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stdatomic.h and atomic_load_explicit()
- From: Sebastian Huber <sebastian dot huber at embedded-brains dot de>
- To: GCC <gcc at gcc dot gnu dot org>
- Date: Wed, 25 Jun 2014 12:59:40 +0200
- Subject: stdatomic.h and atomic_load_explicit()
- Authentication-results: sourceware.org; auth=none
Hello,
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)
I think the inheritance of the volatile qualifier via __typeof__
(*__atomic_load_ptr) is an implementation flaw.
With the FreeBSD version of <stdatomic.h> I don't have this problem:
http://svnweb.freebsd.org/base/head/include/stdatomic.h?revision=234958&view=markup&pathrev=234958#l231
--
Sebastian Huber, embedded brains GmbH
Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone : +49 89 189 47 41-16
Fax : +49 89 189 47 41-09
E-Mail : sebastian.huber@embedded-brains.de
PGP : Public key available on request.
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