tom@xxxxxxxxxx writes:
> What's the right way of writing the 'compute a*b%c' intrinsic?
>
> I can do
>
> #define mulmod(a,b,c,d,e) { asm ("mul %%rbx; div %%rsi;" : "=d" (d),
> "=a" (e) : "a" (a), "b" (b), "S" (c));}
>
> but that causes quite a lot of ugly register-shuffling
>
> If I do
>
> #define mulmod(a,b,c,d,e) { asm ("mul %0; div %1;" : "=d" (d), "=a"
> (e) : "a" (a), "r" (b), "r" (c));}
>
> then the program doesn't work; disassembling
>
> u64 zul(u64 a, u64 b, u64 p)
> {
> u64 x,y;
> mulmod(a,b,p,x,y);
> return x;
> }
>
> gives
>
> 00000000004005b0 <_Z3zulyyy>:
> 4005b0: 48 89 f8 mov %rdi,%rax
> 4005b3: 48 f7 e2 mul %rdx
> 4005b6: 48 f7 f0 div %rax
> 4005b9: 48 89 d0 mov %rdx,%rax
> 4005bc: c3 retq
>
> which is clearly crazy since %rax from 4005b0 will have been
> overwritten by the mul command before it's used.
#define mulmod(a,b,c,d,e) \
{ \
asm ("mul %3\n\tdiv %4\n" \
: "=&d" (d), "=a" (e) \
: "1" (a), "g" (b), "g" (c)); \
}
Andrew.
