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[AMDGPU] Don't implement GCNHazardRecognizer::PreEmitNoops(SUnit *)

Authored by foad on May 6 2020, 3:16 AM.

Description

[AMDGPU] Don't implement GCNHazardRecognizer::PreEmitNoops(SUnit *)

When called from the post-RA scheduler, hazards have already been
handled by getHazardType returning NoopHazard, so PreEmitNoops always
returns zero. Remove it. NFC.

Historical note: PreEmitNoops was added to the hazard recognizer
interface as an optional feature to support dispatch group formation on
the POWER target:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20131202/197470.html
So it seems right that we shouldn't need to implement it.

We do still implement the other overload PreEmitNoops(MachineInstr *)
because that is used by the PostRAHazardRecognizer pass.

Differential Revision: https://reviews.llvm.org/D79476

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