[PATCH] Improve alloca alignment

[Jeff Law]

On 10/17/2017 06:04 AM, Wilco Dijkstra wrote:
> Wilco Dijkstra wrote:
>>
>> Yes STACK_BOUNDARY applies to virtual_stack_dynamic_rtx and all other
>> virtual frame registers. It appears it's main purpose is to enable alignment
>> optimizations since PREFERRED_STACK_BOUNDARY is used to align
>> local and outgoing argument area etc. So if you don't want the alignment
>> optimizations it is feasible to set STACK_BOUNDARY to a lower value
>> without changing the stack layout.
>>
>> There is also STACK_DYNAMIC_OFFSET which computes the total offset
>> from the stack. It's not obvious whether the default version should align (since
>> outgoing arguments are already aligned there is no easy way to record the
>> extra padding), but we could assert if the offset isn't aligned.
> 
> Also there is something odd in the sparc backend:
> 
> /* Given the stack bias, the stack pointer isn't actually aligned.  */
> #define INIT_EXPANDERS                                                   \
>   do {                                                                   \
>     if (crtl->emit.regno_pointer_align && SPARC_STACK_BIAS)      \
>       {                                                                  \
>         REGNO_POINTER_ALIGN (STACK_POINTER_REGNUM) = BITS_PER_UNIT;      \
>         REGNO_POINTER_ALIGN (HARD_FRAME_POINTER_REGNUM) = BITS_PER_UNIT; \
>       }                                                                  \
>   } while (0)
> 
> That lowers the alignment for the stack and frame pointer. So assuming that works
> and blocks alignment optimizations, why isn't this done for the dynamic offset as well?
No clue, but ISTM that it should.  Eric, can you try that and see if it
addresses these problems?  I'd really like to get this wrapped up, but I
don't have access to any sparc systems to test it myself.

Jeff