[PATCH, PR target/69454] Disable TARGET_STV when stack is not properly aligned

[Uros Bizjak]

On Tue, Feb 2, 2016 at 12:53 PM, Jakub Jelinek <jakub@redhat.com> wrote:

>> The bottom line is  ix86_minimum_alignment must return the correct
>> number for DImode or you can just turn off STV.   My suggestion is
>> to use my patch.
>
> Uros, any preferences here?  I mean, it is possible to use
> e.g. the ix86_option_override_internal and have H.J's ix86_minimum_alignment
> change as a safety net, in the usual case for -mpreferred-stack-boundary=2
> we'll just disable TARGET_STV and ix86_minimum_alignment change won't do
> anything, as TARGET_STV will be false, and if for whatever case it gets
> through (target attribute, -mincoming-stack-boundary=, ...)
> ix86_minimum_alignment will be there to ensure enough stack alignment.
> Most of the smaller -mpreferred-stack-boundary= uses are -mno-sse anyway,
> and that is something we don't want to affect.

IMO, we should disable STV when -mpreferred-stack-boundary < 3, as STV
is only an optimization. Perhaps we can also emit a "sorry" for
explicit -mstv in case stack boundary requirement is not satisfied.
*If* there is a need for -mstv with smaller stack boundary, we can
revisit this decision for later gcc versions.

I think disabling STV is less surprising option than increasing stack
boundary behind the user's back.

Uros.