[PATCH] D159193: [AArch64][SME] Disable remat of VL-dependent ops when function changes streaming mode.

[Sander de Smalen via Phabricator via llvm-commits]

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This is a way to prevent the register allocator from inserting instructions
which behave differently for different runtime vector-lengths, inside a
call-sequence which changes the streaming-SVE mode before/after the call.

I've considered using BUNDLEs in Machine IR, but found that using this is
not possible for a few reasons:

- Most passes don't look inside BUNDLEs, but some passes would need to look inside these call-sequence bundles, for example the PrologEpilog pass (to remove the CALLSEQSTART/END), a PostRA pass to remove COPY instructions, or the AArch64PseudoExpand pass.
- Within the streaming-mode-changing call sequence, one of the instructions is a CALLSEQEND. The corresponding CALLSEQBEGIN (AArch64::ADJCALLSTACKUP) is outside this sequence. This means we'd end up with a BUNDLE that has [SMSTART, COPY, BL, ADJCALLSTACKUP, COPY, SMSTOP]. The MachineVerifier doesn't accept this, and we also can't move the CALLSEQSTART into the call sequence.

Maybe in the future we could model this differently by modelling
the runtime vector-length as a value that's used by certain operations
(similar to e.g. NCZV flags) and clobbered by SMSTART/MMSTOP, such that the
register allocator can consider these as actual dependences and avoid
rematerialization. For now we just want to address the immediate problem.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D159193

Files:
  llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
  llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
  llvm/lib/Target/AArch64/AArch64InstrInfo.h
  llvm/lib/Target/AArch64/AArch64MachineFunctionInfo.h
  llvm/test/CodeGen/AArch64/sme-disable-rematerialize-with-streaming-mode-changes.ll

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