Tim Peters <[EMAIL PROTECTED]> wrote:
> > And for 32x32 -> 64, can't this simply be replaced by "(uint64_t)i * j",
> > where uint64_t is as in C99? I'd hope that most compilers would
> > manage to turn this into the appropriate 32x32-bit hardware multiply.
>
> 1. That's C99, not C89, and therefore less portable.
>
> 2. On platforms that support it, this is at least 64x64->64 multiplication,
> potentially much more expensive than the 32x32->64 (or 31x31->62?)
> flavor you /intend/ to move to.
>
> 3. There's no way to know exactly what compilers will do with this short of
> staring at generated code.
I've relied in the past for the compiler generating a 32*32->64 bit
multiply for this code
#include <stdint.h>
uint64_t mul(uint32_t a, uint32_t b)
{
return a*b;
}
Looking at the assembler it produces (x86)
mul:
pushl %ebp
xorl %edx, %edx
movl %esp, %ebp
movl 12(%ebp), %eax
imull 8(%ebp), %eax
popl %ebp
ret
Which I'm pretty sure is a 32x32->64 bit mul (though my x86 assembler
foo is weak).
I think a compiler would have to be pretty stupid not to take this
optimisation... But then there are some pretty stupid compilers out
there!
--
Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick
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