Re: [Google] Add libgcov dummy references for weak symbols

[Xinliang David Li <davidxl at google dot com>]

Ok.

David

On Mon, Mar 10, 2014 at 1:28 PM, Teresa Johnson <tejohnson@google.com> wrote:
> This patch adds dummy references to libgcov for the symbols accessed
> via weak references from application code to ensure they are resolved
> at link time. Passes regression tests. Ok for google-4_8?
>
> Thanks,
> Teresa
>
> 2014-03-10  Teresa Johnson  <tejohnson@google.com>
>
> Google ref b/13338320.
>
> * libgcov-driver.c: Add dummy references to symbols accessed
>         via weak references.
>
> Index: libgcov-driver.c
> ===================================================================
> --- libgcov-driver.c (revision 208402)
> +++ libgcov-driver.c (working copy)
> @@ -57,19 +57,28 @@ extern struct gcov_info *get_gcov_list (void) ATTR
>  /* Create a strong reference to these symbols so that they are
>     unconditionally pulled into the instrumented binary, even when
>     the only reference is a weak reference. This is necessary because
> -   we are using weak references to handle older compilers that
> -   pre-date these new functions. A subtlety of the linker is that
> -   it will only resolve weak references defined within archive libraries
> -   when there is a string reference to something else defined within
> -   the same object file. Since these two functions are defined within
> -   their own object files (using L_gcov_reset and L_gcov_dump), they
> -   would not get resolved. Since there are symbols within the main L_gcov
> -   section that are strongly referenced during -fprofile-generate builds,
> -   these symbols will always need to be resolved.  */
> +   we are using weak references to enable references from code that
> +   may not be linked with libgcov. These are the only symbols that
> +   should be accessed via link references from application code!
> +
> +   A subtlety of the linker is that it will only resolve weak references
> +   defined within archive libraries when there is a strong reference to
> +   something else defined within the same object file. Since these functions
> +   are defined within their own object files, they would not automatically
> +   get resolved. Since there are symbols within the main L_gcov
> +   section that are strongly referenced during -fprofile-generate and
> +   -ftest-coverage builds, these dummy symbols will always need to be
> +   resolved.  */
>  void (*__gcov_dummy_ref1)(void) = &__gcov_reset;
>  void (*__gcov_dummy_ref2)(void) = &__gcov_dump;
>  extern char *__gcov_get_profile_prefix (void);
>  char *(*__gcov_dummy_ref3)(void) = &__gcov_get_profile_prefix;
> +extern void __gcov_set_sampling_period (unsigned int period);
> +char *(*__gcov_dummy_ref4)(void) = &__gcov_set_sampling_period;
> +extern unsigned int __gcov_sampling_enabled (void);
> +char *(*__gcov_dummy_ref5)(void) = &__gcov_sampling_enabled;
> +extern void __gcov_flush (void);
> +char *(*__gcov_dummy_ref6)(void) = &__gcov_flush;
>
>  /* Default callback function for profile instrumentation callback.  */
>  extern void __coverage_callback (gcov_type, int);
>
>
> --
> Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413