如何解决在较高的优化级别上,AVX2 simd相对于标量的性能相对较差
我正在学习和使用SIMD函数,并编写了一个简单的程序,该程序将其可以在 1秒中运行的向量加法指令的数量与正常的标量加法进行了比较。 我发现SIMD在较低的优化级别上表现相对较好,而在较高的优化级别上则始终表现较差,并且我想知道原因。我同时使用MSVC和gcc,这是同一回事。以下结果来自 Ryzen 7 CPU。我也在 Intel 平台上进行了测试,故事几乎相同。
#include <iostream>
#include <numeric>
#include <chrono>
#include <iterator>
#include <thread>
#include <atomic>
#include <vector>
#include <immintrin.h>
int main()
{
const auto threadLimit = std::thread::hardware_concurrency() - 1; //for running main()
for (auto i = 1; i <= threadLimit; ++i)
{
std::cerr << "Testing " << i << " threads: ";
std::atomic<unsigned long long> sumScalar {};
std::atomic<unsigned long long> loopScalar {};
std::atomic<unsigned long long> sumSimd {};
std::atomic<unsigned long long> loopSimd {};
std::atomic_bool stopFlag{ false };
std::vector<std::thread> threads;
threads.reserve(i);
{
for (auto j = 0; j < i; ++j)
threads.emplace_back([&]
{
uint32_t local{};
uint32_t loop{};
while (!stopFlag)
{
++local;
++loop; //removed this(see EDIT)
}
sumScalar += local;
loopScalar += loop;
});
std::this_thread::sleep_for(std::chrono::seconds{ 1 });
stopFlag = true;
for (auto& thread : threads)
thread.join();
}
threads.clear();
stopFlag = false;
{
for (auto j = 0; j < i; ++j)
threads.emplace_back([&]
{
const auto oneVec = _mm256_set1_epi32(1);
auto local = _mm256_set1_epi32(0);
uint32_t inc{};
while (!stopFlag)
{
local = _mm256_add_epi32(oneVec,local);
++inc; //removed this(see EDIT)
}
sumSimd += std::accumulate(reinterpret_cast<uint32_t*>(&local),reinterpret_cast<uint32_t*>(&local) + 8,uint64_t{});
loopSimd += inc;
});
std::this_thread::sleep_for(std::chrono::seconds{ 1 });
stopFlag = true;
for (auto& thread : threads)
thread.join();
}
std::cout << "Sum: "<<sumSimd <<" / "<<sumScalar <<"("<<100.0*sumSimd/sumScalar<<"%)\t"<<"Loop: "<<loopSimd<<" / "<<loopScalar<<"("<< 100.0*loopSimd/loopScalar<<"%)\n";
// SIMD/Scalar,higher value means SIMD better
}
}
有了g++ -O0 -march=native -lpthread
,我得到了:
Testing 1 threads: Sum: 1004405568 / 174344207(576.105%) Loop: 125550696 / 174344207(72.0131%)
Testing 2 threads: Sum: 2001473960 / 348079929(575.004%) Loop: 250184245 / 348079929(71.8755%)
Testing 3 threads: Sum: 2991335152 / 521830834(573.238%) Loop: 373916894 / 521830834(71.6548%)
Testing 4 threads: Sum: 3892119680 / 693704725(561.063%) Loop: 486514960 / 693704725(70.1329%)
Testing 5 threads: Sum: 4957263080 / 802362140(617.834%) Loop: 619657885 / 802362140(77.2292%)
Testing 6 threads: Sum: 5417700112 / 953587414(568.139%) Loop: 677212514 / 953587414(71.0174%)
Testing 7 threads: Sum: 6078496824 / 1067533241(569.396%) Loop: 759812103 / 1067533241(71.1746%)
Testing 8 threads: Sum: 6679841000 / 1196224828(558.41%) Loop: 834980125 / 1196224828(69.8013%)
Testing 9 threads: Sum: 7396623960 / 1308004474(565.489%) Loop: 924577995 / 1308004474(70.6861%)
Testing 10 threads: Sum: 8158849904 / 1416026963(576.179%) Loop: 1019856238 / 1416026963(72.0224%)
Testing 11 threads: Sum: 8868695984 / 1556964234(569.615%) Loop: 1108586998 / 1556964234(71.2018%)
Testing 12 threads: Sum: 9441092968 / 1655554694(570.268%) Loop: 1180136621 / 1655554694(71.2835%)
Testing 13 threads: Sum: 9530295080 / 1689916907(563.951%) Loop: 1191286885 / 1689916907(70.4938%)
Testing 14 threads: Sum: 10444142536 / 1805583762(578.436%) Loop: 1305517817 / 1805583762(72.3045%)
Testing 15 threads: Sum: 10834255144 / 1926575218(562.358%) Loop: 1354281893 / 1926575218(70.2948%)
有了g++ -O3 -march=native -lpthread
,我得到了:
Testing 1 threads: Sum: 2933270968 / 3112671000(94.2365%) Loop: 366658871 / 3112671000(11.7796%)
Testing 2 threads: Sum: 5839842040 / 6177278029(94.5375%) Loop: 729980255 / 6177278029(11.8172%)
Testing 3 threads: Sum: 8775103584 / 9219587924(95.1789%) Loop: 1096887948 / 9219587924(11.8974%)
Testing 4 threads: Sum: 11350253944 / 10210948580(111.158%) Loop: 1418781743 / 10210948580(13.8947%)
Testing 5 threads: Sum: 14487451488 / 14623220822(99.0715%) Loop: 1810931436 / 14623220822(12.3839%)
Testing 6 threads: Sum: 17141556576 / 14437058094(118.733%) Loop: 2142694572 / 14437058094(14.8416%)
Testing 7 threads: Sum: 19883362288 / 18313186637(108.574%) Loop: 2485420286 / 18313186637(13.5718%)
Testing 8 threads: Sum: 22574437968 / 17115166001(131.897%) Loop: 2821804746 / 17115166001(16.4872%)
Testing 9 threads: Sum: 25356792368 / 18332200070(138.318%) Loop: 3169599046 / 18332200070(17.2898%)
Testing 10 threads: Sum: 28079398984 / 20747150935(135.341%) Loop: 3509924873 / 20747150935(16.9176%)
Testing 11 threads: Sum: 30783433560 / 21801526415(141.199%) Loop: 3847929195 / 21801526415(17.6498%)
Testing 12 threads: Sum: 33420443880 / 22794998080(146.613%) Loop: 4177555485 / 22794998080(18.3266%)
Testing 13 threads: Sum: 35989535640 / 23596768252(152.519%) Loop: 4498691955 / 23596768252(19.0649%)
Testing 14 threads: Sum: 38647578408 / 23796083111(162.412%) Loop: 4830947301 / 23796083111(20.3014%)
Testing 15 threads: Sum: 41148330392 / 24252804239(169.664%) Loop: 5143541299 / 24252804239(21.208%)
编辑:删除loop
变量后,在两种情况下仅保留local
(请参见代码编辑),结果仍然相同。
EDIT2:上面的结果是在Ubuntu上使用GCC 9.3。我在Windows(mingw)上切换到GCC 10.2,它显示了很好的缩放比例,如下所示(结果是原始代码)。几乎可以得出结论,这是MSVC和GCC旧版本的问题吗?
Testing 1 threads: Sum: 23752640416 / 3153263747(753.272%) Loop: 2969080052 / 3153263747(94.159%)
Testing 2 threads: Sum: 46533874656 / 6012052456(774.01%) Loop: 5816734332 / 6012052456(96.7512%)
Testing 3 threads: Sum: 66076900784 / 9260324764(713.548%) Loop: 8259612598 / 9260324764(89.1936%)
Testing 4 threads: Sum: 92216030528 / 12229625883(754.038%) Loop: 11527003816 / 12229625883(94.2548%)
Testing 5 threads: Sum: 111822357864 / 14439219677(774.435%) Loop: 13977794733 / 14439219677(96.8044%)
Testing 6 threads: Sum: 122858189272 / 17693796489(694.357%) Loop: 15357273659 / 17693796489(86.7947%)
Testing 7 threads: Sum: 148478021656 / 19618236169(756.837%) Loop: 18559752707 / 19618236169(94.6046%)
Testing 8 threads: Sum: 156931719736 / 19770409566(793.771%) Loop: 19616464967 / 19770409566(99.2213%)
Testing 9 threads: Sum: 143331726552 / 20753115024(690.652%) Loop: 17916465819 / 20753115024(86.3315%)
Testing 10 threads: Sum: 143541178880 / 20331801415(705.993%) Loop: 17942647360 / 20331801415(88.2492%)
Testing 11 threads: Sum: 160425817888 / 22209102603(722.343%) Loop: 20053227236 / 22209102603(90.2928%)
Testing 12 threads: Sum: 157095281392 / 23178532051(677.762%) Loop: 19636910174 / 23178532051(84.7202%)
Testing 13 threads: Sum: 156015224880 / 23818567634(655.015%) Loop: 19501903110 / 23818567634(81.8769%)
Testing 14 threads: Sum: 145464754912 / 23950304389(607.361%) Loop: 18183094364 / 23950304389(75.9201%)
Testing 15 threads: Sum: 149279587872 / 23585183977(632.938%) Loop: 18659948484 / 23585183977(79.1172%)
解决方法
reinterpret_cast<uint32_t*>(&local)
循环后,使GCC9可以在循环内local
存储/重新加载,从而创建存储转发瓶颈。 >
此问题已在GCC10中修复;不需要提交未优化的错误。请勿将指针投射到__m256i
本机上;即使GCC经常使它工作,它也违反了严格混叠,因此it's Undefined Behaviour不带-fno-strict-aliasing
。 (You can point __m256i*
at any other type,but not vice versa。)
gcc9.3(正在使用)正在循环内存储/重新加载向量,但将标量保存在inc eax
的寄存器中!
向量循环因此会限制向量存储转发加vpaddd
的延迟,并且恰好比标量循环慢了8倍。他们的瓶颈无关紧要,接近1倍的总速度只是巧合。
(标量循环大概在Zen1或Skylake上以每次迭代1个周期运行,并且vpaddd
的7个循环存储转发加1听起来是正确的。)
它是由reinterpret_cast<uint32_t*>(&local)
间接引起的,这可能是因为GCC试图宽恕严格混叠的未定义行为违规,或者只是因为您将指针指向本地完全没有。
这不是正常现象,也不是预期结果,但是内循环内部的原子负载和lambda的组合会混淆GCC9导致此错误。 (请注意,GCC9和10正在从循环内部的线程函数arg重新加载stopFlag
的地址,即使是标量,因此将其保存在寄存器中已经有些失败了。)>
在正常的用例中,每次检查停止标志都会进行更多的SIMD工作,并且通常不会在迭代中保持向量状态。通常,您会有一个非原子的arg来告诉您要做多少工作,而不是您在内部循环中检查的停止标志。因此,这个错漏的错误很少是问题。 (除非它即使没有原子标记也会发生?)
可重现的on Godbolt,显示了-DUB_TYPEPUN
与-UUB_TYPEPUN
的对比,其中我使用#ifdef
来使用您的不安全(和错选触发)版本与安全版本一种带有来自Fastest method to calculate sum of all packed 32-bit integers using AVX512 or AVX2的手动矢量随机播放的图片。 (该手动hsum在添加之前不会扩展,因此可能会溢出并包装。但这不是重点;在不严格的条件下使用不同的手动shuffle或_mm256_store_si256
到单独的数组中,可能会得到所需的结果-混淆未定义的行为。)
标量循环为:
# g++9.3 -O3 -march=znver1
.L5: # do{
inc eax # local++
.L3:
mov rdx,QWORD PTR [rdi+8] # load the address of stopFlag from the lambda
movzx edx,BYTE PTR [rdx] # zero-extend *&stopFlag into EDX
test dl,dl
je .L5 # }while(stopFlag == 0)
使用您的-O3 -march=znver1
(即我的源代码版本中的reinterpret_cast
)和g ++ 9.3,-DUB_TYPEPUN
,向量循环:
# g++9.3 -O3 -march=znver1 with your pointer-cast onto the vector
# ... ymm1 = _mm256_set1_epi32(1)
.L10: # do {
vpaddd ymm1,ymm0,YMMWORD PTR [rsp-32] # memory-source add with set1(1)
vmovdqa YMMWORD PTR [rsp-32],ymm1 # store back into stack memory
.L8:
mov rax,QWORD PTR [rdi+8] # load flag address
movzx eax,BYTE PTR [rax] # load stopFlag
test al,al
je .L10 # }while(stopFlag == 0)
... auto-vectorized hsum,zero-extending elements to 64-bit for vpaddq
但是有了一个安全的__m256i
水平和,根本没有指针指向local
的情况下,local
停留在寄存器中。
# ymm1 = _mm256_set1_epi32(1)
.L9:
vpaddd ymm0,ymm1,ymm0 # local += set1(1),staying in a register,ymm0
.L8:
mov rax,QWORD PTR [rdi+8] # same loop overhead,still 3 uops (with fusion of test/je)
movzx eax,BYTE PTR [rax]
test al,al
je .L9
... manually-vectorized 32-bit hsum
在我的Intel Skylake i7-6700k上,使用g ++ 10.1 -O3 -march = skylake,Arch GNU / Linux,energy_performance_preference = balance_power(最大时钟= 3.9),对于每个线程数量,我都能获得预期的800 +-1% GHz,且任何数量的核心都处于活动状态。
标量循环和向量循环具有相同的uops数量,并且没有不同的瓶颈,因此它们以相同的周期/迭代运行。 (4,如果它可以使那些地址->停车标志负载的值链在飞行中,则可能每个周期以1次迭代运行)。
Zen1可能会有所不同,因为vpaddd ymm
是2 oups。但是它的前端足够宽,可能每次迭代仍以1个周期运行该循环,因此您可能还会看到800%。
在未注释++loop
的情况下,我获得〜267%的“ SIMD速度”。在SIMD循环中增加一个inc,它会变成5微妙,并且可能会对Skylake产生一些讨厌的前端影响。
-O0
基准测试通常是没有意义的,它具有不同的瓶颈(通常是将所有内容保存在内存中/从内存中重新加载),并且SIMD内部函数通常在-O0
有很多额外的开销。尽管在这种情况下,甚至-O3
都在SIMD循环的存储/重新加载上遇到了瓶颈。
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