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.
Is there a good way of encoding 'any register other than rax or rdx'?
Many thanks in advance,
Tom
