Re: [dpdk-dev] error: value computed is not used

["Wodkowski, PawelX" <pawelx.wodkowski@intel.com>]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> Sent: Monday, December 15, 2014 11:55 AM
> To: Wodkowski, PawelX; Qiu, Michael
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] error: value computed is not used
> 
> 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.

And something like this?

diff --git a/lib/librte_eal/common/include/arch/x86/rte_memcpy.h b/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
index 290c5cd..906c911 100644
--- a/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
+++ b/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
@@ -168,10 +168,10 @@
 	rte_mov128(dst + 128, src + 128);
 }
 
diff --git a/lib/librte_eal/common/include/arch/x86/rte_memcpy.h b/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
index 290c5cd..c3e8b81 100644
--- a/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
+++ b/lib/librte_eal/common/include/arch/x86/rte_memcpy.h
@@ -169,9 +169,9 @@
 }
 
 #define rte_memcpy(dst, src, n)              \
-	((__builtin_constant_p(n)) ?          \
+	({ (__builtin_constant_p(n)) ?          \
 	memcpy((dst), (src), (n)) :          \
-	rte_memcpy_func((dst), (src), (n)))
+	rte_memcpy_func((dst), (src), (n)); })
 
 static inline void *
 rte_memcpy_func(void *dst, const void *src, size_t n)


Thomas, can you check build with EXTRA_CFLAG='-Wunused-value'. 

/home/pwodkowx/grizzly/dpdk_org_declan_v3_mode4_v2/lib/librte_pmd_ixgbe/ixgbe/ixgbe_common.c: In function 'ixgbe_host_interface_command':
/home/pwodkowx/grizzly/dpdk_org_declan_v3_mode4_v2/lib/librte_pmd_ixgbe/ixgbe/ixgbe_common.c:4429:3: error: statement with no effect [-Werror=unused-value]
/home/pwodkowx/grizzly/dpdk_org_declan_v3_mode4_v2/lib/librte_pmd_ixgbe/ixgbe/ixgbe_common.c:4448:3: error: statement with no effect [-Werror=unused-value]