MC: Store MCRelaxableFragment MCInst out-of-line
Follow-up to #146307
Moved MCInst storage to MCSection, enabling trivial ~MCRelaxableFragment
and eliminating the need for a fragment walk in ~MCSection.
Updated MCRelaxableFragment::getInst to construct an MCInst on demand.
Modified MCAssembler::relaxInstruction's mayNeedRelaxation to accept
opcode and operands instead of an MCInst, avoiding redundant MCInst
creation. Note that MCObjectStreamer::emitInstructionImpl calls
mayNeedRelaxation before determining the target fragment for the MCInst.
Unfortunately, we also have to encode `MCInst::Flags` to support
the EVEX prefix, e.g. `{evex} xorw $foo, %ax`
There is a small decrease in max-rss (stage1-ReleaseLTO-g (link only))
with negligible instructions:u change.
https://llvm-compile-time-tracker.com/compare.php?from=0b533f2d9f0551aaffb13dcac8e0fd0a952185b5&to=f26b57f33bc7ccae749a57dfc841de7ce2acc2ef&stat=max-rss&linkStats=on
Next: Enable MCFragment to store fixed-size data (was MCDataFragment's job)
and optional Opcode/Operands data (was MCRelaxableFragment's job),
and delete MCDataFragment/MCRelaxableFragment.
This will allow re-encoding of Data+Relax+Data+Relax sequences as
Frag+Frag. The saving should outweigh the downside of larger
MCFragment.
Pull Request: https://github.com/llvm/llvm-project/pull/147229
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