Add retain attribute to place symbols in SHF_GNU_RETAIN section

When building Linux kernel, ld in bninutils 2.36 with GCC 11 generates
thousands of

ld: warning: orphan section `.data.event_initcall_finish' from `init/main.o' being placed in section `.data.event_initcall_finish'
ld: warning: orphan section `.data.event_initcall_start' from `init/main.o' being placed in section `.data.event_initcall_start'
ld: warning: orphan section `.data.event_initcall_level' from `init/main.o' being placed in section `.data.event_initcall_level'

Since these sections are marked with SHF_GNU_RETAIN, they are placed in
separate sections.  They become orphan sections since they aren't expected
in the Linux kernel linker script. But orphan sections normally don't work
well with the Linux kernel linker script and the resulting kernel crashed.

Add the "retain" attribute to place symbols in separate SHF_GNU_RETAIN
sections.  Issue a warning if the configured assembler/linker doesn't
support SHF_GNU_RETAIN.

gcc/

	PR target/99113
	* varasm.c (get_section): Replace SUPPORTS_SHF_GNU_RETAIN with
	looking up the retain attribute.
	(resolve_unique_section): Likewise.
	(get_variable_section): Likewise.
	(switch_to_section): Likewise.  Warn when a symbol without the
	retain attribute and a symbol with the retain attribute are
	placed in the section with the same name, instead of the used
	attribute.
	* doc/extend.texi: Document the "retain" attribute.

gcc/c-family/

	PR target/99113
	* c-attribs.c (c_common_attribute_table): Add the "retain"
	attribute.
	(handle_retain_attribute): New function.

gcc/testsuite/

	PR target/99113
	* c-c++-common/attr-retain-1.c: New test.
	* c-c++-common/attr-retain-2.c: Likewise.
	* c-c++-common/attr-retain-3.c: Likewise.
	* c-c++-common/attr-retain-4.c: Likewise.
	* c-c++-common/attr-retain-5.c: Likewise.
	* c-c++-common/attr-retain-6.c: Likewise.
	* c-c++-common/attr-retain-7.c: Likewise.
	* c-c++-common/attr-retain-8.c: Likewise.
	* c-c++-common/attr-retain-9.c: Likewise.
	* c-c++-common/pr99113.c: Likewise.
	* gcc.c-torture/compile/attr-retain-1.c: Likewise.
	* gcc.c-torture/compile/attr-retain-2.c: Likewise.
	* c-c++-common/attr-used.c: Don't expect SHF_GNU_RETAIN section.
	* c-c++-common/attr-used-2.c: Likewise.
	* c-c++-common/attr-used-3.c: Likewise.
	* c-c++-common/attr-used-4.c: Likewise.
	* c-c++-common/attr-used-9.c: Likewise.
	* gcc.c-torture/compile/attr-used-retain-1.c: Likewise.
	* gcc.c-torture/compile/attr-used-retain-2.c: Likewise.
	* c-c++-common/attr-used-5.c: Don't expect warning for the used
	attribute nor SHF_GNU_RETAIN section.
	* c-c++-common/attr-used-6.c: Likewise.
	* c-c++-common/attr-used-7.c: Likewise.
	* c-c++-common/attr-used-8.c: Likewise.

1 files changed, 6 insertions, 2 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 6eb1d32..8bbb937 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -3913,8 +3913,10 @@ When applied to a member function of a C++ class template, the
 attribute also means that the function is instantiated if the
 class itself is instantiated.
 
+@item retain
+@cindex @code{retain} function attribute
 For ELF targets that support the GNU or FreeBSD OSABIs, this attribute
-will also save the function from linker garbage collection.  To support
+will save the function from linker garbage collection.  To support
 this behavior, functions that have not been placed in specific sections
 (e.g. by the @code{section} attribute, or the @code{-ffunction-sections}
 option), will be placed in new, unique sections.
@@ -7504,8 +7506,10 @@ When applied to a static data member of a C++ class template, the
 attribute also means that the member is instantiated if the
 class itself is instantiated.
 
+@item retain
+@cindex @code{retain} variable attribute
 For ELF targets that support the GNU or FreeBSD OSABIs, this attribute
-will also save the variable from linker garbage collection.  To support
+will save the variable from linker garbage collection.  To support
 this behavior, variables that have not been placed in specific sections
 (e.g. by the @code{section} attribute, or the @code{-fdata-sections} option),
 will be placed in new, unique sections.