https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54939
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we now would vectorize this but compute the SLP vectorization as never
profitable. Generated code with -fno-vect-cost-model (-Ofast -march=corei7):
.L4:
movupd (%rdx,%rax), %xmm0
movapd %xmm3, %xmm1
mulpd %xmm0, %xmm1
palignr $8, %xmm0, %xmm0
mulpd %xmm2, %xmm0
addsubpd %xmm0, %xmm1
movupd (%rcx,%rax), %xmm0
addpd %xmm0, %xmm1
movups %xmm1, (%rcx,%rax)
addq $16, %rax
cmpq %rsi, %rax
jne .L4
t.f:6:0: note: Cost model analysis:
Vector inside of loop cost: 15
Vector prologue cost: 10
Vector epilogue cost: 0
Scalar iteration cost: 14
Scalar outside cost: 6
Vector outside cost: 10
prologue iterations: 0
epilogue iterations: 0
t.f:6:0: note: cost model: the vector iteration cost = 15 divided by the scalar
iteration cost = 14 is greater or equal to the vectorization factor = 1.
t.f:6:0: note: not vectorized: vectorization not profitable.
t.f:6:0: note: not vectorized: vector version will never be profitable.
as it is basically equal to basic-block vectorizing the loop body. Note that
we pessimistically handle addsubpd as if it were not present and the code
would really end up as
vect__31.16_42 = vect__27.10_49 - vect__28.15_43;
vect__31.17_41 = vect__27.10_49 + vect__28.15_43;
_40 = VEC_PERM_EXPR <vect__31.16_42, vect__31.17_41, { 0, 3 }>;
which is what the vectorizer handles this with (two vector_stmt plus one
vec_perm cost). The x86 vectorizer cost model would need to be adjusted
for this.
With cost model enabled we fall back to vectorization using interleaving:
.L5:
movupd (%rax), %xmm5
addl $1, %r9d
addq $32, %rax
addq $32, %r8
movupd -32(%r8), %xmm1
movupd -16(%r8), %xmm0
movapd %xmm5, %xmm2
movapd %xmm1, %xmm6
movupd -16(%rax), %xmm9
unpckhpd %xmm0, %xmm1
unpcklpd %xmm0, %xmm6
movapd %xmm6, %xmm0
mulpd %xmm8, %xmm0
unpcklpd %xmm9, %xmm2
unpckhpd %xmm9, %xmm5
mulpd %xmm7, %xmm6
addpd %xmm0, %xmm2
movapd %xmm1, %xmm0
mulpd %xmm7, %xmm0
mulpd %xmm8, %xmm1
subpd %xmm0, %xmm2
movapd %xmm1, %xmm0
addpd %xmm6, %xmm0
movapd %xmm2, %xmm1
addpd %xmm5, %xmm0
unpcklpd %xmm0, %xmm1
unpckhpd %xmm0, %xmm2
movups %xmm1, -32(%rax)
movups %xmm2, -16(%rax)
cmpl %esi, %r9d
jb .L5
t.f:6:0: note: Cost model analysis:
Vector inside of loop cost: 26
Vector prologue cost: 10
Vector epilogue cost: 14
Scalar iteration cost: 14
Scalar outside cost: 6
Vector outside cost: 24
prologue iterations: 0
epilogue iterations: 1
Calculated minimum iters for profitability: 6
t.f:6:0: note: Runtime profitability threshold = 5
t.f:6:0: note: Static estimate profitability threshold = 16
Thus this is now a cost model issue.
