sycl vector add example

在安装好SYCL和ComputeCpp环境之后，按照codeplay发布的Hello SYCL步骤，想要通过测试最简单的向量加法来测试本地开发环境。结果总是Build完成之后遇到Runtime error， 具体参考Issue #230。不得不说同行们的效率还是挺高，在较短的时间内给与了回复，不仅指出了出错原因，也指明了当前Hello SYCL存在的问题。为community维护的社区程序员点赞！
现将此次问题总结如下：

本地开发环境

(base) huang@mlt:~/compute$ /usr/local/computecpp/bin/computecpp_info
********************************************************************************

ComputeCpp Info (CE 1.0.3)

SYCL 1.2.1 revision 3

********************************************************************************

Toolchain information:

GLIBC version: 2.23
GLIBCXX: 20160609
This version of libstdc++ is supported.

********************************************************************************


Device Info:

Discovered 2 devices matching:
  platform    : <any>
  device type : <any>

--------------------------------------------------------------------------------
Device 0:

  Device is supported                     : UNTESTED - Device not tested on this OS
  CL_DEVICE_NAME                          : Intel(R) Gen9 HD Graphics NEO
  CL_DEVICE_VENDOR                        : Intel(R) Corporation
  CL_DRIVER_VERSION                       : 19.20.13008
  CL_DEVICE_TYPE                          : CL_DEVICE_TYPE_GPU
--------------------------------------------------------------------------------
Device 1:

  Device is supported                     : YES - Tested internally by Codeplay Software Ltd.
  CL_DEVICE_NAME                          : Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
  CL_DEVICE_VENDOR                        : Intel(R) Corporation
  CL_DRIVER_VERSION                       : 1.2.0.10
  CL_DEVICE_TYPE                          : CL_DEVICE_TYPE_CPU

If you encounter problems when using any of these OpenCL devices, please consult
this website for known issues:
https://computecpp.codeplay.com/releases/v1.0.3/platform-support-notes

********************************************************************************

Source Code

https://developer.codeplay.com/products/computecpp/ce/guides/sycl-guide/hello-sycl

#include <iostream>
#include <CL/sycl.hpp>

class vector_addition;
namespace sycl = cl::sycl;

int main() {

  sycl::float4 a = {1.0, 2.0, 3.0, 4.0};
  sycl::float4 b = {4.0, 3.0, 2.0, 3.0};
  sycl::float4 c = {.0, 0.0, 0.0, 0.0};

  sycl::default_selector device_selector;

  sycl::queue queue(device_selector);
  std::cout << "Running on "
            << queue.get_device().get_info<sycl::info::device::name>()
            << "\n";
  {
      sycl::buffer<sycl::float4, 1> a_sycl(&a, sycl::range<1>(1));
      sycl::buffer<sycl::float4, 1> b_sycl(&b, sycl::range<1>(1));
      sycl::buffer<sycl::float4, 1> c_sycl(&c, sycl::range<1>(1));

      queue.submit([&] (sycl::handler& cgh) {
      auto a_acc = a_sycl.get_access<sycl::access::mode::read>(cgh);
      auto b_acc = b_sycl.get_access<sycl::access::mode::read>(cgh);
      auto c_acc = c_sycl.get_access<sycl::access::mode::discard_write>(cgh);

      cgh.single_task<class vector_addition>([=] () {
        c_acc[0] = a_acc[0] + b_acc[0];
        });
      });
  }
  std::cout << "  A { " << a.x() << ", " << a.y() << ", " << a.z() << ", " << a.w() << " }\n"
        << "+ B { " << b.x() << ", " << b.y() << ", " << b.z() << ", " << b.w() << " }\n"
        << "------------------\n"
        << "= C { " << c.x() << ", " << c.y() << ", " << c.z() << ", " << c.w() << " }"
        << std::endl;

  return 0;
}

How to build

Build.sh

#!/bin/bash

SOUR_FILE=$1
OBJ_FILE=$2

compute++ -g -I/usr/local/computecpp/include ${SOUR_FILE} -sycl-driver -no-serial-memop -L/usr/local/computecpp/lib -lComputeCpp -o ${OBJ_FILE}

echo "Build Done!"
echo $2 "was generated successfully!"

Note:

• 出错原因就是因为忘记添加-sycl-driver选项，导致kernel在Host端而不是Device端build。
• sample code存在的一个潜在的问题是，计算完的结果（30行）并不会被写回到Host端，一个简单避免这个问题的方式就是添加一个大的括号将这段buffer写操作圈起来（19行，33行），这样可以在退出时buffer的析构函数自动触发copy back到Host端机制。

Run

(base) huang@mlt:~/compute$ ./build.sh hello_sycl.cpp hello
Build Done!
hello was generated successfully!
(base) huang@mlt:~/compute$ ./hello
Running on Intel(R) Gen9 HD Graphics NEO
  A { 1, 2, 3, 4 }
+ B { 4, 3, 2, 3 }
------------------
= C { 5, 5, 5, 7 }