[wellylug] asm stylings?

[Daniel Pittman]

Rob Giltrap <rob at kiwihq.com> writes:
> Daniel Pittman wrote:
>> Rob Giltrap <rob at kiwihq.com> writes:
>>> Daniel Pittman wrote:
>>>
>>>> #define __MULH64(__x, __y)    \
>>>>       	({ uint64 __lo, __hi;        \
>>>> 	__asm__("mulq %3"
>>>> 		: "=a" (__lo), "=d" (__hi) 	/* output */
>>>> 		: "%0" (__x), "rm" (__y)	/* input */)
>>>> 		; __hi; /* return value*/
>>>> 		})
>>>>
>>>> So what we want is a function that multiplies two 64
>>>> bit values together and returns the upper 64 bit value.
>>>>
>>>> The asm becomes an inline for SunStudio as follows:
>>>>
>>>>         .inline __MULH64, 8
>>>>         movq %rdi,%rax
>>>>         mulq %rsi
>>>>         movl %edx,%eax
>>>>         .end
>>>>
>>> So now what I need is to decipher the next three lines...
>>>
>>> #define MUL64x32(  __x,__y,__lo,__hi)    asm("mulq %3" : "=a" (__lo),
>>> "=d" (__hi) : "%0" (__x), "rm" (__y) );
>>
>> I presume you know what mulq means.

[...]

> If this piece of code is multiplying a 64bit number * 32bit number
> then does one of the registers have to be 64 bit e.g movl %eax becomes
> movq %rax?

Sorry, yes.  mulq is a 64-bit instruction, so I should have said 'rax'
and 'rdx' rather than 'eax' and 'edx' in there.  Force of habit; I still
think of the 32-bit full width registers as the "full width registers."

Regards,
        Daniel
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