Floating point exceptions - gcc bug?

Consider the following code:

#include <fenv.h>
#include <stdio.h>
int main()
{
    #pragma STDC FENV_ACCESS ON
    1.0/0.0;
    printf("%x\n", fetestexcept(FE_ALL_EXCEPT));
}

I would expect it to print a nonzero value corresponding to FE_DIVBYZERO, but it prints 0. Changing the second line of main to double x = 1.0/0.0; gives the expected behavior. Is this permitted, or is it a bug?

Edit: For what it's worth, at first it may seem that in most real-world code, the operations which might cause f开发者_Python百科env exceptions to be raised could not be optimized out, so one could safely perform large computations and check at the end whether any overflow, div-by-zero, etc. happened. However, things get messy and a real issue emerges when you consider inlining and optimization. If such a function got inlined in a situation where it would always end up dividing by zero due to constant arguments, gcc might get really smart and optimize the whole inlined function essentially to return INFINITY; without raising any exceptions.

---

This is expected behaviour. gcc doesn't evaluate the expression, because it would have nothing to do with it afterwards.

If you compile with "-Wall", it warns you that the statement has no effect, and that it ignores the pragma statement.

GCC is not fully C99 compliant. For more information, see: http://gcc.gnu.org/c99status.html

For the issue of implementing this behaviour, see: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20785

---

This is something of a gray area. A strict reading of the floating-point environment section of the standard could easily lead one to believe that this is a bug. I suspect that the GCC maintainers will disagree with that reading, however.

For that matter, I'm not sure that GCC even claims to understand the FENV_ACCESS pragma. Certainly earlier versions did not.

---

Compiling with -Wall on gcc 4.6.0 says:

f.c:5:0: warning: ignoring #pragma FENV_ACCESS ON [-Wunknown-pragmas]

According to the GCC info pages:

* `The default state for the `FENV_ACCESS' pragma (C99 7.6.1).'

 This pragma is not implemented, but the default is to "off" unless
 `-frounding-math' is used in which case it is "on".

Unfortuantely, -frounding-math does not seem to have any effect on your program.

Arguably a compiler bug; I would ask on one of the GCC mailing lists.

---

It's possible that your compiler has optimized the original version out. Recognizing that the two constants are not "used" in any non-trivial sense, it may not even exist in the compiled binary.

The second example changes that, by actually assigning the operation to a variable.

---

I think the expression is getting optimised away in the first case, but not in the second. I can reproduce your results with gcc 4.2 with gcc -O0, but if I go to gcc -O3 then I get 0 in both cases.