[PATCH, LTO] add externally_visible attribute when necessary with -fwhole-program and resolution file.

[Bingfeng Mei]

> -----Original Message-----
> From: Richard Guenther [mailto:richard.guenther@gmail.com]
> Sent: 09 June 2010 16:02
> To: Bingfeng Mei
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH, LTO] add externally_visible attribute when
> necessary with -fwhole-program and resolution file.
> 
> On Wed, Jun 9, 2010 at 4:46 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
> > Hi,
> > This patch addresses issue discussed in
> > http://gcc.gnu.org/ml/gcc/2010-05/msg00560.html
> > http://gcc.gnu.org/ml/gcc/2010-06/msg00317.html
> >
> > With the patch, any declaration which is resolved as
> LDPR_PREVAILING_DEF
> > and compiled with -fwhole-program is annotated with
> > attribute "externally_visible" if it doesn't exist already.
> > This eliminates the error-prone process of manual annotation
> > of the attribute when compiling mixed LTO/non-LTO applications.
> >
> > For the following test files:
> > a.c
> >
> > #include <string.h>
> > #include <stdio.h>
> > extern int foo(int);
> > /* Called by b.c, should not be optimized by -fwhole-program */
> > void bar()
> > {
> >  printf("bar\n");
> > }
> > /* Not used by others, should be optimized out by -fwhole-program*/
> > void bar2()
> > {
> >  printf("bar2\n");
> > }
> > extern int src[], dst[];
> > int vvvvvv;
> > int main()
> > {
> >  int ret;
> >  vvvvvv = 12;
> >  ret = foo(20);
> >  bar2();
> >  memcpy(dst, src, 100);
> >  return ret + 3;
> > }
> >
> > b.c
> >
> > #include <stdio.h>
> > int src[100];
> > int dst[100];
> > extern int vvvvvv;
> > extern void bar();
> > int foo(int c)
> > {
> >  printf("Hello world: %d\n", c);
> >  bar();
> >  return 1000 + vvvvvv;
> > }
> >
> > ~/work/install-x86/bin/gcc  a.c -O2 -c  -flto
> > ~/work/install-x86/bin/gcc  b.c -O2 -c
> > ar cru libb.a b.o
> > ~/work/install-x86/bin/gcc -flto a.o -L. -lb -O2 -fuse-linker-plugin
> -o f -fwhole-program
> >
> > The code is compiled and linked correctly. bar & vvvvvv don't become
> static function
> > and cause link errors, whereas bar2 is inlined as expected.
> >
> >
> > Bootstrapped and tested on x86_64-unknown-linux-gnu.
> >
> > Cheers,
> > Bingfeng Mei
> >
> > 2010-06-09  Bingfeng Mei <bmei@broadcom.com>
> >
> >        * lto-symbtab.c (lto_symtab_resolve_symbols): Add
> externally_visible
> >        attribute for declaration of LDPR_PREVAILING_DEF when
> compiling with
> >        -fwhole-program
> >
> >
> > Index: lto-symtab.c
> > ===================================================================
> > --- lto-symtab.c        (revision 160463)
> > +++ lto-symtab.c        (working copy)
> > @@ -476,7 +476,19 @@
> >
> >   /* If the chain is already resolved there is nothing else to
> do.  */
> >   if (e->resolution != LDPR_UNKNOWN)
> > -    return;
> > +    {
> > +      /* Add externally_visible attribute for declaration of
> LDPR_PREVAILING_DEF */
> > +      if (e->resolution == LDPR_PREVAILING_DEF && flag_whole_program)
> > +        {
> > +          if (!lookup_attribute ("externally_visible",
> DECL_ATTRIBUTES (e->decl)))
> > +            {
> > +              DECL_ATTRIBUTES (e->decl)
> > +                = tree_cons (get_identifier ("externally_visible"),
> NULL_TREE,
> > +                             DECL_ATTRIBUTES (e->decl));
> 
> I don't think we need to add an attribute here but we can play
> with some cgraph flags which is cheaper.

Then I also need to change relevant flag for variable declaration. Not sure what 
I should change. 

> 
> Also I think this isn't really correct - not everything that prevails
> needs to be externally visible (in fact, you seem to simply
> remove the effect of -fwhole-program completely).

According to doc, LDPR_PREVAILING_DEF is the definition accessed outside
of LTO IR as opposed to LDPR_PREVAILING_DEF_IRONLY. So I think it should
be externally visible. In my test file, bar2 is the similar as eliminate_me
in your testcase. It is correctly eliminated with -fwhole-program. 

> 
> A testcase that should still work is
> 
> t1.c:
> void foo(void) { bar (); }
> t2.c
> extern void foo (void);
> void bar (void) {}
> void eliminate_me (void) {}
> int main()
> {
>    foo();
> }
> 
> and eliminate_me should still be eliminated with -fwhole-program
> if you do
> 
> gcc -c t1.c
> gcc -O2 t2.c t1.o -flto -fwhole-program -fuse-linker-plugin
> 
> Thus, the resolution file probably does not have the information
> you need (a list of references from outside of the LTO file set).
> 
> Richard.