Re: Simulation of MIPS recip and rsqrt instructions

[Andrew Cagney <ac131313@redhat.com>]

> The mips simulator uses sim_fpu_inv() as a reciprocal function
> but all it really does is negate the exponent.  So we end up
> calculating 1 / (x*2^y) as x*2^-y.
> 
> Test case:
> 
>     #include <stdio.h>
> 
>     int
>     main ()
>     {
>       double d;
>       float f;
> 
>       asm ("recip.s %0,%1" : "=f" (f) : "f" (20.0f));
>       printf ("%g\n", f);
> 
>       asm ("recip.d %0,%1" : "=f" (d) : "f" (10.0));
>       printf ("%g\n", d);
> 
>       asm ("rsqrt.s %0,%1" : "=f" (f) : "f" (0.01f));
>       printf ("%g\n", f);
> 
>       asm ("rsqrt.d %0,%1" : "=f" (d) : "f" (25.0));
>       printf ("%g\n", d);
> 
>       return 0;
>     }

Can this test case be written in strict assembler so that it can be 
added to the simulator testsuite?

> The simulator says:
> 
>     0.078125
>     0.15625
>     25.6
>     0.3125
> 
> while a vr5500 says:
> 
>     0.05
>     0.1
>     10
>     0.2
> 
> Unfortunately, sim_fpu_inv isn't commented and cp1.c seems to be
> its only active user.  Which should change?
> 
> FWIW, the patch below fixes the test case.  Please install if OK.

Hmm, I guess:

	1 / (x * 2^y) == (1/x) * (1/(2^y)) == (1/x) * (2^(-y))

and
	1/x != x

so I think the inv() function should be fixed - that suggests it is 
pretty broken.

Does GLIBC's softfloat code contain an efficient INV implementation that 
could be used?

Andrew