https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69564
--- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
With -Ofast -g -march=native -funroll-loops we actually (for C) beat clang on
LU.
The SOR difference is most likely IVOPTs/scheduling/RA thing, there is nothing
to actually vectorize because of the dependencies.
clang 3.8 (no matter if -funroll-loops is used or not) unrolls the main SOR
loop 4 times and we get roughly:
.align 16, 0x90
.LBB1_12:
vmovsd -24(%r13,%rax,8), %xmm4
vaddsd -24(%r12,%rax,8), %xmm4, %xmm4
vmovsd -24(%rbx,%rax,8), %xmm5
vaddsd %xmm5, %xmm3, %xmm3
vaddsd %xmm3, %xmm4, %xmm3
vmulsd %xmm0, %xmm2, %xmm2
vfmadd231sd %xmm3, %xmm1, %xmm2
vmovsd %xmm2, -32(%rbx,%rax,8)
vmovsd -16(%r13,%rax,8), %xmm3
vaddsd -16(%r12,%rax,8), %xmm3, %xmm3
vmovsd -16(%rbx,%rax,8), %xmm4
vaddsd %xmm4, %xmm3, %xmm3
vaddsd %xmm3, %xmm2, %xmm2
vmulsd %xmm0, %xmm5, %xmm3
vfmadd231sd %xmm2, %xmm1, %xmm3
vmovsd %xmm3, -24(%rbx,%rax,8)
vmovsd -8(%r13,%rax,8), %xmm2
vaddsd -8(%r12,%rax,8), %xmm2, %xmm2
vmovsd -8(%rbx,%rax,8), %xmm5
vaddsd %xmm5, %xmm2, %xmm2
vaddsd %xmm2, %xmm3, %xmm2
vmulsd %xmm0, %xmm4, %xmm3
vfmadd231sd %xmm2, %xmm1, %xmm3
vmovsd %xmm3, -16(%rbx,%rax,8)
vmovsd (%r13,%rax,8), %xmm2
vaddsd (%r12,%rax,8), %xmm2, %xmm4
vmovsd (%rbx,%rax,8), %xmm2
vaddsd %xmm2, %xmm4, %xmm4
vaddsd %xmm4, %xmm3, %xmm4
vmulsd %xmm0, %xmm5, %xmm3
vfmadd231sd %xmm4, %xmm1, %xmm3
vmovsd %xmm3, -8(%rbx,%rax,8)
addq $4, %rax
cmpl %eax, %r15d
jne .LBB1_12
gcc -Ofast -g -march=native unrolls just twice:
.p2align 4,,10
.p2align 3
.L9:
vmovsd -8(%r8,%rdi,8), %xmm5
vmovsd -16(%r9,%rdi,8), %xmm1
vmulsd %xmm4, %xmm2, %xmm4
movslq %edi, %rax
vaddsd -16(%r10,%rdi,8), %xmm1, %xmm1
vaddsd %xmm0, %xmm5, %xmm0
vmulsd %xmm5, %xmm2, %xmm5
vaddsd %xmm0, %xmm1, %xmm0
vmovapd %xmm0, %xmm1
vfmadd132sd %xmm3, %xmm4, %xmm1
vmovsd (%r8,%rdi,8), %xmm4
vmovsd %xmm1, -16(%r8,%rdi,8)
vaddsd %xmm4, %xmm1, %xmm1
vmovsd -8(%r10,%rdi,8), %xmm0
vaddsd -8(%r9,%rdi,8), %xmm0, %xmm0
vaddsd %xmm1, %xmm0, %xmm0
vfmadd132sd %xmm3, %xmm5, %xmm0
vmovsd %xmm0, -8(%r8,%rdi,8)
addq $2, %rdi
cmpq %rcx, %rdi
jne .L9
and with additional -funroll-loops we unroll 8 times instead:
.L9:
vmovsd -8(%rax,%rdi,8), %xmm6
vmovsd -16(%r8,%rdi,8), %xmm5
vmulsd %xmm7, %xmm1, %xmm8
leaq 2(%rdi), %r14
vaddsd -16(%r9,%rdi,8), %xmm5, %xmm9
vmovsd (%rax,%rdi,8), %xmm12
leaq 4(%rdi), %r10
vaddsd %xmm3, %xmm6, %xmm10
vmulsd %xmm6, %xmm1, %xmm7
vmulsd %xmm12, %xmm1, %xmm0
vaddsd %xmm10, %xmm9, %xmm11
vfmadd132sd %xmm2, %xmm8, %xmm11
vmovsd %xmm11, -16(%rax,%rdi,8)
vaddsd %xmm12, %xmm11, %xmm15
vmovsd -8(%r9,%rdi,8), %xmm13
vaddsd -8(%r8,%rdi,8), %xmm13, %xmm14
vaddsd %xmm15, %xmm14, %xmm4
vfmadd132sd %xmm2, %xmm7, %xmm4
vmovsd %xmm4, -8(%rax,%rdi,8)
vmovsd -8(%rax,%r14,8), %xmm6
vmovsd -16(%r8,%r14,8), %xmm3
vaddsd -16(%r9,%r14,8), %xmm3, %xmm8
vmovsd (%rax,%r14,8), %xmm10
vaddsd %xmm4, %xmm6, %xmm5
vmulsd %xmm6, %xmm1, %xmm11
vmulsd %xmm10, %xmm1, %xmm4
vaddsd %xmm5, %xmm8, %xmm9
vfmadd132sd %xmm2, %xmm0, %xmm9
vmovsd %xmm9, -16(%rax,%r14,8)
vaddsd %xmm10, %xmm9, %xmm13
vmovsd -8(%r9,%r14,8), %xmm12
vaddsd -8(%r8,%r14,8), %xmm12, %xmm7
vaddsd %xmm13, %xmm7, %xmm14
vfmadd132sd %xmm2, %xmm11, %xmm14
vmovsd %xmm14, -8(%rax,%r14,8)
vmovsd -8(%rax,%r10,8), %xmm15
vmovsd -16(%r8,%r10,8), %xmm6
vaddsd -16(%r9,%r10,8), %xmm6, %xmm0
vmovsd (%rax,%r10,8), %xmm9
vaddsd %xmm14, %xmm15, %xmm3
vmulsd %xmm15, %xmm1, %xmm5
vmulsd %xmm9, %xmm1, %xmm14
vaddsd %xmm3, %xmm0, %xmm8
vfmadd132sd %xmm2, %xmm4, %xmm8
vmovsd %xmm8, -16(%rax,%r10,8)
vaddsd %xmm9, %xmm8, %xmm12
vmovsd -8(%r9,%r10,8), %xmm10
vaddsd -8(%r8,%r10,8), %xmm10, %xmm11
vaddsd %xmm12, %xmm11, %xmm7
vfmadd132sd %xmm2, %xmm5, %xmm7
vmovsd %xmm7, -8(%rax,%r10,8)
leaq 6(%rdi), %r10
addq $8, %rdi
vmovsd -8(%rax,%r10,8), %xmm13
vmovsd -16(%r8,%r10,8), %xmm15
vaddsd -16(%r9,%r10,8), %xmm15, %xmm6
vaddsd %xmm7, %xmm13, %xmm4
vmovsd (%rax,%r10,8), %xmm7
vmulsd %xmm13, %xmm1, %xmm8
vaddsd %xmm4, %xmm6, %xmm3
vfmadd132sd %xmm2, %xmm14, %xmm3
vmovsd %xmm3, -16(%rax,%r10,8)
vaddsd %xmm7, %xmm3, %xmm5
vmovsd -8(%r9,%r10,8), %xmm0
vaddsd -8(%r8,%r10,8), %xmm0, %xmm9
vaddsd %xmm5, %xmm9, %xmm0
vfmadd132sd %xmm2, %xmm8, %xmm0
vmovapd %xmm0, %xmm3
vmovsd %xmm0, -8(%rax,%r10,8)
cmpq %rcx, %rdi
jne .L9
The number of the v*sd instructions matches in between all 3 versions
proportionally to how many unrolls there are, but the non-unrolled version has
a weird extra move in there (the movslq %edi, %rax) even when it isn't used
anywhere in the loop, and the -funroll-loops version is just too weird, many
leaq insns when the constant could be added into the immediates in the
corresponding addresses.
