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[MC] Use local MCSubtargetInfo in writeNops

Authored by psmith on Aug 9 2021, 3:40 AM.

Description

[MC] Use local MCSubtargetInfo in writeNops

On some architectures such as Arm and X86 the encoding for a nop may
change depending on the subtarget in operation at the time of
encoding. This change replaces the per module MCSubtargetInfo retained
by the targets AsmBackend in favour of passing through the local
MCSubtargetInfo in operation at the time.

On Arm using the architectural NOP instruction can have a performance
benefit on some implementations.

For Arm I've deleted the copy of the AsmBackend's MCSubtargetInfo to
limit the chances of this causing problems in the future. I've not
done this for other targets such as X86 as there is more frequent use
of the MCSubtargetInfo and it looks to be for stable properties that
we would not expect to vary per function.

This change required threading STI through MCNopsFragment and
MCBoundaryAlignFragment.

I've attempted to take into account the in tree experimental backends.

Differential Revision: https://reviews.llvm.org/D45962

Details

Committed
psmithSep 7 2021, 7:46 AM
Differential Revision
D45962: [MC] Use local MCSubtargetInfo in writeNops
Parents
rG5e71839f7793: [MC] Add MCSubtargetInfo to MCAlignFragment
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