[v3] elf: fixes compile error when both enable -Werror and -DNDEBUG
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| dj/TryBot-apply_patch |
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Patch applied to master at the time it was sent
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| dj/TryBot-32bit |
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Build for i686
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Commit Message
Use -Werror and -DNDEBUG at the same time will
causes the following compilation errors:
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(+)
Comments
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.
@@ -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;
assert ((unsigned long long int) (extension_offset - old_offset) < 4);
write_extensions (fd, str_offset, extension_offset);
}