Re: [dpdk-dev] Build is broken in dpdk-next-net

[Stephen Hemminger <stephen@networkplumber.org>]

On Sat, 31 Mar 2018 14:48:55 -0400
Neil Horman <nhorman@tuxdriver.com> wrote:

> On Sat, Mar 31, 2018 at 06:21:41PM +0200, Gaëtan Rivet wrote:
> > On Sat, Mar 31, 2018 at 11:27:55AM -0400, Neil Horman wrote:  
> > > On Sat, Mar 31, 2018 at 05:09:47PM +0200, Gaëtan Rivet wrote:  
> > > > On Sat, Mar 31, 2018 at 09:33:43AM -0400, Neil Horman wrote:  
> > > > > On Fri, Mar 30, 2018 at 10:47:09PM +0800, Tonghao Zhang wrote:  
> > > > > > I rebuild it on ubuntu 17.10 and cash it. I use the 'RTE_SET_USED' to fix it.
> > > > > > 
> > > > > > 
> > > > > > diff --git a/lib/librte_vhost/fd_man.c b/lib/librte_vhost/fd_man.c
> > > > > > index 771675718..f11803191 100644
> > > > > > --- a/lib/librte_vhost/fd_man.c
> > > > > > +++ b/lib/librte_vhost/fd_man.c
> > > > > > @@ -279,7 +279,8 @@ fdset_pipe_read_cb(int readfd, void *dat __rte_unused,
> > > > > >                    int *remove __rte_unused)
> > > > > >  {
> > > > > >         char charbuf[16];
> > > > > > -       read(readfd, charbuf, sizeof(charbuf));
> > > > > > +       int r = read(readfd, charbuf, sizeof(charbuf));
> > > > > > +       RTE_SET_USED(r);
> > > > > >  }
> > > > > > 
> > > > > >  void
> > > > > > @@ -319,5 +320,6 @@ fdset_pipe_init(struct fdset *fdset)
> > > > > >  void
> > > > > >  fdset_pipe_notify(struct fdset *fdset)
> > > > > >  {
> > > > > > -       write(fdset->u.writefd, "1", 1);
> > > > > > +       int r = write(fdset->u.writefd, "1", 1);
> > > > > > +       RTE_SET_USED(r);
> > > > > >  }
> > > > > >   
> > > > > 
> > > > > A better option might be to use _Pragma
> > > > > 
> > > > > Something like this perhaps
> > > > > 
> > > > > #define ALLOW_UNUSED(x) \
> > > > > _Pragma(push) \
> > > > > _Pragma(diagnostic ignored "-Wunused-result") \
> > > > > #x;\
> > > > > _Pragma(pop)
> > > > > 
> > > > > This is of course untested, so it probably needs some tweaking, but this method
> > > > > avoids the need to declare an additional stack variable, which i don't think can
> > > > > be eliminated due to the cast.  I believe that this method should also work
> > > > > accross compilers (the gcc and clang compilers support this, and i think the
> > > > > intel compiler should as well)
> > > > > 
> > > > > Neil
> > > > >   
> > > > 
> > > > It would be nice to avoid the definition of a useless variable.
> > > > An alternative could be
> > > > 
> > > >    if (read() < 0) {
> > > >        /* Failure here is acceptable for such and such reason. */
> > > >    }
> > > > 
> > > > to ensure all-around compatibility, and the definition or another macro.
> > > > Just a suggestion.
> > > >   
> > > That would be a good alternative, but I think its effectiveness is dependent on
> > > when the compiler does with the return value check. Without any code inside the
> > > conditional, the compiler may optimize the check out, meaning the warning will
> > > still be asserted.  If it doesn't optimize the check out, then you have a
> > > useless compare and jump instruction left in the code path.
> > > 
> > > Best
> > > Neil
> > >   
> > 
> > I tested quickly, I see no difference with the three methods:  
> 
> gcc seems to be sufficiently smart to optimize out the conditional, clang not so
> much:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> 
> __attribute__((warn_unused_result))
> int wur(void)
> {
> 	printf("CALLING WUR!\n");
>         return read(0, NULL, 0);
> }
> 
> #define UNUSED_RESULT(x) if (x) {}
> 
> int main(void)
> {
> 	UNUSED_RESULT(wur());
>         return 0;
> }
> 
> [nhorman@neilslaptop ~]$ gcc -g -Wunused-result -Werror ./test.c
> [nhorman@neilslaptop ~]$ objdump -d -S a.out > ./results
> [nhorman@neilslaptop ~]$ cat results
> ... 
> 000000000040054b <main>:
> 
> #define UNUSED_RESULT(x) if (x) {}
> 
> int main(void)
> {
>   40054b:       55                      push   %rbp
>   40054c:       48 89 e5                mov    %rsp,%rbp
>         UNUSED_RESULT(wur());
>   40054f:       e8 d3 ff ff ff          callq  400527 <wur>
>         return 0;
>   400554:       b8 00 00 00 00          mov    $0x0,%eax
> }
>   400559:       5d                      pop    %rbp
>   40055a:       c3                      retq
>   40055b:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 
> 
> [nhorman@neilslaptop ~]$ clang -g -Wunused-result -Werror ./test.c
> [nhorman@neilslaptop ~]$ objdump -d -S a.out > ./results
> [nhorman@neilslaptop ~]$ cat results 
> ...
> 0000000000400570 <main>:
> }
> 
> #define UNUSED_RESULT(x) if (x) {}
> 
> int main(void)
> {
>   400570:       55                      push   %rbp
>   400571:       48 89 e5                mov    %rsp,%rbp
>   400574:       48 83 ec 10             sub    $0x10,%rsp
>   400578:       c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
>         UNUSED_RESULT(wur());
>   40057f:       e8 ac ff ff ff          callq  400530 <wur>
>   400584:       83 f8 00                cmp    $0x0,%eax
>   400587:       0f 84 05 00 00 00       je     400592 <main+0x22>
>   40058d:       e9 00 00 00 00          jmpq   400592 <main+0x22>
>   400592:       31 c0                   xor    %eax,%eax
>         return 0;
>   400594:       48 83 c4 10             add    $0x10,%rsp
>   400598:       5d                      pop    %rbp
>   400599:       c3                      retq
>   40059a:       66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
> 
> 
> There is an additional compare and two jump statements there.  I'm sure
> eventually most compilers will figure out how to eliminate this, and it might
> even do so now with the right optimization flags, but I think its best to just
> organize the source such that no conditional branching is implied.  Assuming the
> intel compiler supports it (which I think it should, can someone with access to
> it confirm), the _Pragma utility is probably the most clear way to do that.
> 
> Regards
> Neil


Rather than wallpapering over the unused result, why not do real error checking?
If the program was run in a non-Linux environment (such as WSL etc), maybe an error
could occur. Best to return an error; or at least call rte_exit().