RFC: Patch to allow spill slot alignment greater than the stack alignment

[H.J. Lu]

On Tue, Oct 6, 2015 at 11:02 AM, Steve Ellcey <sellcey@imgtec.com> wrote:
> On Tue, 2015-10-06 at 17:39 +0200, Bernd Schmidt wrote:
>
>>
>> Did your tag that copy as RTX_FRAME_RELATED? I'd expect dwarf2cfi would
>> ignore instructions with that bit unset. There's even a comment
>> indicating arm does something like this:
>
> Yes, I had marked it as RTX_FRAME_RELATED.  When I took that out things
> looked better (well, maybe just different).
>
> If I remove that and I change Rule #16 to handle an AND with a register
> I get odd looking .cfi stuff.  The AND instruction (which is marked with
> RTX_FRAME_RELATED) seems to generate these cfi_escape macros:
>
>         .cfi_escape 0x10,0x1f,0x2,0x8e,0x7c
>         .cfi_escape 0x10,0x1e,0x2,0x8e,0x78
>         .cfi_escape 0x10,0xc,0x2,0x8e,0x74
>
> which are meaningless to me.  What I found works better is to skip the
> dwarf2cfi.c changes and explicitly attach this note to the AND
> instruction:
>
>       cfi_note = alloc_reg_note (REG_CFA_DEF_CFA, stack_pointer_rtx, NULL_RTX);
>       REG_NOTES (insn) = cfi_note;
>
> When I do this the only unexpected test suite execution failures I see
> are ones that call the C++ set_unexpected function.
>
> FYI: The reason I was copying the stack pointer to another register was
> to use that other register as my argument pointer (needed after the
> stack pointer got aligned) and to use it to restore the stack pointer at
> the end of the function for normal returns.

Does it pass all tests under g++.dg/torture/stackalign?  You need
to implement -mstackrealign and -mpreferred-stack-boundary=
as well as update check_effective_target_automatic_stack_alignment
to run all stack alignment tests.


-- 
H.J.