Use SHF_GNU_RETAIN to preserve symbol definitions

In assemly code, the section flag 'R' sets the SHF_GNU_RETAIN flag to
indicate that the section must be preserved by the linker.

Add SECTION_RETAIN to indicate a section should be retained by the linker
and set SECTION_RETAIN on section for the preserved symbol if assembler
supports SHF_GNU_RETAIN.  All retained symbols are placed in separate
sections with

	.section .data.rel.local.preserved_symbol,"awR"
preserved_symbol:
...
	.section .data.rel.local,"aw"
not_preserved_symbol:
...

to avoid

	.section .data.rel.local,"awR"
preserved_symbol:
...
not_preserved_symbol:
...

which places not_preserved_symbol definition in the SHF_GNU_RETAIN
section.

gcc/

2020-12-01  H.J. Lu  <hjl.tools@gmail.com>

	* configure.ac (HAVE_GAS_SHF_GNU_RETAIN): New.  Define 1 if
	the assembler supports marking sections with SHF_GNU_RETAIN flag.
	* output.h (SECTION_RETAIN): New.  Defined as 0x4000000.
	(SECTION_MACH_DEP): Changed from 0x4000000 to 0x8000000.
	(default_unique_section): Add a bool argument.
	* varasm.c (get_section): Set SECTION_RETAIN for the preserved
	symbol with HAVE_GAS_SHF_GNU_RETAIN.
	(resolve_unique_section): Used named section for the preserved
	symbol if assembler supports SHF_GNU_RETAIN.
	(get_variable_section): Handle the preserved common symbol with
	HAVE_GAS_SHF_GNU_RETAIN.
	(default_elf_asm_named_section): Require the full declaration and
	use the 'R' flag for SECTION_RETAIN.
	* config.in: Regenerated.
	* configure: Likewise.
	* doc/sourcebuild.texi: Document R_flag_in_section.

gcc/testsuite/

2020-12-01  H.J. Lu  <hjl.tools@gmail.com>
	    Jozef Lawrynowicz  <jozef.l@mittosystems.com>

	* c-c++-common/attr-used.c: Check the 'R' flag.
	* c-c++-common/attr-used-2.c: Likewise.
	* c-c++-common/attr-used-3.c: New test.
	* c-c++-common/attr-used-4.c: Likewise.
	* gcc.c-torture/compile/attr-used-retain-1.c: Likewise.
	* gcc.c-torture/compile/attr-used-retain-2.c: Likewise.
	* lib/target-supports.exp
	(check_effective_target_R_flag_in_section): New proc.

1 files changed, 12 insertions, 3 deletions

diff --git a/gcc/varasm.c b/gcc/varasm.c
index b92da26..6c13f52 100644
--- a/gcc/varasm.c
+++ b/gcc/varasm.c
@@ -289,6 +289,10 @@ get_section (const char *name, unsigned int flags, tree decl,
   slot = section_htab->find_slot_with_hash (name, htab_hash_string (name),
 					    INSERT);
   flags |= SECTION_NAMED;
+  if (HAVE_GAS_SHF_GNU_RETAIN
+      && decl != nullptr
+      && DECL_PRESERVE_P (decl))
+    flags |= SECTION_RETAIN;
   if (*slot == NULL)
     {
       sect = ggc_alloc<section> ();
@@ -469,6 +473,7 @@ resolve_unique_section (tree decl, int reloc ATTRIBUTE_UNUSED,
   if (DECL_SECTION_NAME (decl) == NULL
       && targetm_common.have_named_sections
       && (flag_function_or_data_sections
+	  || (HAVE_GAS_SHF_GNU_RETAIN && DECL_PRESERVE_P (decl))
 	  || DECL_COMDAT_GROUP (decl)))
     {
       targetm.asm_out.unique_section (decl, reloc);
@@ -1207,7 +1212,8 @@ get_variable_section (tree decl, bool prefer_noswitch_p)
   if (vnode)
     vnode->get_constructor ();
 
-  if (DECL_COMMON (decl))
+  if (DECL_COMMON (decl)
+      && !(HAVE_GAS_SHF_GNU_RETAIN && DECL_PRESERVE_P (decl)))
     {
       /* If the decl has been given an explicit section name, or it resides
 	 in a non-generic address space, then it isn't common, and shouldn't
@@ -6745,9 +6751,10 @@ default_elf_asm_named_section (const char *name, unsigned int flags,
 
   /* If we have already declared this section, we can use an
      abbreviated form to switch back to it -- unless this section is
-     part of a COMDAT groups, in which case GAS requires the full
-     declaration every time.  */
+     part of a COMDAT groups or with SHF_GNU_RETAIN, in which case GAS
+     requires the full declaration every time.  */
   if (!(HAVE_COMDAT_GROUP && (flags & SECTION_LINKONCE))
+      && !(flags & SECTION_RETAIN)
       && (flags & SECTION_DECLARED))
     {
       fprintf (asm_out_file, "\t.section\t%s\n", name);
@@ -6780,6 +6787,8 @@ default_elf_asm_named_section (const char *name, unsigned int flags,
 	*f++ = TLS_SECTION_ASM_FLAG;
       if (HAVE_COMDAT_GROUP && (flags & SECTION_LINKONCE))
 	*f++ = 'G';
+      if (flags & SECTION_RETAIN)
+	*f++ = 'R';
 #ifdef MACH_DEP_SECTION_ASM_FLAG
       if (flags & SECTION_MACH_DEP)
 	*f++ = MACH_DEP_SECTION_ASM_FLAG;