[PATCH v4][C][ADA] use function descriptors instead of trampolines in C

[Uecker, Martin]

Am Donnerstag, den 13.12.2018, 16:35 -0700 schrieb Jeff Law:
> On 12/12/18 11:12 AM, Uecker, Martin wrote:

...
> > > > diff --git a/gcc/c/c-objc-common.h b/gcc/c/c-objc-common.h
> > > > index 78e768c2366..ef039560eb9 100644
> > > > --- a/gcc/c/c-objc-common.h
> > > > +++ b/gcc/c/c-objc-common.h
> > > > @@ -110,4 +110,7 @@ along with GCC; see the file COPYING3.  If
> > > > not see
> > > >  
> > > >  #undef LANG_HOOKS_TREE_INLINING_VAR_MOD_TYPE_P
> > > >  #define LANG_HOOKS_TREE_INLINING_VAR_MOD_TYPE_P c_vla_unspec_p
> > > > +
> > > > +#undef LANG_HOOKS_CUSTOM_FUNCTION_DESCRIPTORS
> > > > +#define LANG_HOOKS_CUSTOM_FUNCTION_DESCRIPTORS true
> > > >  #endif /* GCC_C_OBJC_COMMON */
> > > 
> > > I wonder if we even need the lang hook anymore.  ISTM that a
> > > front-end
> > > that wants to use the function descriptors can just set
> > > FUNC_ADDR_BY_DESCRIPTOR and we'd use the function descriptor,
> > > else we'll
> > > use the trampoline.  Thoughts?
> > 
> > The lang hook also affects the minimum alignment for function
> > pointers via the FUNCTION_ALIGNMENT macro (gcc/default.h). This
> > does
> > not appear to change the default alignment on any architecture, but
> > it causes a failure in i386/gcc.target/i386/attr-aligned.c when
> > requesting a smaller alignment which is then silently ignored.
> 
> Ugh.  I didn't see that.

The test is new (2019-11-29 Martin Sebor), but one could
argue that we could simply remove this specific test as 'aligned'
is only required to increase alignment. Martin?

> > I am not sure what the best approach is, but my preference
> > would be to remove the lang hook and the FUNCTION_ALIGNMENT
> > logic which will also fix the test case (the requested
> > alignment will be applied).
> > 
> > I would then instead add a warning (or error?) which triggers
> > only with -fno-trampolines if the user requests an alignment
> > which is too small for this mechanism to work.
>
> > Does this sound reasonable?
> 
> So I'm thinking we should wrap the existing patch as-is for the trunk
> (we're well into stage3 after all).  So leave the hook as-is for gcc-
> 9.
> 
> We can then tackle removal of the hook, including twiddling
> FUNCTION_ALIGNMENT for gcc-10.
> 
> Does that sound reasonable to you?

This is fine with me. So just confirm: I should install the 
patch despite the regression?

Best,
Martin