gccclangyoucompleteme

GCC ignores diagnostic pragmas while Clang doesn't

GCC 10.2.0 and Clang 11.1.0.

I have the following piece of code:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-variable"
                uint8_t hour = p[8];
                uint8_t min = p[9];
                uint8_t sec = p[10];
                uint16_t msec = (p[11] * 4);
#pragma GCC diagnostic pop

Clang (which is used by the YouCompleteMe (YCM) Vim plugin) honors these pragmas and doesn't show any warning during file processing. I can confirm that by commenting these pragmas out - YCM diagnostics immediately show me warnings on corresponding lines.

However, GCC ignores them and I get the following in the console log:

packet.c:111:26: warning: unused variable ‘msec’ [-Wunused-variable]
  111 |                 uint16_t msec = (p[11] * 4);
      |                          ^~~~
packet.c:110:25: warning: unused variable ‘sec’ [-Wunused-variable]
  110 |                 uint8_t sec = p[10];
      |                         ^~~
packet.c:109:25: warning: unused variable ‘min’ [-Wunused-variable]
  109 |                 uint8_t min = p[9];
      |                         ^~~
packet.c:108:25: warning: unused variable ‘hour’ [-Wunused-variable]
  108 |                 uint8_t hour = p[8];
      |                         ^~~~

Flags used for build:

CFLAGS="-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -Wall -pedantic -Werror=format-security" CXXFLAGS="-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -Wall -pedantic -Werror=format-security" LDFLAGS="-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now"

What is going on here? It seems like a bug in GCC.

It seems like this is the case even for this simplest program:

int main(int argc, char *argv[]) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-variable"
    int x;
#pragma GCC diagnostic pop

    return 0;
}

Tests:

cd /tmp
clang -Wall main.c
gcc -Wall main.c

main.c: In function ‘main’:
main.c:4:9: warning: unused variable ‘x’ [-Wunused-variable]
    4 |     int x;
      |         ^

Some similar bug is described for g++ and still opened for about nine years!

Solution

  • It seems like it's related to the scoping. When I move push/pop #pragmas outside of a variable's block, GCC behaves well. For example:

    #pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-variable"
                uint8_t hour = p[8];
                uint8_t min = p[9];
                uint8_t sec = p[10];
                uint16_t msec = (p[11] * 4);
#pragma GCC diagnostic pop

    should be changed to the:

    #pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-variable"
            {
                uint8_t hour = p[8];
                uint8_t min = p[9];
                uint8_t sec = p[10];
                uint16_t msec = (p[11] * 4);
            }
#pragma GCC diagnostic pop

    Then both Clang and GCC work fine.