On May 3, 7:13 am, John Nagle <na...@[EMAIL PROTECTED]
> wrote:
> Mathias Gaunard wrote:
> > On 30 avr, 01:38, Le Chaud Lapin <jaibudu...@[EMAIL PROTECTED]
> wrote:
>
> >> With macros, I don't have to worry about return value, or reference
> >> indirection elimination, or prolog/epilog code that I might have to
> >> struggle to get rid of. I simply use one line of code for say,
> >> MULTIPLY, and be done with it:
>
> Of course, you're assuming that the compiler puts the arguments
> in a standard place. This may inhibit optimizations.
Well, technically, I do not assume anything. :) That's the point. With
the inline function method, I have to consider the possibility that
the compiler might add code before and after the inline assembly
inside the inline function. With the macro method, I am certain that
it will not.
> Really, unless you need to emit some instruction you can't
> express at all in C/C++, it's no longer useful to use inline assembly
code.
In this case, there is.
I have never read Knuth 4.3.1 division algorithm, but rumor has it
that he prescribes that programmers implementing a big integer class
have access two double machine-word operations. So for example, if
unsigned int is 32-bits, then...
unsigned int A = (1 << 16); // 65,536
unsigned int B = (1 << 16); // 65,536
unsigned int P = A * B; // 2 ^ 32, will not fit in 32-bit word
That sole high-order bit at position 32 of P cannot be easily captured
in straight C/C++. Imagine trying to get at it in C++ with some
degree of efficiency compared to assembly. The difference in
performance will be dramatic.
Certainly, there is trickery to get at it, as I did to get my member
functions simply working for my big integer class. But due to poor
choice of algorithm, and lack of the optimization, the new, optimized
implementation, soon to be completed, will be 658 times faster than
the old implementation. I do not know how much of the 658 can be
attributed to trying to do in C/C++ what should have been done in
assembly, but my guess is that it's at least 2.5.
-Le Chaud Lapin-
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