Hi! On Thu, Jan 16, 2020 at 07:11:36AM +0100, Christophe Leroy wrote: > I'm trying to see if we could enhance TCP checksum calculations by > splitting inline assembly blocks to give GCC the opportunity to mix it > with other stuff, but I'm getting difficulties with the carry. > > As far as I can read in the documentation, the z constraint represents > '‘XER[CA]’ carry bit (part of the XER register)' > > I've tried the following, but I get errors. Can you help ? > > unsigned long cksum(unsigned long a, unsigned long b, unsigned long c) > { > unsigned long sum; > unsigned long carry; > > asm("addc %0, %2, %3" : "=r"(sum), "=z"(carry) : "r"(a), "r"(b)); > asm("adde %0, %0, %2" : "+r"(sum), "+z"(carry) : "r"(c)); > asm("addze %0, %0" : "+r"(sum) : "z"(carry)); > > return sum; > }
The only register allowed by "z" is a fixed register. You cannot use "z" in inline asm. Just write this as C? It should do a reasonable job of it. If you want *good* code, you need to write it in *actual* assembler code, anyway (hand scheduled and everything). Segher