相同的FLT_EVAL_METHOD，GCC / Clang中的结果不同

Question

The following program (adapted from here ) is giving inconsistent results when compiled with GCC (4.8.2) and Clang (3.5.1). In particular, the GCC result does not change even when FLT_EVAL_METHOD does.

#include <stdio.h>
#include <float.h>

int r1;
double ten = 10.0;

int main(int c, char **v) {
  printf("FLT_EVAL_METHOD = %d\n", FLT_EVAL_METHOD);
  r1 = 0.1 == (1.0 / ten);
  printf("0.1 = %a, 1.0/ten = %a\n", 0.1, 1.0 / ten);
  printf("r1=%d\n", r1);
}

Tests:

$ gcc -std=c99 t.c && ./a.out
FLT_EVAL_METHOD = 0
0.1 = 0x1.999999999999ap-4, 1.0/ten = 0x1.999999999999ap-4
r1=1

$ gcc -std=c99 -mpfmath=387 t.c && ./a.out
FLT_EVAL_METHOD = 2
0.1 = 0x0.0000000000001p-1022, 1.0/ten = 0x0p+0
r1=1

$ clang -std=c99 t.c && ./a.out
FLT_EVAL_METHOD = 0
0.1 = 0x1.999999999999ap-4, 1.0/ten = 0x1.999999999999ap-4
r1=1

$ clang -std=c99 -mfpmath=387 -mno-sse t.c && ./a.out
FLT_EVAL_METHOD = 2
0.1 = 0x0.07fff00000001p-1022, 1.0/ten = 0x0p+0
r1=0

Note that, according to this blog post , GCC 4.4.3 used to output 0 instead of 1 in the second test.

A possibly related question indicates that a bug has been corrected in GCC 4.6, which might explain why GCC's result is different.

I would like to confirm if any of these results would be incorrect, or if some subtle evaluation steps (eg a new preprocessor optimization) would justify the difference between these compilers.

Answer 1

This answer is about something that you should resolve before you go further, because it is going to make reasoning about what happens much harder otherwise:

Surely printing 0.1 = 0x0.07fff00000001p-1022 or 0.1 = 0x0.0000000000001p-1022 can only be a bug on your compilation platform caused by ABI mismatch when using -mfpmath=387 . None of these values can be excused by excess precision.

You could try to include your own conversion-to-readable-format in the test file, so that that conversion is also compiled with -mfpmath=387 . Or make a small stub in another file, not compiled with that option, with a minimalistic call convention:

In other file:

double d;
void print_double(void)
{
  printf("%a", d);
}

In the file compiled with -mfpmath=387 :

extern double d;
d = 0.1;
print_double();

Answer 2

Ignoring the printf problem which Pascal Cuoq addressed, I think GCC is correct here: according to the C99 standard, FLT_EVAL_METHOD == 2 should

evaluate all operations and constants to the range and precision of the long double type.

So, in this case, both 0.1 and 1.0 / ten are being evaluated to an extended precision approximation of 1/10.

I'm not sure what Clang is doing, though this question might provide some help.