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, 40 insertions, 0 deletions

diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index ff6bc5f..9d0a25d 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -10662,3 +10662,43 @@ proc check_effective_target_no_fsanitize_address {} {
     }
     return 0;
 }
+
+# Return 1 if this target supports 'R' flag in .section directive, 0
+# otherwise.  Cache the result.
+
+proc check_effective_target_R_flag_in_section { } {
+    global tool
+    global GCC_UNDER_TEST
+
+    # Need auto-host.h to check linker support.
+    if { ![file exists ../../auto-host.h ] } {
+	return 0
+    }
+
+    return [check_cached_effective_target R_flag_in_section {
+
+	set src pie[pid].c
+	set obj pie[pid].o
+
+	set f [open $src "w"]
+	puts $f "#include \"../../auto-host.h\""
+	puts $f "#if HAVE_GAS_SHF_GNU_RETAIN == 0"
+	puts $f "# error Assembler does not support 'R' flag in .section directive."
+	puts $f "#endif"
+	close $f
+
+	verbose "check_effective_target_R_flag_in_section compiling testfile $src" 2
+	set lines [${tool}_target_compile $src $obj assembly ""]
+
+	file delete $src
+	file delete $obj
+
+	if [string match "" $lines] then {
+	    verbose "check_effective_target_R_flag_in_section testfile compilation passed" 2
+	    return 1
+	} else {
+	    verbose "check_effective_target_R_flag_in_section testfile compilation failed" 2
+	    return 0
+	}
+    }]
+}