Re: [PATCH 2/5] aarch64: Don't use FEAT_MAX as array length

[Andrew Carlotti <andrew.carlotti@arm.com>]

On Tue, Apr 09, 2024 at 04:33:10PM +0100, Richard Sandiford wrote:
> Andrew Carlotti <andrew.carlotti@arm.com> writes:
> > There was an assumption in some places that the aarch64_fmv_feature_data
> > array contained FEAT_MAX elements.  While this assumption held up till
> > now, it is safer and more flexible to use the array size directly.
> >
> > gcc/ChangeLog:
> >
> > 	* config/aarch64/aarch64.cc (compare_feature_masks):
> > 	Use ARRAY_SIZE to determine iteration bounds.
> > 	(aarch64_mangle_decl_assembler_name): Ditto.
> >
> >
> > diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> > index 1ea84c8bd7386e399f6ffa3a5e36408cf8831fc6..5de842fcc212c78beba1fa99639e79562d718579 100644
> > --- a/gcc/config/aarch64/aarch64.cc
> > +++ b/gcc/config/aarch64/aarch64.cc
> > @@ -19899,7 +19899,8 @@ compare_feature_masks (aarch64_fmv_feature_mask mask1,
> >    auto diff_mask = mask1 ^ mask2;
> >    if (diff_mask == 0ULL)
> >      return 0;
> > -  for (int i = FEAT_MAX - 1; i > 0; i--)
> > +  static const int num_features = ARRAY_SIZE (aarch64_fmv_feature_data);
> 
> There doesn't seem any need for this to be static (or const).  Same for
> the second hunk.

Agreed - I'll fix that, and the other instance I added in a previous patch.

I originally copied this pattern from my driver-aarch64.c:252, which was added
by Kyrill back in 2015.

> > +  for (int i = num_features - 1; i > 0; i--)
> 
> Pre-existing, but is > 0 rather than >= 0 deliberate?  Shouldn't we look
> at index 0 as well?

That was probably left over from when "default" was handled as part of the
list.  I think a different instance of this mistake was mentioned in a previous
review.  I'll fix this mistake and add a test.

> LGTM otherwise.
> 
> Thanks,
> Richard
> 
> >      {
> >        auto bit_mask = aarch64_fmv_feature_data[i].feature_mask;
> >        if (diff_mask & bit_mask)
> > @@ -19982,7 +19983,8 @@ aarch64_mangle_decl_assembler_name (tree decl, tree id)
> >  
> >        name += "._";
> >  
> > -      for (int i = 0; i < FEAT_MAX; i++)
> > +      static const int num_features = ARRAY_SIZE (aarch64_fmv_feature_data);
> > +      for (int i = 0; i < num_features; i++)
> >  	{
> >  	  if (feature_mask & aarch64_fmv_feature_data[i].feature_mask)
> >  	    {