efriedma added inline comments.
================
Comment at: llvm/lib/Target/AArch64/SVEInstrFormats.td:5333
+ // We need a layer of indirection because early machine code passes balk at
+ // physical register (i.e. FFR) uses that have no previous definition.
+ let hasSideEffects = 1, hasNoSchedulingInfo = 1, mayLoad = 1 in {
----------------
kmclaughlin wrote:
> efriedma wrote:
> > This is depending on hasSideEffects to preserve the correct ordering with
> > instructions that read/write FFR? That probably works. I guess the
> > alternative is to insert an IMPLICIT_DEF of FFR in the entry block of each
> > function.
> >
> > What are the calling convention rules for FFR? Is it callee-save? If not,
> > we might need to do some work to make FFR reads/writes do something sane
> > across calls inserted by the compiler.
> The FFR is not callee-saved. We will need to add support to save & restore it
> where appropriate at the point the compiler starts generating reads to the
> FFR, but for the purpose of the ACLE the user will be required to do this if
> necessary.
How can the user write correct code to save/restore the FFR? The compiler can
move arbitrary readnone/argmemonly calls between the definition and the use.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D71698/new/
https://reviews.llvm.org/D71698
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