Options of fixing biggest alignment in PR target/38736

[Ye, Joey]

From: Ian Lance Taylor [mailto:iant@google.com]:
> Therefore, I propose that we do the following:
> 
> 1) Introduce __attribute__ ((aligned (scalar))).  This will be
>    documented as having a fixed value for each ABI.  The value will be
>    guaranteed to be sufficient to hold any ordinary non-vector type.
>    The default will be BIGGEST_ALIGNMENT.  The value for the
>    x86/x86_64 will be 128.
> 
> 2) Introduce __attribute__ ((aligned (max))).  This will be documented
>    as having the largest value available for any version of the
>    architecture, and thus in particular it may change if new versions
>    of the architecture are released.  The value will not change based
>    on command line options which do not change the ABI; that is, if it
>    is possible to link together two files compiled with different set
>    of command line options and expect the result to work, then those
>    command line options must not change the value of this attribute.
>    The value will be guaranteed to be sufficient to hold any type,
>    including any vector type.  The default will be BIGGEST_ALIGNMENT.
>    The value for the x86/x86_64 will (presumably) be 256.
To me "new version of x86 architecture are released" usually means 
"change based on command line option". How about the default value
grow to 512 or even higher in future?

Thanks - Joey