https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57534
--- Comment #34 from bin cheng <amker at gcc dot gnu.org> ---
So we could have three different addressing modes here.
1. What we have now:
leaq 0(,%rbp,8), %rax
movsd 8(%rbx,%rax), %xmm0
addsd (%rbx,%rbp,8), %xmm0
addq $8, %rbp
addsd 16(%rbx,%rax), %xmm0
addsd 24(%rbx,%rax), %xmm0
addsd %xmm0, %xmm1
movsd 32(%rbx,%rax), %xmm0
addsd 40(%rbx,%rax), %xmm0
addsd 48(%rbx,%rax), %xmm0
addsd 56(%rbx,%rax), %xmm0
addsd %xmm0, %xmm2
cmpq %rsi, %rbp
2. GCC-4.7:
fldl (%esi,%ebx,8)
lea 0x8(%ebx),%eax
faddl 0x8(%esi,%ebx,8)
cmp %eax,%edi
faddl 0x10(%esi,%ebx,8)
faddl 0x18(%esi,%ebx,8)
faddp %st,%st(2)
fldl 0x20(%esi,%ebx,8)
faddl 0x28(%esi,%ebx,8)
faddl 0x30(%esi,%ebx,8)
faddl 0x38(%esi,%ebx,8)
faddp %st,%st(1)
3. With slsr change:
leaq 0(%rbp,%rbx,8), %rax
addq $8, %rbx
movsd (%rax), %xmm0
addsd 8(%rax), %xmm0
addsd 16(%rax), %xmm0
addsd 24(%rax), %xmm0
addsd %xmm0, %xmm1
movsd 32(%rax), %xmm0
addsd 40(%rax), %xmm0
addsd 48(%rax), %xmm0
addsd 56(%rax), %xmm0
addsd %xmm0, %xmm2
cmpq %rsi, %rbx
This was reported that 2. is better than 1. Also Jeff recommended 3.
What I don't understand are:
A) why 2. is better than 1.? It seems to have more computations in address.
B) Is 3. the best one? It has the simplest addressing mode, but does require
one additional lea because of strength reduction.
Thanks.
