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Make -fwhole-program to work with incremental LTO linking

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Update documentation of -fwhole-program which was wrongly
claiming that it is useless with LTO whole it is useful for LTO without plugin
and extends -fwhole-program to also work with incremental linking.
This is useful when building kernel where the incremental link is de-facto fina
binary and only some explicitly marked symbols needs to remain.

Bootstrapped/regtested x86_64-linux, comitted.

gcc/ChangeLog:

2022-12-21  Jan Hubicka  <hubicka@ucw.cz>

	* doc/invoke.texi: Fix documentation of -fwhole-program with LTO
	and document behaviour for incremental linking.

gcc/lto/ChangeLog:

2022-12-21  Jan Hubicka  <hubicka@ucw.cz>

	* lto-common.cc (lto_resolution_read): With incremental linking
	and whole program ignore turn LDPR_PREVAILING_DEF_IRONLY to
	LDPR_PREVAILING_DEF_IRONLY_EXP
	* lto-lang.cc (lto_post_options): Do not clear flag_whole_program
	for incremental link
This commit is contained in:
Jan Hubicka 2022-12-21 12:41:25 +01:00
parent 88709c4a1e
commit 39ebd3a9f5
3 changed files with 17 additions and 4 deletions
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9
gcc/doc/invoke.texi
View File

@ -13719,9 +13719,12 @@ compiled. All public functions and variables with the exception of @code{main}
and those merged by attribute @code{externally_visible} become static functions
and in effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with @option{-flto}.
Instead relying on a linker plugin should provide safer and more precise
information.
With @option{-flto} this option has a limited use. In most cases the
precise list of symbols used or exported from the binary is known the
resolution info passed to the link-time optimizer by the linker plugin. It is
still useful if no linker plugin is used or during incremental link step when
final code is produced (with @option{-flto}
@option{-flinker-output=nolto-rel}).
@item -flto[=@var{n}]
@opindex flto

11
gcc/lto/lto-common.cc
View File

@ -2118,6 +2118,17 @@ lto_resolution_read (splay_tree file_ids, FILE *resolution, lto_file *file)
if (strcmp (lto_resolution_str[j], r_str) == 0)
{
r = (enum ld_plugin_symbol_resolution) j;
/* Incremental linking together with -fwhole-program may seem
somewhat contradictionary (as the point of incremental linking
is to allow re-linking with more symbols later) but it is
used to build LTO kernel. We want to hide all symbols that
are not explicitely marked as exported and thus turn
LDPR_PREVAILING_DEF_IRONLY_EXP
to LDPR_PREVAILING_DEF_IRONLY. */
if (flag_whole_program
&& flag_incremental_link == INCREMENTAL_LINK_NOLTO
&& r == LDPR_PREVAILING_DEF_IRONLY_EXP)
r = LDPR_PREVAILING_DEF_IRONLY;
break;
}
}

1
gcc/lto/lto-lang.cc
View File

@ -901,7 +901,6 @@ lto_post_options (const char **pfilename ATTRIBUTE_UNUSED)
break;
case LTO_LINKER_OUTPUT_NOLTOREL: /* .o: incremental link producing asm */
flag_whole_program = 0;
flag_incremental_link = INCREMENTAL_LINK_NOLTO;
break;