Re: PATCH 3.3 0203 i386/{i386.c,{i386elf,ptx4-i,sysv4}.h} forUnixWare build failure.

[Zack Weinberg <zack at codesourcery dot com>]

Jan Hubicka <jh@suse.cz> writes:
>
> It is not clear to my why I am "moving to using
> DEFAULT_PCC_STRUCT_RETURN over RETURN_IN_MEMORY".  I just have to
> disable flag_pcc_struct_return for x86-64 that is enabled by default for
> Linux. That was the only reason for that change.
> I didn't intended to change anything else to be safe.
>
> Currently flag_pcc_struct_return controls the PS-ABI compiliant behaviour
> where each structure is returned in memory (used by Linux for instance), 
> and the "gcc" way of returning small structs in registers (used by BSD).
>
> I agree that it would be cleaner to have this decided in
> x86_return_in_memory, but I wanted to first ensure whether you are
> discussing some bug I introduced by the change, or future cleanup.

Okay, I see where the confusion is now.  I had three different things
tangled up in my original post.

You did sort of introduce a bug, in that you induced new requirements
on ix86 subtargets.  They now should not define RETURN_IN_MEMORY, and
need to provide some definition of DEFAULT_PCC_STRUCT_RETURN, or
i386.c will not compile.  Rodney's patch was addressing that problem.

Independent of that, I observe that the 32-bit case of ix86_return_in_memory
reads

      if (TYPE_MODE (type) == BLKmode
	  || (VECTOR_MODE_P (TYPE_MODE (type))
	      && int_size_in_bytes (type) == 8)
	  || (int_size_in_bytes (type) > 12 && TYPE_MODE (type) != TImode
	      && TYPE_MODE (type) != TFmode
	      && !VECTOR_MODE_P (TYPE_MODE (type))))
	return 1;
      return 0;

Now it is my understanding that "TYPE_MODE (type) == BLKmode" and
"AGGREGATE_TYPE_P (type)" wind up being equivalent tests even though
they base their test on different data.  I could be wrong about that.
If I am correct, then the setting of DEFAULT_PCC_STRUCT_RETURN makes
no difference on any 32-bit ix86 target, which means that we could
make life easier for subtarget maintainers by putting an unconditional

#define DEFAULT_PCC_STRUCT_RETURN 0

in i386.h, removing all subtarget definitions of
DEFAULT_PCC_STRUCT_RETURN and RETURN_IN_MEMORY, and removing the code
in i386.c that diddles flag_pcc_struct_return.

Independent of *that*, I want to see -fpcc-struct-return go away and
all targets use RETURN_IN_MEMORY exclusively to specify this part of
their ABI.

zw