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Re: [PATCH] Introduce libgcov.so run-time library (PR gcov-profile/84107).
On 08/29/2018 11:17 AM, Richard Biener wrote:
> On Wed, Aug 29, 2018 at 10:31 AM Martin Liška <mliska@suse.cz> wrote:
>>
>> Hello.
>>
>> Moving the thread from gcc ML into gcc-patches. That's first implementation
>> of shared libgcov library. Currently inclusion of t-libgcov is added only
>> to *-linux targets. Is it fine to add to all configurations that already
>> include 't-slibgcc t-slibgcc-elf-ver'?
>>
>> Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.
>
> I understand this is to make profiling work with multiple DSOs (all
> instrumented).
Yes.
> But does this also make the case work when those DSOs are dlopened/dlclosed
> multiple times?
That works fine even now, when a DSO is loaded, __gcov_init is called and
it registers gcov_info into __gcov_root.
Following sample:
#include <dlfcn.h>
#include <assert.h>
#include <unistd.h>
int main()
{
for (int i = 0; i < 10; i++)
{
void *handle = dlopen ("libfoo.so", RTLD_NOW);
assert (handle);
void (*fn) (void) = dlsym (handle, "foo");
assert (fn);
fn ();
sleep (1);
}
return 0;
}
has:
$ gcov-dump -l foo.gcda
foo.gcda:data:magic `gcda':version `A82*'
foo.gcda:stamp 2235622999
foo.gcda: a3000000: 22:PROGRAM_SUMMARY checksum=0x02bfe640
foo.gcda: counts=2, runs=1, sum_all=20, run_max=10, sum_max=10
foo.gcda: counter histogram:
foo.gcda: 9: num counts=2, min counter=10, cum_counter=20
foo.gcda: 01000000: 3:FUNCTION ident=1636255671, lineno_checksum=0x2172766e, cfg_checksum=0xc0bbb23e
foo.gcda: 01a10000: 4:COUNTERS arcs 2 counts
foo.gcda: 0: 10 10
foo.gcda: 01af0000: 2:COUNTERS time_profiler 1 counts
foo.gcda: 0: 1
which is fine, runs == 1 and arcs counter executed 10x.
I understand that with the shared libgcov implementation this
> library must be finalized last?
If you do profiling, then you depend on libgcov, thus it's loaded before a DSO (or executable)
is loaded.
Wouldn't it be better to have some additional
> object that lives in an executable only? Or do we want profiled DSOs without
> profiling the executable using it?
That should be rare, but it will be possible as __gcov_exit will be eventually called.
Does that case work with the shared libgcov
> (with dlopen/dlclose)?
Yes, I've just tested that.
Does it work when DSO1 is compiled with GCC N
> and DSO2 is compiled with GCC N+1 where the libgcov ABI changed?
For such case we have:
/* Exactly one of these will be live in the process image. */
struct gcov_master __gcov_master =
{GCOV_VERSION, 0};
...
and it's check in __gcov_init.
>
> I think we need a better understanding of the situation before jumping to
> the conclusion that a shared libgcov is the solution. Maybe better isolating
> the individual instances is better?
It's undesired as seen in the PR. When doing profiling of indirect jumps
we need have exactly one __gcov_indirect_call_callee. That's because one can
do indirect calls that cross DSO boundaries.
Martin
>
> Richard.
>
>> Martin
>>
>> libgcc/ChangeLog:
>>
>> 2018-08-28 Martin Liska <mliska@suse.cz>
>>
>> PR gcov-profile/84107
>> * Makefile.in: Build libgcov.so objects separately,
>> add rule for libgcov.map and how the library
>> is linked.
>> * config.host: Add t-libgcov and t-libgcov-elf-ver
>> to *-linux targets.
>> * config/t-libgcov: New file.
>> * config/t-libgcov-elf-ver: New file.
>> * libgcov-driver.c: Declare function if new L_gcov_shared
>> is defined.
>> * libgcov-interface.c: Likewise.
>> * libgcov-merge.c: Likewise.
>> * libgcov-profiler.c: Likewise.
>> * libgcov-std.ver.in: New file.
>> * libgcov.h (__gcov_init): Remove ATTRIBUTE_HIDDEN.
>> (__gcov_exit): Likewise.
>> (__gcov_merge_add): Likewise.
>> (__gcov_merge_time_profile): Likewise.
>> (__gcov_merge_single): Likewise.
>> (__gcov_merge_ior): Likewise.
>> (__gcov_merge_icall_topn): Likewise.
>> (gcov_sort_n_vals): Likewise.
>> (__gcov_fork): Likewise.
>> (__gcov_execl): Likewise.
>> (__gcov_execlp): Likewise.
>> (__gcov_execle): Likewise.
>> (__gcov_execv): Likewise.
>> (__gcov_execvp): Likewise.
>> (__gcov_execve): Likewise.
>> ---
>> libgcc/Makefile.in | 56 ++++++++++++++++++++++++++++++---
>> libgcc/config.host | 2 +-
>> libgcc/config/t-libgcov | 46 +++++++++++++++++++++++++++
>> libgcc/config/t-libgcov-elf-ver | 3 ++
>> libgcc/libgcov-driver.c | 4 +--
>> libgcc/libgcov-interface.c | 28 ++++++++---------
>> libgcc/libgcov-merge.c | 14 ++++-----
>> libgcc/libgcov-profiler.c | 34 ++++++++++----------
>> libgcc/libgcov-std.ver.in | 53 +++++++++++++++++++++++++++++++
>> libgcc/libgcov.h | 31 +++++++++---------
>> 10 files changed, 209 insertions(+), 62 deletions(-)
>> create mode 100644 libgcc/config/t-libgcov
>> create mode 100644 libgcc/config/t-libgcov-elf-ver
>> create mode 100644 libgcc/libgcov-std.ver.in
>>
>>