[gcc r13-4825] Make -fwhole-program to work with incremental LTO linking

[gcc r13-4825] Make -fwhole-program to work with incremental LTO linking

[Jan Hubicka]

https://gcc.gnu.org/g:39ebd3a9f5cb8555bcb0422086e646ce833028a4

commit r13-4825-g39ebd3a9f5cb8555bcb0422086e646ce833028a4
Author: Jan Hubicka <jh@suse.cz>
Date:   Wed Dec 21 12:41:25 2022 +0100

    Make -fwhole-program to work with incremental LTO linking
    
    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

Diff:
---
 gcc/doc/invoke.texi   |  9 ++++++---
 gcc/lto/lto-common.cc | 11 +++++++++++
 gcc/lto/lto-lang.cc   |  1 -
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index d8a20797603..da9ad1068fb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -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
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;