Hello,
For fun, I'm trying to write code propagating carries in bignums.
gcc-trunk generates incorrect code, but I suspect that's
because my asm template is invalid (it's a byzantine syntax)
typedef unsigned long long u64;
void testcase(u64 *acc, u64 a, u64 b)
{
asm("add %[LO], %[D0]\n\t" "adc %[HI], %[D1]\n"
"1:\n\t" "adc $0, %[D2]\n\t" "lea %[D1], %[ACC]\n\t" "jc 1b" :
[D0] "+m" (acc[0]), [D1] "+m" (acc[1]), [D2] "+m" (acc[2]), [ACC] "+r" (acc) :
[LO] "r" (a), [HI] "r" (b) : "cc");
}
void foo(u64 *acc, u64 a, u64 b)
{
testcase(acc+0, a, b);
testcase(acc+1, a, b);
}
$ gcc-trunk -O3 -march=znver1 -S
testcase:
add %rsi, (%rdi) # acc[0] += a
adc %rdx, 8(%rdi) # acc[1] += b + cf
1:
adc $0, 16(%rdi) # acc[2] += cf
lea 8(%rdi), %rdi # ++acc
jc 1b # loop until cf = 0
ret
foo:
leaq 8(%rdi), %rcx # rcx = acc+1
movq %rdi, %rax # rax = acc
add %rsi, (%rax) # acc[0] += a
adc %rdx, 8(%rax) # acc[1] += b + cf
1:
adc $0, 16(%rax) # acc[2] += cf
lea 8(%rax), %rax # ++acc
jc 1b
# So far, so good
movq %rcx, %rax # rax = acc+1
add %rsi, (%rax) # acc[1] += a
# Not sure why we switch to rdi instead of rax at this point, but why not...
adc %rdx, 16(%rdi) # acc[2] += b + cf
1:
adc $0, 24(%rdi) # acc[3] += cf
# BROKEN: increments wrong reg, and by 2
lea 16(%rdi), %rax
jc 1b
ret
I guess I'm using the wrong constraints?
Regards
