[Bug gcov-profile/89468] New: [GCOV] wrong frequencies when the block is empty in a if statement

[yangyibiao at nju dot edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89468

            Bug ID: 89468
           Summary: [GCOV] wrong frequencies when the block is empty in a
                    if statement
           Product: gcc
           Version: 8.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: gcov-profile
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yangyibiao at nju dot edu.cn
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
8.2.0-1ubuntu2~18.04' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-8
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 8.2.0 (Ubuntu 8.2.0-1ubuntu2~18.04)

$ cat small.c
int main()
{
  int ret = 0;
  int a[2] = {1955, -1957};
  for (int i = 0; i < 2; ++i)
  {
    if (i % 2 == 1 && a[i] != -2 * i - 1955) {
      ret = 1;
    }
    else if (i % 2 == 0 && a[i] != 1955 + 2 * i) {
      // ret = 2;
    }
  }
  return ret;
}

$ gcc -O0 --coverage small.c -w; ./a.out; gcov small.c; cat small.c.gcov
File 'small.c'
Lines executed:87.50% of 8
Creating 'small.c.gcov'

        -:    0:Source:small.c
        -:    0:Graph:small.gcno
        -:    0:Data:small.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        1:    1:int main()
        -:    2:{
        1:    3:  int ret = 0;
        1:    4:  int a[2] = {1955, -1957};
        3:    5:  for (int i = 0; i < 2; ++i)
        -:    6:  {
        2:    7:    if (i % 2 == 1 && a[i] != -2 * i - 1955) {
    #####:    8:      ret = 1;
        -:    9:    }
       1*:   10:    else if (i % 2 == 0 && a[i] != 1955 + 2 * i) {
        -:   11:      // ret = 2;
        -:   12:    }
        -:   13:  }
        1:   14:  return ret;
        -:   15:}

Line #10 should be also executed twice as Line #7.

When Line #11 is not removed, the result is correct as:
$ gcc -O0 --coverage small.c -w; ./a.out; gcov small.c; cat small.c.gcov
File 'small.c'
Lines executed:77.78% of 9
Creating 'small.c.gcov'

        -:    0:Source:small.c
        -:    0:Graph:small.gcno
        -:    0:Data:small.gcda
        -:    0:Runs:1
        -:    0:Programs:1
        1:    1:int main()
        -:    2:{
        1:    3:  int ret = 0;
        1:    4:  int a[2] = {1955, -1957};
        3:    5:  for (int i = 0; i < 2; ++i)
        -:    6:  {
        2:    7:    if (i % 2 == 1 && a[i] != -2 * i - 1955) {
    #####:    8:      ret = 1;
        -:    9:    }
        2:   10:    else if (i % 2 == 0 && a[i] != 1955 + 2 * i) {
    #####:   11:      ret = 2;
        -:   12:    }
        -:   13:  }
        1:   14:  return ret;
        -:   15:}