This is a v2 which addresses feedback from v1, posted here:
https://gcc.gnu.org/pipermail/gcc-patches/2024-January/642313.html

Bootstrapped/regtested on aarch64-linux-gnu, OK for trunk?

Thanks,
Alex

-- >8 --

In r14-6604-gd7ee988c491cde43d04fe25f2b3dbad9d85ded45 we changed the CFI notes
attached to callee saves (in aarch64_save_callee_saves).  That patch changed
the ldp/stp representation to use unspecs instead of PARALLEL moves.  This meant
that we needed to attach CFI notes to all frame-related pair saves such that
dwarf2cfi could still emit the appropriate CFI (it cannot interpret the unspecs
directly).  The patch also attached REG_CFA_OFFSET notes to individual saves so
that the ldp/stp pass could easily preserve them when forming stps.

In that change I chose to use REG_CFA_OFFSET, but as the PR shows, that
choice was problematic in that REG_CFA_OFFSET requires the attached
store to be expressed in terms of the current CFA register at all times.
This means that even scheduling of frame-related insns can break this
invariant, leading to ICEs in dwarf2cfi.

The old behaviour (before that change) allowed dwarf2cfi to interpret the RTL
directly for sp-relative saves.  This change restores that behaviour by using
REG_FRAME_RELATED_EXPR instead of REG_CFA_OFFSET.  REG_FRAME_RELATED_EXPR
effectively just gives a different pattern for dwarf2cfi to look at instead of
the main insn pattern.  That allows us to attach the old-style PARALLEL move
representation in a REG_FRAME_RELATED_EXPR note and means we are free to always
express the save addresses in terms of the stack pointer.

Since the ldp/stp fusion pass can combine frame-related stores, this patch also
updates it to preserve REG_FRAME_RELATED_EXPR notes, and additionally gives it
the ability to synthesize those notes when combining sp-relative saves into an
stp (the latter always needs a note due to the unspec representation, the former
does not).

gcc/ChangeLog:

        PR target/113077
        * config/aarch64/aarch64-ldp-fusion.cc (filter_notes): Add fr_expr param to
        extract REG_FRAME_RELATED_EXPR notes.
        (combine_reg_notes): Handle REG_FRAME_RELATED_EXPR notes, and
        synthesize these if needed.  Update caller ...
        (ldp_bb_info::fuse_pair): ... here.
        * config/aarch64/aarch64.cc (aarch64_save_callee_saves): Use
        REG_FRAME_RELATED_EXPR instead of REG_CFA_OFFSET.

gcc/testsuite/ChangeLog:

        PR target/113077
        * gcc.target/aarch64/pr113077.c: New test.