[LLVMdev] question about -coverage
Bob Wilson
bob.wilson at apple.com
Fri Oct 4 12:33:29 PDT 2013
The instrumentation that I have proposed (on cfe-dev) for PGO is also intended to provide the necessary info for code coverage. I have not yet measured the performance of the code to write out the data, but it ought to be quite a bit faster than what we have now.
On Oct 4, 2013, at 1:40 AM, Kostya Serebryany <kcc at google.com> wrote:
> Another question is about the performance of coverage's at-exit actions (dumping coverage data on disk).
> I've built chromium's base_unittests with -fprofile-arcs -ftest-coverage and the coverage's at-exit hook takes 22 seconds,
> which is 44x more than I am willing to pay.
> Most of the time is spent here:
> #0 0x00007ffff3b034cd in msync () at ../sysdeps/unix/syscall-template.S:82
> #1 0x0000000003a8c818 in llvm_gcda_end_file ()
> #2 0x0000000003a8c914 in llvm_writeout_files ()
> #3 0x00007ffff2f5e901 in __run_exit_handlers
> The test depends on ~700 source files and so the profiling library calls msync ~700 times.
> Full chromium depends on ~12000 source files, so we'll be dumping the coverage data for 5 minutes this way.
> I understand that we have to support the lcov/gcov format (broken in may ways) and this may be the reason for being slow.
> But I really need something much faster (and maybe simpler).
>
> Is anyone planing any work on coverage in the nearest months?
> If no, we'll probably cook something simple and gcov-independent.
> Thoughts?
>
> --kcc
>
>
> On Thu, Oct 3, 2013 at 6:47 PM, Kostya Serebryany <kcc at google.com> wrote:
> Hello,
>
> I have few questions about coverage.
>
> Is there any user-facing documentation for clang's "-coverage" flag?
> The coverage instrumentation seems to happen before asan, and so if asan is also enabled
> asan will instrument accesses to @__llvm_gcov_ctr.
> This is undesirable and so we'd like to skip these accesses.
> Looks like GEP around @__llvm_gcov_ctr have special metadata attached:
> %2 = getelementptr inbounds [4 x i64]* @__llvm_gcov_ctr, i64 0, i64 %1
> %3 = load i64* %2, align 8
> %4 = add i64 %3, 1
> store i64 %4, i64* %2, align 8
> ...
> !1 = metadata !{...; [ DW_TAG_compile_unit ] ... /home/kcc/tmp/cond.cc] [DW_LANG_C_plus_plus]
>
> Can we rely on having this metadata attached to @__llvm_gcov_ctr?
> Should we attach some metadata to the actual accesses as well, or simply find the corresponding GEP?
>
> Finally, does anyone have performance numbers for coverage?
> As of today it seems completely thread-hostile since __llvm_gcov_ctr is not thread-local.
> A simple stress test shows that coverage slows down by 50x!
> % cat ~/tmp/coverage_mt.cc
> #include <pthread.h>
> __thread int x;
> __attribute__((noinline))
> void foo() {
> x++;
> }
>
> void *Thread(void *) {
> for (int i = 0; i < 100000000; i++)
> foo();
> return 0;
> }
>
> int main() {
> static const int kNumThreads = 16;
> pthread_t t[kNumThreads];
> for (int i = 0; i < kNumThreads; i++)
> pthread_create(&t[i], 0, Thread, 0);
> for (int i = 0; i < kNumThreads; i++)
> pthread_join(t[i], 0);
> return 0;
> }
>
> % clang -O2 ~/tmp/coverage_mt.cc -lpthread ; time ./a.out
> TIME: real: 0.284; user: 3.560; system: 0.000
> % clang -O2 ~/tmp/coverage_mt.cc -lpthread -coverage ; time ./a.out
> TIME: real: 13.327; user: 174.510; system: 0.000
>
> Any principal objections against making __llvm_gcov_ctr thread-local, perhaps under a flag?
>
> If anyone is curious, my intent is to enable running coverage and asan in one process.
>
> Thanks,
> --kcc
>
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