[Bug c/64439] New: Incorrect location of -Wunused-value or false negative

[Bug c/64439] New: Incorrect location of -Wunused-value or false negative

[chengniansun at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64439

            Bug ID: 64439
           Summary: Incorrect location of -Wunused-value or false negative
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chengniansun at gmail dot com

For the expression "a && ((b = 0) != 0)", GCC warns that the expression value
"((b = 0) != 0)" is not used. Should it point to the whole expression rather
this sub-expression? Or this is a false negative of -Wunused-value?


$: cat s.c 
void f(int a, int b) { 
  a && ((b = 0) != 0); 
}
$: 
$: gcc-trunk -c -Wunused-value s.c 
s.c: In function ‘f’:
s.c:2:17: warning: value computed is not used [-Wunused-value]
   a && ((b = 0) != 0); 
                 ^
$: 
$: clang-trunk -c -Wunused-value s.c
s.c:2:5: warning: expression result unused [-Wunused-value]
  a && ((b = 0) != 0); 
  ~ ^  ~~~~~~~~~~~~~~
1 warning generated.
$: 
$:
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From: "anlauf at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/64432] [5 Regression] SYSTEM_CLOCK(COUNT_RATE=rate) wrong result for integer(4)::rate
Date: Mon, 29 Dec 2014 19:54:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd432

--- Comment #7 from Harald Anlauf <anlauf at gmx dot de> ---
(In reply to Harald Anlauf from comment #5)
> (In reply to Francois-Xavier Coudert from comment #4)
> > If you have another idea, please post a list of what you think should happen
> > in all various cases (all possible combinations of arguments have to be
> > allowed).
>
> Let's see:
>
> - For any number of arguments present (1, 2 or 3)
>   - always integer(4): msec resolution (as before)
>   - always integer(>=8): usec resolution (or whatever is possible)
>   - always real: I don't care, but I think it might be a good idea
>     to use the same as for integer of a compatible kind.
>   - different types and/or kinds: I don't care, since one should
>     expect problems (wrapping or truncation) anyway.
>
> But presence of non-presence should never make a difference
> if consistent types and kinds are used.

I played around with other compilers and posted the result at:

https://groups.google.com/forum/?hl=en#!topic/comp.lang.fortran/5aoFFRNBP0g

The only compiler that allows for different timer resolutions and
produces an at least self-consistent result appears to be Intel.
It actually treats each argument separately: if it is a 4-byte
argument, it returns values referring to the low-res (0.1 msec) version;
for an 8-byte argument one gets the high-res (1 usec) version.
(Confirmed by looking at the explicit assembler code).

That would be even simpler than any of the versions I discussed above.

The actual implementation then might generate a call to
_gfortran_system_clock_{4,8} for each present argument,
with the respective kind taken into account.

[Bug c/64439] Incorrect location of -Wunused-value or false negative

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64439

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
It doesn't seem a false negative to me. 

It is a bit strange that we get different locations for equivalent expressions:

s.c:2:17: warning: value computed is not used [-Wunused-value]
   a && ((b = 0) != 0); 
                 ^
s.c:3:18: warning: value computed is not used [-Wunused-value]
   ((b = 0) != 0) && a;
                  ^

But I'm not sure if it is a bug. Someone would need to look at gcc while
compiling the testcase under a debugger to understand why the difference exists
at all.
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From: "dark_footix at yahoo dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug regression/64437] hang with iconv on the configure : "checking whether the C compiler works"
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd437

fredm <dark_footix at yahoo dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |INVALID

--- Comment #5 from fredm <dark_footix at yahoo dot fr> ---
Sorry, I did a mismatch of pre build lib with my previous toolchain

This bug is closed