$target.h vs $target-protos.h

[Joseph Myers]

On Sun, 18 Feb 2018, Sandra Loosemore wrote:

> So what is the purpose of having a separate $target-protos.h?

It looks like the original explanation was 
<https://gcc.gnu.org/ml/gcc-patches/1999-09n/msg00866.html>, "Because 
tm_p.h needs to be included after tree.h and rtl.h.".

It's certainly still true that rtl.h requires tm.h to have been included 
first.  It's also true that tm_p.h depends on types beyond those for which 
simple forward declarations suffice - for example, the machine_mode enum 
(and now the various classes related to machine modes).  So maybe in 1999 
there would have been circular dependencies on types from tree.h and 
rtl.h.

Now more things are included in coretypes.h (including e.g. the 
machine_mode enum via insn-modes.h).  I don't know if circular dependency 
issues would still arise; any global move of prototypes into tm.h would 
require building cc1 for all architectures to locate any such problems.  
(Presumably the prototypes would also all need to be disabled for 
USED_FOR_TARGET.)

An alternative to adding more tm_p.h includes or moving prototypes into 
tm.h would be, if there are only a few target macros involved, converting 
those target macros into hooks.

-- 
Joseph S. Myers
joseph@codesourcery.com