When the evaluation of a __builtin_constant_p argument would have a side effect, this argument should not be regarded as a constant because in practice, it may be used in an expression that evaluates the argument several times (for instance, think of a macro to compute min(x,y), avoiding a function call when x and y are constants). The current tcc (8fb8d88 in mob branch) fails to behave correctly, as shown on the following example.

#include <stdio.h>

#define FOO(X) (__builtin_constant_p (X) ? ((X), (X), (X)) : (X))

int main (void)
{
  int c = 0, i;

  i = FOO ((c++, 7));
  printf ("%d %d\n", c, i);
  return c == 1 && i == 7 ? 0 : 1;
}

With GCC and Clang, I get "1 7" since FOO ((c++, 7)) is replaced by ((c++, 7)).

But with tcc, I get "3 7".