首先，我是SSE的新手。我决定加速我的代码，但看起来它的运行速度比我的本机代码慢。
这是一个示例，它计算平方和。在我的Intel i7-6700HQ上，本机代码需要0.43s，SSE需要0.52s。那么，瓶颈在哪里？
inline float squared_sum(const float x, const float y)
{
return x * x + y * y;
}
#define USE_SIMD
void calculations()
{
high_resolution_clock::time_point t1, t2;
int result_v = 0;
t1 = high_resolution_clock::now();
alignas(16) float data_x[4];
alignas(16) float data_y[4];
alignas(16) float result[4];
__m128 v_x, v_y, v_res;
for (int y = 0; y < 5120; y++)
{
data_y[0] = y;
data_y[1] = y + 1;
data_y[2] = y + 2;
data_y[3] = y + 3;
for (int x = 0; x < 5120; x++)
{
data_x[0] = x;
data_x[1] = x + 1;
data_x[2] = x + 2;
data_x[3] = x + 3;
#ifdef USE_SIMD
v_x = _mm_load_ps(data_x);
v_y = _mm_load_ps(data_y);
v_x = _mm_mul_ps(v_x, v_x);
v_y = _mm_mul_ps(v_y, v_y);
v_res = _mm_add_ps(v_x, v_y);
_mm_store_ps(result, v_res);
#else
result[0] = squared_sum(data_x[0], data_y[0]);
result[1] = squared_sum(data_x[1], data_y[1]);
result[2] = squared_sum(data_x[2], data_y[2]);
result[3] = squared_sum(data_x[3], data_y[3]);
#endif
result_v += (int)(result[0] + result[1] + result[2] + result[3]);
}
}
t2 = high_resolution_clock::now();
duration<double> time_span1 = duration_cast<duration<double>>(t2 - t1);
std::cout << "Exec time:\t" << time_span1.count() << " s\n";
}
我正在使用Visual Studio2017。针对x64编译。
编译器会生成已优化的代码，因此如今很难进一步加速它。您可以做的一件事就是提高并行化速度，以提高代码速度。
感谢您的回答。它们基本上是相同的，所以我接受SørenV. Poulsen的回答，因为它是第一个。
答案 0 :(得分：3)
现代编译是令人难以置信的机器，并且如果可能的话（并且带有正确的编译标志）将已经使用SIMD指令。
确定编译器正在做什么的一种通用策略是查看代码的反汇编。如果您不想在自己的计算机上执行此操作，则可以使用在线服务，例如Godbolt：https://gcc.godbolt.org/z/T6GooQ。
一个技巧是避免像您在此处那样atomic存储中间结果。原子值用于确保线程之间的同步，相对而言，这可能会花费很高的计算成本。
答案 1 :(得分：2)
遍历程序集以查找基于编译器的代码（无SIMD内容），
calculations():
pxor xmm2, xmm2
xor edx, edx
movdqa xmm0, XMMWORD PTR .LC0[rip]
movdqa xmm11, XMMWORD PTR .LC1[rip]
movdqa xmm9, XMMWORD PTR .LC2[rip]
movdqa xmm8, XMMWORD PTR .LC3[rip]
movdqa xmm7, XMMWORD PTR .LC4[rip]
.L4:
movdqa xmm5, xmm0
movdqa xmm4, xmm0
cvtdq2ps xmm6, xmm0
movdqa xmm10, xmm0
paddd xmm0, xmm7
cvtdq2ps xmm3, xmm0
paddd xmm5, xmm9
paddd xmm4, xmm8
cvtdq2ps xmm5, xmm5
cvtdq2ps xmm4, xmm4
mulps xmm6, xmm6
mov eax, 5120
paddd xmm10, xmm11
mulps xmm5, xmm5
mulps xmm4, xmm4
mulps xmm3, xmm3
pxor xmm12, xmm12
.L2:
movdqa xmm1, xmm12
cvtdq2ps xmm14, xmm12
mulps xmm14, xmm14
movdqa xmm13, xmm12
paddd xmm12, xmm7
cvtdq2ps xmm12, xmm12
paddd xmm1, xmm9
cvtdq2ps xmm0, xmm1
mulps xmm0, xmm0
paddd xmm13, xmm8
cvtdq2ps xmm13, xmm13
sub eax, 1
mulps xmm13, xmm13
addps xmm14, xmm6
mulps xmm12, xmm12
addps xmm0, xmm5
addps xmm13, xmm4
addps xmm12, xmm3
addps xmm0, xmm14
addps xmm0, xmm13
addps xmm0, xmm12
movdqa xmm12, xmm1
cvttps2dq xmm0, xmm0
paddd xmm2, xmm0
jne .L2
add edx, 1
movdqa xmm0, xmm10
cmp edx, 1280
jne .L4
movdqa xmm0, xmm2
psrldq xmm0, 8
paddd xmm2, xmm0
movdqa xmm0, xmm2
psrldq xmm0, 4
paddd xmm2, xmm0
movd eax, xmm2
ret
main:
xor eax, eax
ret
_GLOBAL__sub_I_calculations():
sub rsp, 8
mov edi, OFFSET FLAT:_ZStL8__ioinit
call std::ios_base::Init::Init() [complete object constructor]
mov edx, OFFSET FLAT:__dso_handle
mov esi, OFFSET FLAT:_ZStL8__ioinit
mov edi, OFFSET FLAT:_ZNSt8ios_base4InitD1Ev
add rsp, 8
jmp __cxa_atexit
.LC0:
.long 0
.long 1
.long 2
.long 3
.LC1:
.long 4
.long 4
.long 4
.long 4
.LC2:
.long 1
.long 1
.long 1
.long 1
.LC3:
.long 2
.long 2
.long 2
.long 2
.LC4:
.long 3
.long 3
.long 3
.long 3
您的SIMD代码生成：
calculations():
pxor xmm5, xmm5
xor eax, eax
mov r8d, 1
movabs rdi, -4294967296
cvtsi2ss xmm5, eax
.L4:
mov r9d, r8d
mov esi, 1
movd edx, xmm5
pxor xmm5, xmm5
pxor xmm4, xmm4
mov ecx, edx
mov rdx, QWORD PTR [rsp-24]
cvtsi2ss xmm5, r8d
add r8d, 1
cvtsi2ss xmm4, r8d
and rdx, rdi
or rdx, rcx
pxor xmm2, xmm2
mov edx, edx
movd ecx, xmm5
sal rcx, 32
or rdx, rcx
mov QWORD PTR [rsp-24], rdx
movd edx, xmm4
pxor xmm4, xmm4
mov ecx, edx
mov rdx, QWORD PTR [rsp-16]
and rdx, rdi
or rdx, rcx
lea ecx, [r9+2]
mov edx, edx
cvtsi2ss xmm4, ecx
movd ecx, xmm4
sal rcx, 32
or rdx, rcx
mov QWORD PTR [rsp-16], rdx
movaps xmm4, XMMWORD PTR [rsp-24]
mulps xmm4, xmm4
.L2:
movd edx, xmm2
mov r10d, esi
pxor xmm2, xmm2
pxor xmm7, xmm7
mov ecx, edx
mov rdx, QWORD PTR [rsp-40]
cvtsi2ss xmm2, esi
add esi, 1
and rdx, rdi
cvtsi2ss xmm7, esi
or rdx, rcx
mov ecx, edx
movd r11d, xmm2
movd edx, xmm7
sal r11, 32
or rcx, r11
pxor xmm7, xmm7
mov QWORD PTR [rsp-40], rcx
mov ecx, edx
mov rdx, QWORD PTR [rsp-32]
and rdx, rdi
or rdx, rcx
lea ecx, [r10+2]
mov edx, edx
cvtsi2ss xmm7, ecx
movd ecx, xmm7
sal rcx, 32
or rdx, rcx
mov QWORD PTR [rsp-32], rdx
movaps xmm0, XMMWORD PTR [rsp-40]
mulps xmm0, xmm0
addps xmm0, xmm4
movaps xmm3, xmm0
movaps xmm1, xmm0
shufps xmm3, xmm0, 85
addss xmm1, xmm3
movaps xmm3, xmm0
unpckhps xmm3, xmm0
shufps xmm0, xmm0, 255
addss xmm1, xmm3
addss xmm0, xmm1
cvttss2si edx, xmm0
add eax, edx
cmp r10d, 5120
jne .L2
cmp r9d, 5120
jne .L4
rep ret
main:
xor eax, eax
ret
_GLOBAL__sub_I_calculations():
sub rsp, 8
mov edi, OFFSET FLAT:_ZStL8__ioinit
call std::ios_base::Init::Init() [complete object constructor]
mov edx, OFFSET FLAT:__dso_handle
mov esi, OFFSET FLAT:_ZStL8__ioinit
mov edi, OFFSET FLAT:_ZNSt8ios_base4InitD1Ev
add rsp, 8
jmp __cxa_atexit
请注意，编译器的版本正在使用cvtdq2ps，paddd，cvtdq2ps，mulps，addps和cvttps2dq。所有这些都是SIMD指令。通过有效地组合它们，编译器可以生成快速代码。
相反，您的代码会生成大量add，and，cvtsi2ss，lea，mov，movd，{{ 1}}，or，pxor，它们不是SIMD指令。
我怀疑编译器在处理数据类型转换和数据重排方面比您做得更好，并且这可以使编译器更有效地安排数学运算。