GCC provides its own version of stdatomic.h since GCC 4.9. Here we
have:
#define atomic_load_explicit(PTR, MO) \
__extension__ \
({ \
__auto_type __atomic_load_ptr = (PTR); \
__typeof__ (*__atomic_load_ptr) __atomic_load_tmp; \
__atomic_load (__atomic_load_ptr, &__atomic_load_tmp, (MO)); \
__atomic_load_tmp; \
})
According to
http://en.cppreference.com/w/c/atomic/atomic_load
(or in the standard "7.17.7.2 The atomic_load generic functions") we
have
C atomic_load_explicit( volatile A* obj, memory_order order );
This test case
#include <stdatomic.h>
int ld(volatile atomic_int *i)
{
return atomic_load_explicit(i, memory_order_relaxed);
}
yields on ARM
arm-rtems4.11-gcc -march=armv7-a -O2 test.c -S && cat test.s
.arch armv7-a
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "test.c"
.text
.align 2
.global ld
.type ld, %function
ld:
@ args = 0, pretend = 0, frame = 8
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldr r3, [r0]
sub sp, sp, #8
str r3, [sp, #4]
ldr r0, [sp, #4]
add sp, sp, #8
@ sp needed
bx lr
.size ld, .-ld
.ident "GCC: (GNU) 4.9.1 20140515 (prerelease)
To solve this performance issue discard all qualifiers in __typeof__ and
__auto_type for atomic types.
With this patch we have
rm-rtems4.11-gcc -march=armv7-a -O2 test.c -S && cat test.s
.arch armv7-a
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "test.c"
.text
.align 2
.global ld
.type ld, %function
ld:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldr r0, [r0]
bx lr
.size ld, .-ld
.ident "GCC: (GNU) 4.9.1 20140625 (prerelease)
gcc/c/ChangeLog
2014-06-25 Sebastian Huber <sebastian.hu...@embedded-brains.de>
* c-parser.c (c_parser_declaration_or_fndef): Discard all type
qualifiers in __auto_type for atomic types.
(c_parser_typeof_specifier): Discard all type qualifiers in
__typeof__ for atomic types.
---
gcc/c/c-parser.c | 21 ++++++---------------
1 files changed, 6 insertions(+), 15 deletions(-)
diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 99ff546..037da03 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -1707,14 +1707,10 @@ c_parser_declaration_or_fndef (c_parser *parser, bool
fndef_ok,
" initializer");
init = convert_lvalue_to_rvalue (init_loc, init, true, true);
tree init_type = TREE_TYPE (init.value);
- /* As with typeof, remove _Atomic and const
- qualifiers from atomic types. */
+ /* As with typeof, remove all qualifiers from atomic types.
*/
if (init_type != error_mark_node && TYPE_ATOMIC (init_type))
init_type
- = c_build_qualified_type (init_type,
- (TYPE_QUALS (init_type)
- & ~(TYPE_QUAL_ATOMIC
- | TYPE_QUAL_CONST)));
+ = c_build_qualified_type (init_type, TYPE_UNQUALIFIED);
bool vm_type = variably_modified_type_p (init_type,
NULL_TREE);
if (vm_type)
@@ -3011,16 +3007,11 @@ c_parser_typeof_specifier (c_parser *parser)
if (was_vm)
ret.expr = c_fully_fold (expr.value, false, &ret.expr_const_operands);
pop_maybe_used (was_vm);
- /* For use in macros such as those in <stdatomic.h>, remove
- _Atomic and const qualifiers from atomic types. (Possibly
- all qualifiers should be removed; const can be an issue for
- more macros using typeof than just the <stdatomic.h>
- ones.) */
+ /* For use in macros such as those in <stdatomic.h>, remove all
+ qualifiers from atomic types. (const can be an issue for more macros
+ using typeof than just the <stdatomic.h> ones.) */
if (ret.spec != error_mark_node && TYPE_ATOMIC (ret.spec))
- ret.spec = c_build_qualified_type (ret.spec,
- (TYPE_QUALS (ret.spec)
- & ~(TYPE_QUAL_ATOMIC
- | TYPE_QUAL_CONST)));
+ ret.spec = c_build_qualified_type (ret.spec, TYPE_UNQUALIFIED);
}
c_parser_skip_until_found (parser, CPP_CLOSE_PAREN, "expected %<)%>");
return ret;
--
1.7.7
