[PATCH v3] elf: fixes compile error when both enable -Werror and -DNDEBUG

[Carlos O'Donell]

On 4/2/22 05:51, Yang Yanchao via Libc-alpha wrote:
> Use -Werror and -DNDEBUG at the same time will
> causes the following compilation errors:

Thank you for following up with a v3. A little more work and I think
this will be ready. Thank you for testing with -Werror and -DNDEBUG.
This is a configuration we used to test in Fedora, but have since
stopped using -DNDEBUG to keep the asserts in place. I need to consider
adding such an option to build-many-glibcs testing.

> cache.c: In function 'save_cache':
> cache.c:758:15: error: unused variable 'old_offset' [-Werror=unused-variable]
>   758 |       off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
>       |               ^~~~~~~~~~
> 
> -DNDEBUG will disables the assertion.
> Therefore, only the variables used by assertions do not take effect.
> Fooling the Compiler with Type Conversions.
> ---
>  elf/cache.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/elf/cache.c b/elf/cache.c
> index dbf4c83a7a..68cd4d0828 100644
> --- a/elf/cache.c
> +++ b/elf/cache.c
> @@ -754,6 +754,7 @@ save_cache (const char *cache_name)
>      {
>        /* Align file position to 4.  */
>        off64_t old_offset = lseek64 (fd, extension_offset, SEEK_SET);
> +      (void) old_offset;

The compiler unused-variable warnings are very aggressive, and this may not fix all issues.

In cases like these we try to improve the situation by using specific pragmas.

e.g.

/* With GCC X.Y when compiling with -Werror, -DNDEBUG, and optimizations, the
   compiler may be able to eliminate the use of extension_offset and so we
   get an unused variable warning for extension_offset.  We want to continue to
   support the assert, so we disable this warning for this code.  */
DIAG_PUSH_NEEDS_COMMENT;
DIAG_IGNORE_NEEDS_COMMENT (X, "-Werror=unused-variable")
assert ((unsigned long long int) (extension_offset - old_offset) < 4);
DIAG_POP_NEEDS_COMMENT;

This is just an example, with X being the major version of gcc, and y being the minor.

You'll need to test this out, and double check CI/CD passes:
https://patchwork.sourceware.org/project/glibc/list/
You can view your posts there with success/warn/fail results.

>        assert ((unsigned long long int) (extension_offset - old_offset) < 4);
>        write_extensions (fd, str_offset, extension_offset);
>      }

Looking forward to a v4.

-- 
Cheers,
Carlos.