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Summary

  1. Revert 651f07908a1 "[AArch64] Don't combine callee-save and local stack (details)
Commit c2443155a0fb245c8f17f2c1c72b6ea391e86e81 by hans
Revert 651f07908a1 "[AArch64] Don't combine callee-save and local stack
adjustment when optimizing for size"
This caused asserts (and perhaps also miscompiles) while building for
Windows on AArch64. See the discussion on D68530 for details and
reproducer.
Reverting until this can be investigated and fixed.
> For arm64, D18619 introduced the ability to combine bumping the stack
pointer
> upfront in case it needs to be bumped for both the callee-save area as
well as
> the local stack area.
>
> That diff already remarks that "This change can cause an increase in
> instructions", but argues that even when that happens, it should be
still be a
> performance benefit because the number of micro-ops is reduced.
>
> We have observed that this code-size increase can be significant in
practice.
> This diff disables combining stack bumping for methods that are marked
as
> optimize-for-size.
>
> Example of a prologue with the behavior before this diff (combining
stack bumping when possible):
>   sub        sp, sp, #0x40
>   stp        d9, d8, [sp, #0x10]
>   stp        x20, x19, [sp, #0x20]
>   stp        x29, x30, [sp, #0x30]
>   add        x29, sp, #0x30
>   [... compute x8 somehow ...]
>   stp        x0, x8, [sp]
>
> And after this  diff, if the method is marked as optimize-for-size:
>   stp        d9, d8, [sp, #-0x30]!
>   stp        x20, x19, [sp, #0x10]
>   stp        x29, x30, [sp, #0x20]
>   add        x29, sp, #0x20
>   [... compute x8 somehow ...]
>   stp        x0, x8, [sp, #-0x10]!
>
> Note that without combining the stack bump there are two
auto-decrements,
> nicely folded into the stp instructions, whereas otherwise there is a
single
> sub sp, ... instruction, but not folded.
>
> Patch by Nikolai Tillmann!
>
> Differential Revision: https://reviews.llvm.org/D68530
The file was removedllvm/test/CodeGen/AArch64/arm64-never-combine-csr-local-stack-bump-for-size.ll
The file was modifiedllvm/lib/Target/AArch64/AArch64FrameLowering.cpp