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From: Thomas Monjalon <thomas.monjalon@6wind.com>
To: "Wodkowski, PawelX" <pawelx.wodkowski@intel.com>,
	"Qiu, Michael" <michael.qiu@intel.com>
Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] error: value computed is not used
Date: Mon, 15 Dec 2014 11:54:33 +0100
Message-ID: <19703944.gAxOdpYSXo@xps13> (raw)
In-Reply-To: <F6F2A6264E145F47A18AB6DF8E87425D12B67782@IRSMSX102.ger.corp.intel.com>

2014-12-08 15:26, Wodkowski, PawelX:
> From: Qiu, Michael
> > On 2014/12/8 19:00, Wodkowski, PawelX wrote:
> > >> lib/librte_pmd_enic/enic_main.c: In function 'enic_set_rsskey':
> > >> lib/librte_pmd_enic/enic_main.c:862:2: error: value computed is not used
> > >>
> > >> I dig out that, it was ome issue of  the macros rte_memcpy()
> > >> #define rte_memcpy(dst, src, n)              \
> > >>         ((__builtin_constant_p(n)) ?          \
> > >>         memcpy((dst), (src), (n)) :          \
> > >>         rte_memcpy_func((dst), (src), (n)))
> > >>
> > >> When I use only (n) instead of (__builtin_constant_p(n), it will pass( I
> > >> know that it was incorrect, just a experiment).
> > >>
> > >> But I try to use inline function instead of macros:
> > >> static inline void * rte_memcpy(void *dst, const void *src, size_t n)
> > >> {
> > >>         return __builtin_constant_p(n) ? memcpy(dst, src, n) :
> > >>                                          rte_memcpy_func(dst, src, n);
> > >> }
> > >>
> > >> It will pass:), and works, this could be one potential workaround fix.
> > >>
> > >> Who knows why? The root cause is what?
> > >>
> > >> I've no idea about this.
> > >>
> > > I got the same issue while ago. I don't remember exactly everything
> > > but my conclusion was that there was some bug in compiler. I think,
> > > when 'n' I constant and/or small compiler is inlining memcpy and throwing
> > > everything else (including returned value). In that case error is not
> > > produced (I think this is a bug in compiler). In other case it is computing
> > > some value calling memcpy or rte_ memcpy and you should at least
> > > explicitly throw it away by casting to void. I like solution with static
> > 
> > Actually, I try to pass "n" as a Int value like 4, it still report this
> > error :)
> 
> My workaround was:
> (void) rte_memcpy(...);
> 
> But this is only a workaround.

It's not so bad.

> > > inline but someone else should spoke about possible side effects.
> > 
> > Yes, but as I know inline is better than macros.

>From the GCC manual:
"
You may use this built-in function in either a macro or an inline function.
However, if you use it in an inlined function and pass an argument of the
function as the argument to the built-in, GCC never returns 1 when you call
the inline function with a string constant or compound literal and does not
return 1 when you pass a constant numeric value to the inline function unless
you specify the -O option.
"

It seems the "inline fix" cannot be used.

I'm going to send a patch with Pawel's workaround.

-- 
Thomas

  reply	other threads:[~2014-12-15 10:54 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-12-08  9:05 Qiu, Michael
2014-12-08 11:00 ` Wodkowski, PawelX
2014-12-08 15:23   ` Qiu, Michael
2014-12-08 15:26     ` Wodkowski, PawelX
2014-12-15 10:54       ` Thomas Monjalon [this message]
2014-12-15 11:07         ` [dpdk-dev] [PATCH] enic: fix build on SUSE 11 Thomas Monjalon
2014-12-15 11:27         ` [dpdk-dev] error: value computed is not used Wodkowski, PawelX
2014-12-15 13:26           ` Thomas Monjalon
2014-12-15 13:47             ` Wodkowski, PawelX
2014-12-15 14:16               ` Thomas Monjalon
2014-12-15 15:44                 ` Wodkowski, PawelX
2014-12-15 16:35                   ` Ananyev, Konstantin
2014-12-15 16:00                 ` Ananyev, Konstantin
2014-12-15 16:40                   ` Thomas Monjalon
2014-12-15 17:03                 ` Jastrzebski, MichalX K
2014-12-16  0:49         ` Qiu, Michael
2014-12-09  9:19 ` Qiu, Michael
2014-12-10  9:26 ` Qiu, Michael

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