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Re: How to define a built-in 24-bit type?


On Fri, 20 Jan 2012, Georg-Johann Lay wrote:

> Hi.
> 
> avr-gcc implements a 24-bit scalar integer types __int24 and __uint24 in
> avr.c:TARGET_INIT_BUILTINS like so:
> 
>   tree int24_type  = make_signed_type (GET_MODE_BITSIZE (PSImode));
>   tree uint24_type = make_unsigned_type (GET_MODE_BITSIZE (PSImode));
> 
>   (*lang_hooks.types.register_builtin_type) (int24_type, "__int24");
>   (*lang_hooks.types.register_builtin_type) (uint24_type, "__uint24");
> 
> PSImode is defined in avr-modes.c:
> 
> FRACTIONAL_INT_MODE (PSI, 24, 3);
> 
> Is this the right definition of a built-in type?

FRACTIONAL_INT_MODE should work after Bernd's patch series from last July 
relating to 40-bit types, though it's certainly possible there are issues 
that appear with 24-bit types but not 40-bit types.

> The question is because __int24 shreds the compiler, see PR51527
> 
> So the question is if there is something missing or broken in the definition
> above or if it's actually a flaw in the front-end.
> 
> For the __int128 there is much more code sprinkled over the compiler sources,
> so maybe it's not that easy to introduce a new, built-in type completely in the
> back-end?

See my discussions with Bernd from last July.  In essence, I don't think 
we should spread such code across the compiler for each non-power-of-2 
size; ultimately we should go the other way, stop having any TImode or 
__int128 references in files outside config/, libgcc/config/ etc. and have 
targets using those modes and types choose to use the relevant source 
files for them.  (And, further along, the existence of HImode, SImode, 
DImode ought to be target-dependent as well, with files in config/ that 
are used by targets with 8-bit bytes but maybe not by any targets with 
wider bytes.)

-- 
Joseph S. Myers
joseph@codesourcery.com


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