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

[Wodkowski, PawelX]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
> Sent: Monday, December 15, 2014 2:27 PM
> To: Wodkowski, PawelX
> Cc: Qiu, Michael; dev at dpdk.org
> Subject: Re: [dpdk-dev] error: value computed is not used
> 
> 2014-12-15 11:27, Wodkowski, PawelX:
> > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.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.
> >
> > And something like this?
> >
> >  #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)); })
> 
> What happens to the returned value after this change?
> ptr = rte_memcpy(dst, src, n) + offset:
> 
https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html#Statement-Exprs

Whole expression should be 'void *' type (like *memcpy()) and it should work
as usual (see maxint() example in above link). It is GCC extension. 

> > Thomas, can you check build with EXTRA_CFLAG='-Wunused-value'.
> 
> You mean EXTRA_CFLAGS (with a S).
> It fails in many locations.
> What's your point?

I am just asking if this is an typo, error or intend to do statements with no effects like bellow.

ixgbe_common.c:4429:3: error: statement with no effect [-Werror=unused-value]

4426:	/* first pull in the header so we know the buffer length */
4427:	for (bi = 0; bi < dword_len; bi++) {
4428:		buffer[bi] = IXGBE_READ_REG_ARRAY(hw, IXGBE_FLEX_MNG, bi);
4429:		IXGBE_LE32_TO_CPUS(&buffer[bi]); // <------ here
4430	}


> Do you to support -Wunused-value?
No, I just turned this on to check above change and was surprised what happened.


-- 
Pawel