On Sat, 31 Mar 2018 14:48:55 -0400
Neil Horman <nhor...@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().
