https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99971
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
Ever confirmed|0 |1
Last reconfirmed| |2021-04-09
Status|UNCONFIRMED |ASSIGNED
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. While we manage to analyze for the "perfect" solution" we fail
because dependence testing doesn't handle a piece, this throws away half
of the vectorization. We do actually see that we'll retain the scalar
loads and computations but still doing three vector loads and a vector add
seems cheaper than doing four scalar stores:
0x1fdb5a0 x_2(D)->a 1 times unaligned_load (misalign -1) costs 12 in body
0x1fdb5a0 y1_3(D)->a 1 times unaligned_load (misalign -1) costs 12 in body
0x1fdb5a0 _13 + _14 1 times vector_stmt costs 4 in body
0x1fdb5a0 _15 1 times unaligned_store (misalign -1) costs 12 in body
0x1fddcb0 _15 1 times scalar_store costs 12 in body
0x1fddcb0 _18 1 times scalar_store costs 12 in body
0x1fddcb0 _21 1 times scalar_store costs 12 in body
0x1fddcb0 _24 1 times scalar_store costs 12 in body
t.C:28:1: note: Cost model analysis:
Vector inside of basic block cost: 40
Vector prologue cost: 0
Vector epilogue cost: 0
Scalar cost of basic block: 48
t.C:28:1: note: Basic block will be vectorized using SLP
now, fortunately GCC 11 will improve on this [a bit] and we'll produce
_Z4testR1ARKS_S2_:
.LFB2:
.cfi_startproc
movdqu (%rsi), %xmm0
movdqu (%rdi), %xmm1
paddd %xmm1, %xmm0
movups %xmm0, (%rdi)
movd %xmm0, %eax
subl (%rdx), %eax
movl %eax, (%rdi)
pextrd $1, %xmm0, %eax
subl 4(%rdx), %eax
movl %eax, 4(%rdi)
pextrd $2, %xmm0, %eax
subl 8(%rdx), %eax
movl %eax, 8(%rdi)
pextrd $3, %xmm0, %eax
subl 12(%rdx), %eax
movl %eax, 12(%rdi)
ret
which is not re-doing the scalar loads/adds but instead uses the vector
result. Still the same dependence issue is present:
t.C:16:11: missed: can't determine dependence between y1_3(D)->b and
x_2(D)->a
t.C:16:11: note: removing SLP instance operations starting from: x_2(D)->a =
_6;
the scalar code before vectorization looks like
<bb 2> [local count: 1073741824]:
_13 = x_2(D)->a;
_14 = y1_3(D)->a;
_15 = _13 + _14;
x_2(D)->a = _15;
_16 = x_2(D)->b;
_17 = y1_3(D)->b; <---
_18 = _16 + _17;
x_2(D)->b = _18;
_19 = x_2(D)->c;
_20 = y1_3(D)->c;
_21 = _19 + _20;
x_2(D)->c = _21;
_22 = x_2(D)->d;
_23 = y1_3(D)->d;
_24 = _22 + _23;
x_2(D)->d = _24;
_5 = y2_4(D)->a;
_6 = _15 - _5;
x_2(D)->a = _6; <---
_7 = y2_4(D)->b;
_8 = _18 - _7;
x_2(D)->b = _8;
_9 = y2_4(D)->c;
_10 = _21 - _9;
x_2(D)->c = _10;
_11 = y2_4(D)->d;
_12 = _24 - _11;
x_2(D)->d = _12;
return;
Using
void test(A& __restrict x, A const& y1, A const& y2)
{
x += y1;
x -= y2;
}
produces optimal assembly even with GCC 10:
_Z4testR1ARKS_S2_:
.LFB2:
.cfi_startproc
movdqu (%rsi), %xmm0
movdqu (%rdx), %xmm1
movdqu (%rdi), %xmm2
psubd %xmm1, %xmm0
paddd %xmm2, %xmm0
movups %xmm0, (%rdi)
ret
note that I think we should be able to handle the dependences even without
the __restrict annotation.
