Re: [Qemu-devel] [PATCH v3 04/10] tcg: Introduce atomic helpers for integer min/max

[Peter Maydell]

On 8 May 2018 at 19:27, Peter Maydell <address@hidden> wrote:
> On 8 May 2018 at 18:49, Peter Maydell <address@hidden> wrote:
>> [weird compiler errors]
>
> This fixes them:
>
> --- a/include/qemu/atomic.h
> +++ b/include/qemu/atomic.h
> @@ -187,7 +187,7 @@
>  /* Returns the eventual value, failed or not */
>  #define atomic_cmpxchg__nocheck(ptr, old, new)    ({                    \
>      typeof_strip_qual(*ptr) _old = (old);                               \
> -    __atomic_compare_exchange_n(ptr, &_old, new, false,                 \
> +    (void)__atomic_compare_exchange_n(ptr, &_old, new, false,           \
>                                __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \
>      _old;                                                               \
>  })
>
> Seems pretty clearly a compiler bug -- rth says newer gcc don't
> do this, so presumably fixed upstream somewhere between gcc 5 and 6.

Standalone testcase, fwiw:

===begin===
/*
 * Weirdly, this compiler will complain about FOO(int8_t) but not
 * the others:
 * $ gcc -g -Wall -Wunused-value -o zz9.o -c zz9.c
 * zz9.c: In function ‘foo_int8_t’:
 * zz9.c:12:5: warning: value computed is not used [-Wunused-value]
 *     __atomic_compare_exchange_n(p, exp, des, 0, __ATOMIC_SEQ_CST,
__ATOMIC_SEQ_CST); \
 *     ^
 * /tmp/zz9.c:17:1: note: in expansion of macro ‘FOO’
 * FOO(int8_t)
 * ^
 *
 * This is gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
 */

typedef unsigned char uint8_t;
typedef signed char int8_t;

#define FOO(TYPE) \
void foo_##TYPE(TYPE *p, TYPE *exp, TYPE *des) { \
    __atomic_compare_exchange_n(p, exp, des, 0, __ATOMIC_SEQ_CST,
__ATOMIC_SEQ_CST); \
}

FOO(int)
FOO(uint8_t)
FOO(int8_t)
===endit===

thanks
-- PMM