https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110104
Bug ID: 110104
Summary: gcc produces sub-optimal code for _addcarry_u64 chain
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: slash.tmp at free dot fr
Target Milestone: ---
Consider the following code:
#include <x86intrin.h>
typedef unsigned long long u64;
typedef unsigned __int128 u128;
void testcase1(u64 *acc, u64 a, u64 b)
{
u128 res = (u128)a*b;
u64 lo = res, hi = res >> 64;
unsigned char cf = 0;
cf = _addcarry_u64(cf, lo, acc[0], acc+0);
cf = _addcarry_u64(cf, hi, acc[1], acc+1);
cf = _addcarry_u64(cf, 0, acc[2], acc+2);
}
void testcase2(u64 *acc, u64 a, u64 b)
{
u128 res = (u128)a * b;
u64 lo = res, hi = res >> 64;
asm("add %[LO], %[D0]\n\t" "adc %[HI], %[D1]\n\t" "adc $0, %[D2]" :
[D0] "+m" (acc[0]), [D1] "+m" (acc[1]), [D2] "+m" (acc[2]) :
[LO] "r" (lo), [HI] "r" (hi) : "cc");
}
Compilation with either
gcc-trunk -Wall -Wextra -O3 -S testcase.c
gcc-trunk -Wall -Wextra -Os -S testcase.c
generate the same code:
// rdi = acc, rsi = a, rdx = b
testcase1:
movq %rsi, %rax
mulq %rdx
addq %rax, (%rdi)
movq %rdx, %rax
adcq 8(%rdi), %rax
adcq $0, 16(%rdi)
movq %rax, 8(%rdi)
ret
testcase2:
movq %rsi, %rax ; rax = rsi = a
mulq %rdx ; rdx:rax = rax*rdx = a*b
add %rax, (%rdi) ; acc[0] += lo
adc %rdx, 8(%rdi) ; acc[1] += hi + cf
adc $0, 16(%rdi) ; acc[2] += cf
ret
Conclusion:
gcc generates the expected code for testcase2.
However, the code for testcase1 is sub-optimal.
movq %rdx, %rax
adcq 8(%rdi), %rax
movq %rax, 8(%rdi)
instead of
adc %rdx, 8(%rdi) ; acc[1] += hi + cf
The copy of rdx to rax is useless.
The (load/add+store) ops can be merged into an load/add/store op.
