What's the right way of telling gcc that an input register is
clobbered?
I've two asm statements in the same function, both of which have "a"
(a) as an input constraint; the code corrupts RAX, and the optimiser is
not reloading a into RAX before starting the second bit of code.
If I add "rax" to the clobbers list, I get an error
modp2.cpp:55: error: can't find a register in class ___AREG___ while
reloading ___asm___
Also: how do I refer to r8 through r15 in register constraints on
x64? If I replace "r" (p) by "r9" (p) in the second asm statement, I
get an error
modp2.cpp:64: error: matching constraint references invalid operand
number
--- the function
asm("mulq %2\n mov %%rdx, %%r12 \n mov %%rax, %%r13 \n" \
"mov %3,%%rax \n mulq %%r13 \n mov %%rdx, %%r10 \n mov %%rax, %%
r11 \n" \
"mov %4,%%rax \n mulq %%r12 \n mov %%rdx, %%r8 \n mov %%rax, %%
r9 \n" \
"mov %3,%%rax \n mulq %%r12 \n add %%rax, %%r10 \n adc %%rdx, %%
r9 \n adc $0, %%r8 \n" \
"mov %4,%%rax \n mulq %%r13 \n add %%rax, %%r10 \n adc %%rdx, %%
r9 \n adc $0, %%r8 \n" \
"shrd %%cl, %%r8, %%r9\n mov %%r9, %%rdx"
: "=&d" (aus)
: "a" (a), "r" (b), "r" (il), "r" (ih), "c" (pis)
: "r8", "r9", "r10", "r11", "r12", "r13");
quotient=aus;
u64 rem;
asm("mulq %2\nmov %%rdx,%%r12 \n mov %%rax, %%r13 \n" \
"mov %3,%%rax\nmulq %4\n" \
"sub %%rax,%%r13 \n" \
"mov %%r13, %%rdx"
: "=&d" (rem)
: "a" (a), "r" (b), "c" (aus), "r" (p)
: "r12", "r13");
