Re: Changing the treatment of the MIPS HI and LO registers

[Maxim Kuvyrkov <maxim@codesourcery.com>]

Richard Sandiford wrote:

...

> +    <li>The MIPS port no longer recognizes the <code>h</code>
> +    <code>asm</code> constraint.  It was necessary to remove
> +    this constraint in order to avoid generating unpredictable
> +    code sequences.
> +
> +    <p>One of the main uses of the <code>h</code> constraint
> +    was to extract the high part of a multiplication on
> +    64-bit targets.  For example:</p>
> +    <pre>
> +    asm ("dmultu\t%1,%2" : "=h" (result) : "r" (x), "r" (y));</pre>
> +    <p>You can now achieve the same effect using 128-bit types:</p>
> +    <pre>
> +    typedef unsigned int uint128_t __attribute__((mode(TI)));
> +    result = ((uint128_t) x * y) >> 64;</pre>
> +    <p>The second sequence is better in many ways.  For example,
> +    if <code>x</code> and <code>y</code> are constants, the
> +    compiler can perform the multiplication at compile time.
> +    If <code>x</code> and <code>y</code> are not constants,
> +    the compiler can schedule the runtime multiplication
> +    better than it can schedule an <code>asm</code> statement.</p>
> +    </li>
>   </ul>

Hi,

GLIBC contains the following code in stdlib/longlong.h:
<snip>
#if defined (__mips__) && W_TYPE_SIZE == 32
#define umul_ppmm(w1, w0, u, v) \
   __asm__ ("multu %2,%3"						\
	   : "=l" ((USItype) (w0)),					\
	     "=h" ((USItype) (w1))					\
	   : "d" ((USItype) (u)),					\
	     "d" ((USItype) (v)))
#define UMUL_TIME 10
#define UDIV_TIME 100
#endif /* __mips__ */
</snip>

What would be a correct fix in this case?  Something like this:
<snip>
#define umul_ppmm(w1, w0, u, v)					\
   ({unsigned int __attribute__((mode(DI))) __xx;		\
     __xx = (unsigned int __attribute__((mode(DI)))) u * v;	\
     w0 = __xx & ((1 << 32) - 1);				\
     w1 = __xx >> 32;})
</snip>

Or is there a better way?


Thanks,

Maxim