[coverage] support source-based static destructor coverage

[justincady]

It appears executed static destructors are not reported for code coverage:

// coverage.cc
#include <cstdio>

struct C {
  C() {
   printf("ctor called\n");
  }
  ~C() {
   printf("dtor called\n");
  }
};

static C c;

int main() {
  printf("main\n");
  return 0;
}

# build-cxx.sh
/usr/bin/clang++ --version | /bin/grep "clang version"
/usr/bin/clang++ -fprofile-instr-generate -fcoverage-mapping coverage.cc -o coverage
./coverage
/usr/bin/llvm-profdata merge -sparse default.profraw -o default.profdata
/usr/bin/llvm-cov show ./coverage -instr-profile=default.profdata

$ ./build-cxx.sh coverage.cc
clang version 22.0.0
ctor called
main
dtor called
    1|       |#include <cstdio>
    2|       |
    3|       |struct C {
    4|      1|  C() {
    5|      1|   printf("ctor called\n");
    6|      1|  }
    7|      0|  ~C() {
    8|      0|   printf("dtor called\n");
    9|      0|  }
   10|       |};
   11|       |
   12|       |static C c;
   13|       |
   14|      1|int main() {
   15|      1|  printf("main\n");
   16|      1|  return 0;
   17|      1|}

I believe this is because ~C() runs after static destruction triggers coverage output to be written via __llvm_profile_write_file (checked with GDB).

It seems this was fixed for gcov (--coverage) years ago:

• Support collecting coverage for static destructors #37750
• https://reviews.llvm.org/D82253

@MaskRay could we extend this fix to source-based code coverage?

[justincady]

So, the "problem" is in compiler-rt/lib/profile/InstrProfilingRuntime.cpp:

 13 static int RegisterRuntime() {
 14   __llvm_profile_initialize();
 15 #ifdef _AIX
 16   extern COMPILER_RT_VISIBILITY void *__llvm_profile_keep[];
 17   (void)*(void *volatile *)__llvm_profile_keep;
 18 #endif
 19   return 0;
 20 }
 21
 22 /* int __llvm_profile_runtime  */
 23 COMPILER_RT_VISIBILITY int INSTR_PROF_PROFILE_RUNTIME_VAR = RegisterRuntime();

• __llvm_profile_initialize registers the atexit handler for dumping coverage
• Any static constructor invoked before RegisterRuntime is invoked during static init is going to have its destructor run after coverage is dumped

It would be nice to prioritize the RegisterRuntime call via a constructor attribute but it's unclear to me how deeply AIX is dependent on the INSTR_PROF_PROFILE_RUNTIME_VAR initialization remaining exactly as is.

Another option might be to insert libprofile earlier on the link line (when it is implicitly added via -fprofile-instr-generate)?

A workaround: explicitly link -lclang_rt.profile prior to any other objects on the link line (e.g. adjusting the build script to use -fprofile-instr-generate -fcoverage-mapping -lclang_rt.profile coverage.cc -o coverage):

$ ./build-cxx.sh coverage.cc
clang version 22.0.0
ctor called
main
dtor called
    1|       |#include <cstdio>
    2|       |
    3|       |struct C {
    4|      1|  C() {
    5|      1|   printf("ctor called\n");
    6|      1|  }
    7|      1|  ~C() {
    8|      1|   printf("dtor called\n");
    9|      1|  }
   10|       |};
   11|       |
   12|       |static C c;
   13|       |
   14|      1|int main() {
   15|      1|  printf("main\n");
   16|      1|  return 0;
   17|      1|}