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

[Richard Guenther <richard dot guenther at gmail dot com>]

On Mon, Jun 14, 2010 at 10:39 AM, Bingfeng Mei <bmei@broadcom.com> wrote:
> Hi, Richard,
> Here is the updated patch. The flags are set instead of attributes now.
> The check is moved to the end of lto_symtab_merge_decls_1. For the DECL_COMM,
> since internal resolver is always used due to your workaround for gold plugin,
> These variables would still need explicit externally_visible attributes.

Can you amend the docs that talk about -flto + -fwhole-program
accordingly?

I'd like Honza to go over the cgraph related changes, otherwise
the patch looks good with ...

> Bootstrapped and tested.
>
> Cheers,
> Bingfeng.
>
> 2010-06-11 ?Bingfeng Mei <bmei@broadcom.com>
>
> ? ? ? ?* lto-symbtab.c (lto_symtab_merge_decls_1): Set externally_visible
> ? ? ? ?flags for symbols of LDPR_PREVAILING_DEF when compiling with
> ? ? ? ?-fwhole-program
> ? ? ? ?(lto_symtab_resolve_symbols) Use LDPR_PREVAILING_DEF_IRONLY for
> ? ? ? ?internal resolver
> ? ? ? ?* ipa.c (function_and_variable_visibility): Set externally_visible
> ? ? ? ?flags only if they are false. This allows flags to be passed from
>
>
> Index: lto-symtab.c
> ===================================================================
> --- lto-symtab.c ? ? ? ?(revision 160529)
> +++ lto-symtab.c ? ? ? ?(working copy)
> @@ -530,11 +530,21 @@
> ? ? return;
>
> ?found:
> - ?if (TREE_CODE (prevailing->decl) == VAR_DECL
> - ? ? ?&& TREE_READONLY (prevailing->decl))
> + ?/* If current lto files represent the whole program,
> + ? ?it is correct to use LDPR_PREVALING_DEF_IRONLY.
> + ? ?If current lto files are part of whole program, internal
> + ? ?resolver doesn't know if it is LDPR_PREVAILING_DEF
> + ? ?or LDPR_PREVAILING_DEF_IRONLY. Use IRONLY conforms to
> + ? ?using -fwhole-program. Otherwise, it doesn't
> + ? ?matter using either LDPR_PREVAILING_DEF or
> + ? ?LDPR_PREVAILING_DEF_IRONLY
> +
> + ? ?FIXME: above workaround due to gold plugin makes some
> + ? ?variables IRONLY, which are indeed PREVAILING_DEF in
> + ? ?resolution file. These variables still need manual
> + ? ?externally_visible attribute
> + ? ?*/

Full-stop at the end of the sentence, */ not on the next line.  Also
two spaces after each '.'

> ? ? prevailing->resolution = LDPR_PREVAILING_DEF_IRONLY;
> - ?else
> - ? ?prevailing->resolution = LDPR_PREVAILING_DEF;
> ?}
>
> ?/* Merge all decls in the symbol table chain to the prevailing decl and
> @@ -698,6 +708,25 @@
> ? ? ? && TREE_CODE (prevailing->decl) != VAR_DECL)
> ? ? prevailing->next = NULL;
>
> +

No extra vertical space please.

> + ?/* Add externally_visible attribute for declaration of LDPR_PREVAILING_DEF */

Adjust the comment, to sth like "In whole-program mode mark
LDPR_PREVAILING_DEFs as externally visible. "

> + ?if (flag_whole_program)
> + ? ?{
> + ? ? ?if (prevailing->resolution == LDPR_PREVAILING_DEF)
> + ? ? ? ?{
> + ? ? ? ? ?if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
> + ? ? ? ? ? ?prevailing->node->local.externally_visible = true;
> + ? ? ? ? ?else
> + ? ? ? ? ? ?prevailing->vnode->externally_visible = true;
> + ? ? ? ?}
> + ? ? ?else if (prevailing->resolution == LDPR_PREVAILING_DEF_IRONLY)
> + ? ? ? ?{
> + ? ? ? ? ?if (TREE_CODE (prevailing->decl) == FUNCTION_DECL)
> + ? ? ? ? ? ?prevailing->node->local.externally_visible = false;
> + ? ? ? ? ?else
> + ? ? ? ? ? ?prevailing->vnode->externally_visible = false;
> + ? ? ? ?}
> + ? ?}

Honza will tell you if the above is correct, I am not 100% sure.

Did you verify we generate the same code with and without your
patch when all symbols are resolved inside the IL?

Thanks,
Richard.

> ? return 1;
> ?}
>
> Index: ipa.c
> ===================================================================
> --- ipa.c ? ? ? (revision 160529)
> +++ ipa.c ? ? ? (working copy)
> @@ -665,13 +665,12 @@
> ? ? ? ?}
> ? ? ? gcc_assert ((!DECL_WEAK (node->decl) && !DECL_COMDAT (node->decl))
> ? ? ? ? ? ? ? ? ?|| TREE_PUBLIC (node->decl) || DECL_EXTERNAL (node->decl));
> - ? ? ?if (cgraph_externally_visible_p (node, whole_program))
> + ? ? ?if (!node->local.externally_visible
> + ? ? ? ? ?&& cgraph_externally_visible_p (node, whole_program))
> ? ? ? ? {
> ? ? ? ? ?gcc_assert (!node->global.inlined_to);
> ? ? ? ? ?node->local.externally_visible = true;
> ? ? ? ?}
> - ? ? ?else
> - ? ? ? node->local.externally_visible = false;
> ? ? ? if (!node->local.externally_visible && node->analyzed
> ? ? ? ? ?&& !DECL_EXTERNAL (node->decl))
> ? ? ? ?{
> @@ -721,7 +720,8 @@
> ? ? {
> ? ? ? if (!vnode->finalized)
> ? ? ? ? continue;
> - ? ? ?if (vnode->needed
> + ? ? ?if (!vnode->externally_visible
> + ? ? ? ? ?&& vnode->needed
> ? ? ? ? ?&& (DECL_COMDAT (vnode->decl) || TREE_PUBLIC (vnode->decl))
> ? ? ? ? ?&& (!whole_program
> ? ? ? ? ? ? ?/* We can privatize comdat readonly variables whose address is not taken,
> @@ -732,8 +732,6 @@
> ? ? ? ? ? ? ?|| lookup_attribute ("externally_visible",
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? DECL_ATTRIBUTES (vnode->decl))))
> ? ? ? ?vnode->externally_visible = true;
> - ? ? ?else
> - ? ? ? ?vnode->externally_visible = false;
> ? ? ? if (!vnode->externally_visible)
> ? ? ? ?{
> ? ? ? ? ?gcc_assert (whole_program || !TREE_PUBLIC (vnode->decl));
>
>> -----Original Message-----
>> From: Richard Guenther [mailto:richard.guenther@gmail.com]
>> Sent: 09 June 2010 16:26
>> 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 5:20 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
>> > I added an attribute because -fwhole-program/externally_visible is
>> handled in ipa.c
>> >
>> > ...
>> > ?if (lookup_attribute ("externally_visible", DECL_ATTRIBUTES (node-
>> >decl)))
>> > ? ?return true;
>> > ...
>> >
>> > Adding attribute seems cleaner than changing flags, otherwise I need
>> to change
>> > handling in ipa.c as well.
>>
>> True, but there is an externally_visible flag in cgraph_node,
>> so I guess that attribute lookup is bogus.
>>
>> Richard.
>>
>> > Bingfeng
>> >
>> >> -----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.
>> >>
>> >> 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).
>> >>
>> >> 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.
>> >
>> >
>> >
>
>
>