any COPY instruction whose result is the subject of CSE will have its DBG_VALUEs copy-propagated too, even if the COPY lives on, which is a bit overkill. I believe this propagation is only necessary if the COPY is to be deleted -- hence in this patch, the DBG_VALUE updating is moved to only occur if the COPY is deleted.

While we're here, alter changeDebugValuesDefReg to find DBG_VALUEs by enumerating vreg users, which catches DBG_VALUEs that don't immediately follow the COPY. MachineCSE is the only remaining user of this function (assuming D56265 <https://reviews.llvm.org/D56265> makes it in).

I've added a test that stimulates both behaviours -- first where a COPY is deleted (and thus all DBG_VALUE users must update) and one where it doesn't.

A clang-3.4 build with this patch ticks up a fractional amount, and the covered scope bytes ticks down a fraction too. The ticking down is likely because vreg liveness interacting with DBG_VALUEs is still an open problem.


Repository:
  rL LLVM

https://reviews.llvm.org/D58450

Files:
  include/llvm/CodeGen/MachineInstr.h
  lib/CodeGen/MachineCSE.cpp
  lib/CodeGen/MachineInstr.cpp
  test/DebugInfo/MIR/X86/machine-cse.mir

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