Impossible Register Constraint in asm

Asked Oct 20 '14 at 14:24

Active Oct 20 '14 at 14:24

Viewed 336 times

I am trying to use this code for multiplication in a C++ file.

LL mul(LL a, LL b, LL m)
{
   LL q;
   LL r;
   asm( "mulq %3;"
   "divq %4;"
   : "=a"(q), "=d"(r)
   : "a"(a), "rm"(b), "rm"(m));
   return r;
}

But this is showing impossible register constraint in asm error.

asked Oct 20 '14 at 14:24

user1043936

which compiler are you using and how are you instantiating this? – Marco A. Oct 20 '14 at 14:47
I am using gcc 4.6.3 on Ubuntu 12 – user1043936 Oct 20 '14 at 14:57
For the input operand, try using the same-place constraint, that is `"0"(a)`. – Jester Oct 20 '14 at 15:01
Tried, still same error – user1043936 Oct 20 '14 at 15:04
Just tested using gcc 4.6.4 on [gcc.godbolt.org](http://gcc.godbolt.org/), it seems to work fine either way. I doubt the minor number makes much of a difference. I assume `LL` is `long`. That said, I don't see much point in using asm for this. – Jester Oct 20 '14 at 15:06
I am just trying to learn asm. Not using it in a project as such. It is compiling fine on 4.6.4. Thanks for the help – user1043936 Oct 20 '14 at 15:10
Which processor is the assembly language for? Processors can have different assembly languages. – Thomas Matthews Oct 20 '14 at 17:17
It is on Intel i3-2100 – user1043936 Oct 20 '14 at 17:41
I'm not getting "impossible constraint" either, although I can get "inconsistent operand constraints" if I compile the code for 32bit (I use `#define LL long long`). Is there anything interesting in the calling code? – David Wohlferd Oct 20 '14 at 20:27

0 Answers0