Re: [PATCH] c, c++: Add new value for vector types for __builtin_classify_type (type)

[Joseph Myers <joseph@codesourcery.com>]

On Thu, 16 Nov 2023, Jason Merrill wrote:

> On 11/11/23 03:22, Jakub Jelinek wrote:
> > Hi!
> > 
> > While filing a clang request to return 18 on _BitInts for
> > __builtin_classify_type instead of -1 they return currently, I've
> > noticed that we return -1 for vector types.  I'm not convinced it is a good
> > idea to change behavior of __builtin_classify_type (vector_expression)
> > after 22 years it behaved one way (returned -1), but the
> > __builtin_classify_type (type) form is a new extension added for GCC 14,
> > so this patch returns 19 for vectors just in that second form.  Many other
> > return values are only accessible from the second form as well (mostly
> > because
> > of argument promotions), so I think it is fine like that.
> > 
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> The C++ changes are OK (and obvious).  I'm skeptical of the choice to keep
> returning -1 for the expression form, it seems more likely to cause problems
> (due to it disagreeing with the type form) than changing it (due to old code
> somehow relying on -1?).  But people who are more familiar with the use of
> __builtin_classify_type should make the call.

I'm also doubtful of keeping returning -1 for vectors in expression form 
(I'd be surprised if people are actually using __builtin_classify_type 
with vectors).  The C changes are OK (but the front-end changes wouldn't 
be needed at all if the vector and type argument cases aren't 
distinguished).

-- 
Joseph S. Myers
joseph@codesourcery.com