The attached patch implements TARGET_ASM_MARK_DECL_PRESERVED for ELF GNU
OSABI targets, so that declarations that have the "used" attribute
applied will be saved from linker garbage collection.
TARGET_ASM_MARK_DECL_PRESERVED will emit an assembler ".retain"
directive for the decl, and the assembler will apply the SHF_GNU_RETAIN
flag to the section containing the decl.
The linker will not garbage collect sections marked with the
SHF_GNU_RETAIN flag.
SHF_GNU_RETAIN is a GNU OSABI ELF extension, and it was discussed on the
GNU gABI mailing list here:
https://sourceware.org/pipermail/gnu-gabi/2020q3/000429.html
The Binutils patch to implement .retain and other SHF_GNU_RETAIN
handling is posted here:
https://sourceware.org/pipermail/binutils/2020-November/113993.html
Successfully bootstrapped and regtested for x86_64-pc-linux-gnu, and
regtested for arm-none-eabi.
Ok for trunk?
Thanks,
Jozef
>From 0827e28480b7edd07cda4f938bdd14b1cbdf1fa2 Mon Sep 17 00:00:00 2001
From: Jozef Lawrynowicz <joze...@mittosystems.com>
Date: Thu, 29 Oct 2020 21:00:07 +0000
Subject: [PATCH] Implement TARGET_MARK_DECL_PRESERVED for ELF GNU OSABI
targets
The GAS .retain directive will apply the SHF_GNU_RETAIN flag to the
section containing the symbol that must be preserved.
gcc/ChangeLog:
* config.in: Regenerate.
* config/elfos.h (TARGET_ASM_MARK_DECL_PRESERVED): Define for
HAVE_GAS_RETAIN.
* configure: Regenerate.
* configure.ac: Define HAVE_GAS_RETAIN.
* doc/extend.texi (used attribute): Document saving from linker garbage
collection.
* doc/sourcebuild.texi: Document "retain" effective target keyword.
* doc/tm.texi: Regenerate.
* output.h (default_elf_mark_decl_preserved): New.
* target.def (mark_decl_preserved): Mention GAS .retain directive.
* varasm.c (default_elf_mark_decl_preserved): New.
gcc/testsuite/ChangeLog:
* c-c++-common/attr-used-2.c: Test for .retain in assembler output.
* c-c++-common/attr-used.c: Likewise.
* lib/target-supports.exp (check_effective_target_retain): New.
---
gcc/config.in | 6 ++++
gcc/config/elfos.h | 7 +++++
gcc/configure | 35 ++++++++++++++++++++++++
gcc/configure.ac | 8 ++++++
gcc/doc/extend.texi | 6 ++++
gcc/doc/sourcebuild.texi | 3 ++
gcc/doc/tm.texi | 2 +-
gcc/output.h | 4 +++
gcc/target.def | 2 +-
gcc/testsuite/c-c++-common/attr-used-2.c | 1 +
gcc/testsuite/c-c++-common/attr-used.c | 2 ++
gcc/testsuite/lib/target-supports.exp | 9 ++++++
gcc/varasm.c | 13 +++++++++
13 files changed, 96 insertions(+), 2 deletions(-)
diff --git a/gcc/config.in b/gcc/config.in
index 3657c46f349..8ef075a0ff3 100644
--- a/gcc/config.in
+++ b/gcc/config.in
@@ -1346,6 +1346,12 @@
#endif
+/* Define if your assembler supports the .retain directive. */
+#ifndef USED_FOR_TARGET
+#undef HAVE_GAS_RETAIN
+#endif
+
+
/* Define if your assembler supports specifying the exclude section flag. */
#ifndef USED_FOR_TARGET
#undef HAVE_GAS_SECTION_EXCLUDE
diff --git a/gcc/config/elfos.h b/gcc/config/elfos.h
index 74a3eafda6b..fab7b0e8ea4 100644
--- a/gcc/config/elfos.h
+++ b/gcc/config/elfos.h
@@ -474,3 +474,10 @@ see the files COPYING3 and COPYING.RUNTIME respectively.
If not, see
#undef TARGET_LIBC_HAS_FUNCTION
#define TARGET_LIBC_HAS_FUNCTION no_c99_libc_has_function
+
+/* If the assembler supports the .retain directive for saving a symbol
+ from linker garbage collection, define this macro. */
+#if HAVE_GAS_RETAIN
+#undef TARGET_ASM_MARK_DECL_PRESERVED
+#define TARGET_ASM_MARK_DECL_PRESERVED default_elf_mark_decl_preserved
+#endif
diff --git a/gcc/configure b/gcc/configure
index abff47d30eb..37488eac25d 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -24223,6 +24223,41 @@ cat >>confdefs.h <<_ACEOF
_ACEOF
+# Test if the assembler supports the .retain directive for saving a symbol from
+# linker garbage collection.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking assembler for retain
directive" >&5
+$as_echo_n "checking assembler for retain directive... " >&6; }
+if ${gcc_cv_as_retain_r+:} false; then :
+ $as_echo_n "(cached) " >&6
+else
+ gcc_cv_as_retain_r=no
+ if test x$gcc_cv_as != x; then
+ $as_echo '.retain retain_sym' > conftest.s
+ if { ac_try='$gcc_cv_as $gcc_cv_as_flags -o conftest.o conftest.s >&5'
+ { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+ (eval $ac_try) 2>&5
+ ac_status=$?
+ $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+ test $ac_status = 0; }; }
+ then
+ gcc_cv_as_retain_r=yes
+ else
+ echo "configure: failed program was" >&5
+ cat conftest.s >&5
+ fi
+ rm -f conftest.o conftest.s
+ fi
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $gcc_cv_as_retain_r" >&5
+$as_echo "$gcc_cv_as_retain_r" >&6; }
+
+
+
+cat >>confdefs.h <<_ACEOF
+#define HAVE_GAS_RETAIN `if test $gcc_cv_as_retain_r = yes; then echo 1; else
echo 0; fi`
+_ACEOF
+
+
{ $as_echo "$as_me:${as_lineno-$LINENO}: checking assembler for section
merging support" >&5
$as_echo_n "checking assembler for section merging support... " >&6; }
if ${gcc_cv_as_shf_merge+:} false; then :
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 26a5d8e3619..08b38d894a3 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -3216,6 +3216,14 @@ AC_DEFINE_UNQUOTED(HAVE_GAS_SECTION_EXCLUDE,
[`if test $gcc_cv_as_section_exclude_e = yes || test
$gcc_cv_as_section_exclude_hash = yes; then echo 1; else echo 0; fi`],
[Define if your assembler supports specifying the exclude section flag.])
+# Test if the assembler supports the .retain directive for saving a symbol from
+# linker garbage collection.
+gcc_GAS_CHECK_FEATURE([retain directive], gcc_cv_as_retain_r,,,
+ [.retain retain_sym])
+AC_DEFINE_UNQUOTED(HAVE_GAS_RETAIN,
+ [`if test $gcc_cv_as_retain_r = yes; then echo 1; else echo 0; fi`],
+[Define if your assembler supports the .retain directive.])
+
gcc_GAS_CHECK_FEATURE(section merging support, gcc_cv_as_shf_merge,
[elf,2,12,0], [--fatal-warnings],
[.section .rodata.str, "aMS", @progbits, 1])
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 62549b02452..4f77a5c0229 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -3810,6 +3810,9 @@ 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.
+As a GNU ELF extension, functions with this attribute will not be
+garbage collected by the linker.
+
@item visibility ("@var{visibility_type}")
@cindex @code{visibility} function attribute
This attribute affects the linkage of the declaration to which it is attached.
@@ -7269,6 +7272,9 @@ 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.
+As a GNU ELF extension, variables with this attribute will not be
+garbage collected by the linker.
+
@item vector_size (@var{bytes})
@cindex @code{vector_size} variable attribute
This attribute specifies the vector size for the type of the declared
diff --git a/gcc/doc/sourcebuild.texi b/gcc/doc/sourcebuild.texi
index 49316a5d0ff..7fe77e7f09e 100644
--- a/gcc/doc/sourcebuild.texi
+++ b/gcc/doc/sourcebuild.texi
@@ -2551,6 +2551,9 @@ Target supports @option{-pie}, @option{-fpie} and
@option{-fPIE}.
@item rdynamic
Target supports @option{-rdynamic}.
+@item retain
+Target supports the @code{.retain} assembler directive.
+
@item scalar_all_fma
Target supports all four fused multiply-add optabs for both @code{float}
and @code{double}. These optabs are: @code{fma_optab}, @code{fms_optab},
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 97437e8274f..b074b2ff75b 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -8773,7 +8773,7 @@ library function is given by @var{symref}, which is a
@code{symbol_ref}.
@deftypefn {Target Hook} void TARGET_ASM_MARK_DECL_PRESERVED (const char
*@var{symbol})
This target hook is a function to output to @var{asm_out_file} an assembler
directive to annotate @var{symbol} as used. The Darwin target uses the
-.no_dead_code_strip directive.
+.no_dead_code_strip directive, and ELF targets use the .retain directive.
@end deftypefn
@defmac ASM_OUTPUT_LABELREF (@var{stream}, @var{name})
diff --git a/gcc/output.h b/gcc/output.h
index eb253c50329..c0eba372c5d 100644
--- a/gcc/output.h
+++ b/gcc/output.h
@@ -609,6 +609,10 @@ extern void default_elf_init_array_asm_out_constructor
(rtx, int);
extern void default_elf_fini_array_asm_out_destructor (rtx, int);
extern int maybe_assemble_visibility (tree);
+#if HAVE_GAS_RETAIN
+void default_elf_mark_decl_preserved (const char *);
+#endif
+
extern int default_address_cost (rtx, machine_mode, addr_space_t, bool);
/* Stack usage. */
diff --git a/gcc/target.def b/gcc/target.def
index ed2da154e30..12164792b00 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -737,7 +737,7 @@ DEFHOOK
(mark_decl_preserved,
"This target hook is a function to output to @var{asm_out_file} an
assembler\n\
directive to annotate @var{symbol} as used. The Darwin target uses the\n\
-.no_dead_code_strip directive.",
+.no_dead_code_strip directive, and ELF targets use the .retain directive.",
void, (const char *symbol),
hook_void_constcharptr)
diff --git a/gcc/testsuite/c-c++-common/attr-used-2.c
b/gcc/testsuite/c-c++-common/attr-used-2.c
index f78b94b53a9..cf1d25a5b27 100644
--- a/gcc/testsuite/c-c++-common/attr-used-2.c
+++ b/gcc/testsuite/c-c++-common/attr-used-2.c
@@ -9,3 +9,4 @@ void foo()
}
/* { dg-final { scan-assembler "xyzzy" } } */
+/* { dg-final { scan-assembler "\\.retain\t\.*xyzzy" { target retain } } } */
diff --git a/gcc/testsuite/c-c++-common/attr-used.c
b/gcc/testsuite/c-c++-common/attr-used.c
index ba7705aaa77..65a2f029698 100644
--- a/gcc/testsuite/c-c++-common/attr-used.c
+++ b/gcc/testsuite/c-c++-common/attr-used.c
@@ -11,3 +11,5 @@ static void function_declaration_after(void)
__attribute__((__used__));
/* { dg-final { scan-assembler "function_declaration_before" } } */
/* { dg-final { scan-assembler "function_declaration_after" } } */
+/* { dg-final { scan-assembler "\\.retain\t\.*function_declaration_before" {
target retain } } } */
+/* { dg-final { scan-assembler "\\.retain\t\.*function_declaration_after" {
target retain } } } */
diff --git a/gcc/testsuite/lib/target-supports.exp
b/gcc/testsuite/lib/target-supports.exp
index 8439720baea..7c0e925f7b4 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -380,6 +380,15 @@ proc check_effective_target_noinit { } {
return 0
}
+# The .retain assembler directive is only supported by some targets.
+# This proc returns 1 if it's supported, 0 if it's not.
+
+proc check_effective_target_retain { } {
+ return [check_no_compiler_messages retain_available object {
+ __asm__(".retain used_var");
+ }]
+}
+
###############################
# proc check_visibility_available { what_kind }
###############################
diff --git a/gcc/varasm.c b/gcc/varasm.c
index ea0b59cf44a..c38640456c4 100644
--- a/gcc/varasm.c
+++ b/gcc/varasm.c
@@ -8276,6 +8276,19 @@ default_elf_fini_array_asm_out_destructor (rtx symbol,
int priority)
assemble_addr_to_section (symbol, sec);
}
+
+#if HAVE_GAS_RETAIN
+/* Implement TARGET_ASM_MARK_DECL_PRESERVED for ELF targets that support the
+ .retain assembler directive. */
+void
+default_elf_mark_decl_preserved (const char *name)
+{
+ fprintf (asm_out_file, "\t.retain\t");
+ assemble_name (asm_out_file, name);
+ fputc ('\n', asm_out_file);
+}
+#endif
+
/* Default TARGET_ASM_OUTPUT_IDENT hook.
This is a bit of a cheat. The real default is a no-op, but this
--
2.28.0
