Re: gcc corrections for better pie support

[Richard Sandiford <rsandifo at redhat dot com>]

"Peter S. Mazinger" <ps.m@gmx.net> writes:
> On Tue, 9 Nov 2004, Richard Sandiford wrote:
>> flag_shlib || flag_pie should make sure that the macros are only
>> defined if -fpic/-fPIC/-fpie/-fPIE are passed.
>
> that would change the current behaviour, where it was defined all the 
> time and that would mean some trouble with apps (all that checked 
> for __PIC__ and -fPIC was not used in CFLAGS)

Yeah, I know, but...

This is just such a rat's nest.  The problem is that -fno-pic doesn't
achieve anything on its own.  It gets unconditionally overridden by
the MIPS options handling code unless you also specify -mno-abicalls.
And if you specify -mno-abicalls, then -fno-pic is redundant, because
the MIPS port no longer has any form of non-abicalls PIC (embedded PIC
having been removed, and never having being an issue for GNU/Linux anyway).

So the current handling of __PIC__ is inconsistent.  What does it mean
for mips*-linux-gnu targets?  Does it mean:

   (1) That position independent code has been explicitly selected?
       If so, then flag_pie || flag_shlib seems like the right condition.

   (2) That abicalls code is selected?  If so, then the current specs are
       wrong, because "gcc -fno-pic" generates abicalls code.

But I suppose the current definition is closest to (2).  If (2) is indeed
what we want, then another alternative would be to guard the __PIC__
definition in linux.h with TARGET_ABICALLS.

>> >> > @@ -123,7 +121,7 @@
>> >> >  #define SUBTARGET_ASM_SPEC "\
>> >> >  %{mabi=64: -64} \
>> >> >  %{!fno-PIC:%{!fno-pic:-KPIC}} \
>> >> > -%{fno-PIC:-non_shared} %{fno-pic:-non_shared}"
>> >> > +%{fno-PIC|fno-pic:-non_shared}"
>> >> 
>> >> If this is causing trouble, you should be able to get rid of all the
>> >> PIC stuff here too.  GAS assumes -non_shared by default and GCC uses
>> >> .abicalls if it wants -KPIC.
>> 
>> Sorry, just ignore this comment.  I was thinking only about gcc's use
>> of the assembler.
>> 
>> We need to pass -KPIC by default when using the gcc driver to assemble
>> user .S and .s files, so we do need to keep the existing options.
>
> so -non_shared could go away and I think the other one would be more 
> correct as
> %{!fno-PIC:%{!fno-pic:%{!fno-PIE:%{!fno-pie:-K PIC}}}}

No space between -K and PIC.  But what effect do you want -fno-pie to have?
The change you suggest means that:

   gcc -S foo.S
   gcc -S -fno-pie bar.S
   gcc foo.o bar.o

might fail.  foo.o will use abicalls and, unless it has an explicit
.abicalls directive, bar.o won't.  Is that what you want?

Again, I find the use of -fno-pic a bit suspicious here.
It seems strange that "mips-linux-gnu-gcc -fno-pic" generates
abicalls code when compiling C code but doesn't when compiling asm.
I'd be happy enough with:

   %{!mno-abicalls:-KPIC}

but let's see what others say...

Richard