[SYCL][Doc] Add FAQ section

sycl/doc/FAQ.md

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+# Frequently Asked Questions
+
+**Table of contents**
+
+1. [Developing with SYCL](#developing-with-sycl)
+1. [Using applications built with SYCL](#using-applications-built-with-sycl)
+1. [Common issues](#common-issues)
+1. [Device specific questions and issues](#device-specific-questions-and-issues)
+
+
+## Developing with SYCL
+
+### Q: What do I need to start developing with SYCL?
+**A:** To get the full SYCL experience you need a SYCL-capable compiler. Intel
+SYCL compiler provides you with both host and device side compilation. Another
+requirement for code offloading to specialized devices is a compatible OpenCL
+runtime. Our [Get Started Guide](GetStartedWithSYCLCompiler.md) will help you
+set up a proper environment. To learn more about using the SYCL compiler, please
+refer to [User Manual](SYCLCompilerUserManual.md). If using a special compiler
+is not an option for you and/or you would like to experiment without offloading
+code to non-host devices, you can exploit SYCL's host device feature. This gives
+you the ability to use any C++11 compiler. You will need to link your
+application with the SYCL Runtime library and provide a path to the SYCL headers
+directory. Please, refer to your compiler manual to learn about specific build
+options.
+
+### Q: How are SYCL compilation phases different from those of a usual C++ compiler? Can I customize this flow for my applications?
+**A:** Due to the fact that both host and device code need to be compiled and
+linked into the final binary, the compilation steps sequence is more complicated
+compared to the usual C++ flow.
+
+In general, we encourage our users to rely on the SYCL Compiler for handling all
+of the compilation phases "under the hood". However, thorough understanding of
+the above-described steps may allow you to customize your compilation by invoking
+different phases manually. As an example, you could:
+1. preprocess your host code with another C++-capable compiler;
+2. turn to the SYCL compiler for generating the integration header and compiling
+the device code for the needed target(s);
+3. use your preferred host compiler from 1) to compile your preprocessed host
+code and the integration header into a host object file;
+4. link the host object file and the device image(s) into the final executable.
+
+To learn more about the concepts behind this flow, and the SYCL Compiler
+internals as such, we welcome you to study our
+[SYCL Compiler and Runtime architecture design](SYCLCompilerAndRuntimeDesign.md)
+document.
+
+
+## Using applications built with SYCL
+
+### Q: What happens if I run my application on a machine without OpenCL?
+**A:** If you use the default SYCL device selector (or any other selector that
+allows host device), then a fallback to the host device will take place.
+Otherwise, an exception will be thrown.
+
+
+## Common issues
+
+### Q: SYCL application complains about missing libsycl.so (or sycl.dll) library.
+Linux:
+```
+$ ./app
+$ ./app: error while loading shared libraries: libsycl.so: cannot open shared object file: No such file or directory
+```
+Windows:
+
+![Error screen](images/missing_sycl_dll.png)
+
+*The code execution cannot proceed because sycl.dll was not found. Reinstalling the program may fix this problem.*
+
+**A:** The SYCL Runtime library is required to run SYCL-enabled applications.
+While compiler driver is able to find the library and link against it, your
+operating system may struggle. Make sure that the location of the SYCL Runtime
+library is listed in the correct environment variable: `LD_LIBRARY_PATH`
+(for Linux) or `LIB` (for Windows).
+
+### Q: SYCL fails to compile device code that uses STD functions.
+Example error message:
+```
+In file included from example.cpp:1:
+In file included from C:\compiler\lib\clang\10.0.0\include\CL/sycl.hpp:11:
+In file included from C:\compiler\lib\clang\10.0.0\include\CL/sycl/accessor.hpp:13:
+In file included from C:\compiler\lib\clang\10.0.0\include\CL/sycl/buffer.hpp:10:
+In file included from C:\compiler\lib\clang\10.0.0\include\CL/sycl/detail/buffer_impl.hpp:14:
+In file included from C:\compiler\lib\clang\10.0.0\include\CL/sycl/detail/aligned_allocator.hpp:16:
+C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.22.27905\include\algorithm(4493,9): error: SYCL kernel cannot call a
+      dllimport function
+        _STL_ASSERT(!(_Left < _Right), "invalid comparator");
+        ^
+C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.22.27905\include\yvals.h(193,33): note: expanded from macro '_STL_ASSERT'
+#define _STL_ASSERT(cond, mesg) _STL_VERIFY(cond, mesg)
+                                ^
+C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.22.27905\include\yvals.h(186,13): note: expanded from macro '_STL_VERIFY'
+            _STL_REPORT_ERROR(mesg);                                \
+            ^
+C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.22.27905\include\yvals.h(178,9): note: expanded from macro
+      '_STL_REPORT_ERROR'
+        _RPTF0(_CRT_ASSERT, mesg);           \
+        ^
+C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\ucrt\crtdbg.h(760,37): note: expanded from macro '_RPTF0'
+    #define _RPTF0(rptno, msg)      _RPT_BASE(rptno, __FILE__, __LINE__, NULL, "%s", msg)
+                                    ^
+C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\ucrt\crtdbg.h(747,23): note: expanded from macro '_RPT_BASE'
+        (void) ((1 != _CrtDbgReport(__VA_ARGS__)) || \
+                      ^
+C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\ucrt\crtdbg.h(607,26): note: '_CrtDbgReport' declared here
+    _ACRTIMP int __cdecl _CrtDbgReport(
+```
+
+**A:** Section 6.3 of the SYCL specification says:
+>  Implementations are not required to support any library routines in kernels
+>  beyond those explicitly mentioned as usable in kernels in this spec.
+
+Replace usage of STD built-ins with SYCL-defined math built-ins. Please, note
+that you have to explicitly specify built-in namespace (i.e. `cl::sycl::fmin`).
+The full list of SYCL math built-ins is provided in section 4.13.3 of the
+specification.
+
+
+## Device specific questions and issues
+
+### Q: What devices are supported by Intel SYCL compiler?
+**A:** By design, SYCL is closely connected to OpenCL, which is used to offload
+code to specialized devices. Intel SYCL compiler currently makes use of SPIR-V,
+a portable intermediate representation format. It is a core feature of
+OpenCL 2.1, so any device, capable of OpenCL 2.1, should be supported.
+Otherwise, your OpenCL device must support `cl_khr_il_program` extension.
+
+Apart from that, there's also the so-called host device, which can be used when
+no suitable OpenCL device is found. The host device will use your host CPU as
+the offload target for kernel execution. Since the device code is also compiled
+for the host CPU and no JIT is required, you can easily use any classic C++
+debugging tools of your choice for the host device code.
+
+Furthermore, developers can extend capabilities of the SYCL Runtime to
+non-OpenCL devices by writing correspondent plugins. To learn more, please check
+out our [Plugin Interface Guide](SYCLPluginInterface.md).
+
+### Q: SYCL applications hang on Intel GPUs while working well on other devices
+**A:** One of the common reasons is Intel GPUs feature called "hang check".
+If your workload runs for more than a certain amount of time, it will be killed
+by hardware. From the application point of view this looks like a hang. To allow
+heavy kernels to be executed, disable hang check. **Please, note that other apps
+on your system may contain bugs, and disabling "hang check" may lead to real
+hangs.**
+
+You can find out more about hang check and how to disable it on
+[this page](https://software.intel.com/en-us/articles/installation-guide-for-intel-oneapi-toolkits).