diff mbox series

Make -fwhole-program to work with incremental LTO linking

Message ID Y6LxPU/HBAJ0fFQl@kam.mff.cuni.cz
State New
Headers show
Series Make -fwhole-program to work with incremental LTO linking | expand

Commit Message

Jan Hubicka Dec. 21, 2022, 11:42 a.m. UTC 
Hi,
this patches updates 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 when
non-LTO code is produced.
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
diff mbox series

Patch

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 330da6eb5d4..8c01160e2bb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -13648,9 +13648,12 @@  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
diff --git a/gcc/lto/lto-common.cc b/gcc/lto/lto-common.cc
index f64309731fa..125064ba47e 100644
--- a/gcc/lto/lto-common.cc
+++ b/gcc/lto/lto-common.cc
@@ -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;
 	    }
 	}
diff --git a/gcc/lto/lto-lang.cc b/gcc/lto/lto-lang.cc
index d36453ba25d..7018dfae4a5 100644
--- a/gcc/lto/lto-lang.cc
+++ b/gcc/lto/lto-lang.cc
@@ -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;