https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111166
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2023-08-28
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. We vectorize the scalar code:
> ./cc1 -quiet t.i -O2 -fopt-info-vec -fdump-tree-slp-details
weird_gcc_behaviour.c:15:41: optimized: basic block part vectorized using 16
byte vectors
generating
uint64_t turn_into_struct (u32 a, u32 b, u32 c, u32 d)
{
u32 * vectp.4;
vector(4) unsigned int * vectp.3;
struct quad_u32 D.2865;
uint64_t _11;
vector(4) unsigned int _13;
<bb 2> [local count: 1073741824]:
_13 = {a_2(D), b_4(D), c_6(D), d_8(D)};
MEM <vector(4) unsigned int> [(unsigned int *)&D.2865] = _13;
_11 = do_smth_with_4_u32 (D.2865);
D.2865 ={v} {CLOBBER(eol)};
return _11;
and
weird_gcc_behaviour.c:15:41: note: Cost model analysis:
a_2(D) 1 times scalar_store costs 12 in body
b_4(D) 1 times scalar_store costs 12 in body
c_6(D) 1 times scalar_store costs 12 in body
d_8(D) 1 times scalar_store costs 12 in body
a_2(D) 1 times vector_store costs 12 in body
node 0x5d35f70 1 times vec_construct costs 36 in prologue
weird_gcc_behaviour.c:15:41: note: Cost model analysis for part in loop 0:
Vector cost: 48
Scalar cost: 48
weird_gcc_behaviour.c:15:41: note: Basic block will be vectorized using SLP
we are choosing the vector side at same cost because we assume it would win
on code size. Practically a vector store instead of a scalar store is
also good for store forwarding.
We get
turn_into_struct:
.LFB0:
.cfi_startproc
movd %edi, %xmm1
movd %esi, %xmm4
movd %edx, %xmm0
movd %ecx, %xmm3
punpckldq %xmm4, %xmm1
punpckldq %xmm3, %xmm0
movdqa %xmm1, %xmm2
punpcklqdq %xmm0, %xmm2
movaps %xmm2, -24(%rsp)
movq -24(%rsp), %rdi
movq -16(%rsp), %rsi
jmp do_smth_with_4_u32
instead of (-fno-tree-vectorize)
turn_into_struct:
.LFB0:
.cfi_startproc
xorl %eax, %eax
movl %ecx, %r8d
movl %edi, %ecx
movl %edx, %r9d
movabsq $-4294967296, %r10
movq %rax, %rdi
xorl %edx, %edx
salq $32, %r8
andq %r10, %rdi
orq %rcx, %rdi
movq %rsi, %rcx
salq $32, %rcx
movl %edi, %esi
orq %rcx, %rsi
movq %rdx, %rcx
andq %r10, %rcx
movq %rsi, %rdi
orq %r9, %rcx
movl %ecx, %ecx
orq %r8, %rcx
movq %rcx, %rsi
jmp do_smth_with_4_u32
and our guess for code-size is correct (47 bytes for vector, 67 for scalar).
The latency for the scalar code is also quite a bit bigger. The spilling
should be OK, the store should forward nicely.
Unless you can come up with an actual benchmark showing the vector code is
slower I'd say it's not. Given it's smaller it should win on the icache
side if not executed frequently as well.
So - not a bug?
The spilling could be avoided by using movq, movhlps + movq, but it's
call handling so possibly difficult to achieve.
