`-Wl,--gc-sections` removes codecov related symbols

[mati865]

Forwarding taiki-e/cargo-llvm-cov#254 (comment) here with a simple C reproducer.

Code:

#include <stdio.h>

int main(){
  printf("Hello");
  return 0;
}

1. Running without --gc-sections:

$ clang hello.c -fprofile-instr-generate -fcoverage-mapping

$ ./a.exe
Hello

$ llvm-profdata merge -sparse default.profraw -o hello.profdata

$ llvm-cov show a.exe -instr-profile=hello.profdata
    1|       |#include <stdio.h>
    2|       |
    3|      1|int main(){
    4|      1|  printf("Hello");
    5|      1|  return 0;
    6|      1|}

2. Running the same commands with --gc-sections:

$ clang hello.c -fprofile-instr-generate -fcoverage-mapping -Wl,--gc-sections

$ ./a.exe
Hello

$ llvm-profdata merge -sparse default.profraw -o hello.profdata

$ llvm-cov show a.exe -instr-profile=hello.profdata

(llvm-cov show prints nothing)

3. Reusing .profdata and .profraw from step 2 and binary without sections GC:

$ clang hello.c -fprofile-instr-generate -fcoverage-mapping

$ llvm-cov show a.exe -instr-profile=hello.profdata
warning: a.exe: profile data may be out of date - object is newer
    1|       |#include <stdio.h>
    2|       |
    3|      1|int main(){
    4|      1|  printf("Hello");
    5|      1|  return 0;
    6|      1|}

Only 2nd step is missing coverage info, which leads me to believe llvm-cov is looking for a symbol or section removed by --gc-sections. Especially removal of .lcovfun section by section GC is suspicious.