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Windows 10
IDE
Visual Studio 19
Compiler
Intel(R) oneAPI DPC++ compiler
Toolkit
oneAPI Base Toolkit version 2021.1
Problem
- Random call instructions are generated in critical loops when using parallel_for.
- No loop optimization is applied when using single_task.
Example program,
#include <CL/sycl.hpp>
#include <iostream>
namespace {
__declspec(noinline) void add(
float const* a, size_t a_stride,
float const* b, size_t b_stride,
float* c, size_t c_stride,
size_t w, size_t h
) {
for (size_t y = 0; y < h; ++y) {
for (size_t x = 0; x < w; ++x) {
c[y * c_stride + x] = a[y * a_stride + x] + b[y * b_stride + x];
}
}
}
__declspec(noinline) void add_single_task(
sycl::queue& q,
float const* a, size_t a_stride,
float const* b, size_t b_stride,
float* c, size_t c_stride,
size_t w, size_t h
) {
auto e = q.submit([&](sycl::handler& cgh) {
cgh.single_task([=]() {
for (size_t y = 0; y < h; ++y) {
for (size_t x = 0; x < w; ++x) {
c[y * c_stride + x] = a[y * a_stride + x] + b[y * b_stride + x];
}
}
});
});
e.wait_and_throw();
}
__declspec(noinline) void add_parallel_for(
sycl::queue& q,
float const* a, size_t a_stride,
float const* b, size_t b_stride,
float* c, size_t c_stride,
size_t w, size_t h
) {
auto e = q.submit([&](sycl::handler& cgh) {
cgh.parallel_for(sycl::range<2>(h, w), [=](sycl::id<2> index) {
auto y = index[0];
auto x = index[1];
c[y * c_stride + x] = a[y * a_stride + x] + b[y * b_stride + x];
});
});
e.wait_and_throw();
}
sycl::queue get_queue() {
// Note:
// Initialization of a queue is very slow.
// Hence, we'll be caching it.
static auto q = sycl::queue(sycl::cpu_selector());
return q;
}
} // namespace
int main(int argc, char* argv[]) {
try {
auto q = get_queue();
}
catch (std::exception const& e) {
std::cout << "Queue initialization failed with " << e.what() << '\n';
return -1;
}
try {
constexpr auto w = size_t(1024);
constexpr auto h = size_t(1024);
auto a = std::vector<float>(w * h, 1);
auto b = std::vector<float>(w * h, 2);
auto c = std::vector<float>(w * h);
auto q = get_queue();
for (size_t i = 0; i < 30; ++i) {
add(a.data(), w, b.data(), w, c.data(), w, w, h);
add_single_task(q, a.data(), w, b.data(), w, c.data(), w, w, h);
add_parallel_for(q, a.data(), w, b.data(), w, c.data(), w, w, h);
}
}
catch (std::exception const& e) {
std::cout << "Add failed with " << e.what() << '\n';
return -1;
}
return 0;
}
Intel Advisor reports,
- add_parallel_for,
- call tree,
- assembly, - add_single_task,
- source insights,
Are those call instructions necessary and is there a way to request the AOT/JIT compiler to not generate them? And for single_task, shouldn't optimization be applied as well (was simply interested to see how it fare against the host compiler optimization, add)?
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Thanks for sharing!
/Zi tells the compiler to generate full debugging information and call instructions are generated when it enabled. Without this option, there are no callq instructions. Please, note that /Zi is not recommended to use in the release code. So, this is not a bug.
Also, please, note that O0-O3 options are also supported by dpcpp \ icx compilers as well.
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Hi,
Thanks for reaching out to us!
We are escalating this thread to the Subject Matter Expert(SME) who will guide you further.
Have a Good day
Thanks & Regards
Goutham
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I reproduced the second issue (No loop optimization is applied when using single_task). Since this is a missed performance opportunity, I suppose it might be escalated as a feature request. I am presently investigating it and will get back to you shortly with an update.
However, I wasn't able to reproduce callq inside add_parallel_for. Could you please share the compiler options you used? Which compiler driver, dpcpp or icx\icpx, did you use?
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Using default settings on release mode in Visual Studio.
/O2 which should translate to -O3 by clang-cl.
Was reproducible in,
-
Intel(R) Core(TM) i5-7300HQ
-
Intel(R) Xeon(R) Gold 6230N
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If it's still not reproducible, try removing the add inside main, leaving only add_single_task and add_parallel_for.
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No, it is not reproducible, even with O3. Could you please share a compiler command line and other steps to reproduce?
What do you mean by 'which should translate to -O3 by clang-cl'?
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Steps:
- Create a new project, select Empty Project.
- Change Platform Toolset from Visual Studio 2019 (v142) to Intel(R) oneAPI DPC++ Compiler.
- Change x86 to x64 and Debug to Release.
- Paste the script and build.
* DPC++ Console Application template project should work too (without having to go through steps 1 and 2).
For reference, these are the arguments passed to the compiler.
/O2 /Zi /D "NDEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /WX- /MD /EHsc /W1 /nologo /Fo"x64\Release\"
As to "which should translate to -O3 by clang-cl", as seen above, I'm using /O2 which is a msvc option, not a gcc/clang option. However, clang-cl should translate it to -O3.
I have also attached a zipped file which contains the project and executable along with Intel Advisor's report.
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Thanks for sharing!
/Zi tells the compiler to generate full debugging information and call instructions are generated when it enabled. Without this option, there are no callq instructions. Please, note that /Zi is not recommended to use in the release code. So, this is not a bug.
Also, please, note that O0-O3 options are also supported by dpcpp \ icx compilers as well.
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Yes, that works.
And the generated assembly is much much better, see attached image.
Finally, is there any way to use intel loop directives FPGA loop directives for a CPU/GPU kernel, like [[intel::invdep]]?
Using the same example, c[y * c_stride + x] = a[y * a_stride + x] + b[y * b_stride + x], I believe the optimizer detects possible memory aliasing here (y * stride), and disables other loop transformations, like loop unrolling, permutation, tiling, etc.
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Hi,
Thanks for the confirmation!
As this issue has been resolved, we will no longer respond to this thread.
If you require any additional assistance from Intel, please start a new thread.
Any further interaction in this thread will be considered community only.
Have a Good day.
Thanks & Regards
Goutham
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