On Thu, 28 May 2015, James Cowgill wrote:
> Your test case is wrong. If compiled without optimization, GCC call
> 'sqrt' from glibc instead of using the sqrt.s MIPS instruction. With
> optimization GCC will remove the call altogether. This is different to
> fortran because in fortran SQRT is a builtin intrinsic.
>
> Try this instead (compile with -O2 -lm):
>
> #include <math.h>
> float x = 1.1342362e-39;
> int main(void) {
> x = sqrt(x);
> return 0;
> }
Use:
#include <inttypes.h>
#include <stdio.h>
int main(void)
{
union {
float f;
uint32_t i;
} x = { .f = 1.1342362e-39 }, y;
printf("x: %08" PRIx32 ", %e\n", x.i, (double) x.f);
fflush(stdout);
asm volatile(
" sqrt.s %0, %1"
: "=f" (y.f) : "f" (x.f));
printf("y: %08" PRIx32 ", %e\n", y.i, (double) y.f);
fflush(stdout);
return 0;
}
or suchlike to make sure the required instruction is produced (at any
optimisation level), and to report the input and output bit patterns.
Substitute the initialisation of `x' as required (you can assign to
`x.f' or `x.i' as required). No need to link against -lm.
I have just tried this program with an R4400 (MIPS III) processor that
does trap on denormals, and Linux 4.1.0-rc4, so emulation itself is fine:
x: 000c59ca, 1.134236e-39
y: 1f1f0ab7, 3.367842e-20
For comparison you can run the kernel with the `nofpu' parameter so that
FPU hardware is permanently disabled and all FP instructions are emulated.
Maciej
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