Placeholder should be used for link executable

[frengels]

When setting the dpcpp executable directly it bypasses any additional flags which cmake would set. For example, this currently prevents cmake from setting --target= when cross-compiling the SYCL-CTS. Since CMAKE_CXX_COMPILER is already set to DPCPP_CXX_EXECUTABLE this will have the same desired effect.

[CLAassistant]

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[keryell]

Thanks, that makes sense.

[bader]

When setting the dpcpp executable directly it bypasses any additional flags which cmake would set.

I think this was done intentionally. CMake configuration was inherited from the Codeplay ComputeCPP implementation, which has an option to use system C++ compiler as a host compiler for the SYCL code. DPC++ has this option as well, but it's not a default one.

For example, this currently prevents cmake from setting --target= when cross-compiling the SYCL-CTS

@frengels, did you considered using DPCPP_FLAGS for setting DPC++ compiler options?

Since CMAKE_CXX_COMPILER is already set to DPCPP_CXX_EXECUTABLE this will have the same desired effect.

Considering that ComputeCPP is EOLed, we should consider dropping DPC++ specific variables (i.e. DPCPP_CXX_EXECUTABLE and DPCPP_FLAGS) and use SYCL compiler as standard C++ host compiler as well. This will require more clean-up.

@frengels, what cmake options do use for cross-compiling SYCL-CTS?

[frengels]

Sorry, I hadn't hit send.

I think this was done intentionally. CMake configuration was inherited from the Codeplay ComputeCPP implementation, which has an option to use system C++ compiler as a host compiler for the SYCL code. DPC++ has this option as well, but it's not a default one.

I have no issue with it changing from some of the host compiler flags. It's just that this would've been an unforeseen one that is added for cross-compilation with a toolchain file.

@frengels, did you considered using DPCPP_FLAGS for setting DPC++ compiler options?

No, I wanted it to remain as vanilla as possible, only having to change the toolchain file.

@frengels, what cmake options do use for cross-compiling SYCL-CTS?

I'm using the following

"SYCL_IMPLEMENTATION": "DPCPP",
"SYCL_CTS_CTEST_DEVICE": "GPU",
"SYCL_CTS_ENABLE_DOUBLE_TESTS": "OFF",
"SYCL_CTS_ENABLE_HALF_TESTS": "ON",
"SYCL_CTS_ENABLE_OPENCL_INTEROP_TESTS": "OFF",
"DPCPP_INSTALL_DIR": "$penv{DPCPP_INSTALL_DIR}",
"CMAKE_POLICY_VERSION_MINIMUM": "3.5"

And my toolchain file contains

set(CMAKE_SYSTEM Windows)
set(CMAKE_SYSTEM_PROCESSOR ARM64)

set(_TARGET "aarch64-pc-windows-msvc")

set(_DPCPP_INSTALL_DIR $ENV{DPCPP_INSTALL_DIR})

set(CMAKE_C_COMPILER_TARGET ${_TARGET})
set(CMAKE_CXX_COMPILER_TARGET ${_TARGET})
set(CMAKE_ASM_COMPILER_TARGET ${_TARGET})
set(CMAKE_C_COMPILER "${_DPCPP_INSTALL_DIR}/bin/clang.exe")
set(CMAKE_CXX_COMPILER "${_DPCPP_INSTALL_DIR}/bin/clang++.exe")
set(CMAKE_ASM_COMPILER "${_DPCPP_INSTALL_DIR}/bin/clang.exe")

[bader]

To be honest, setting SYCL compiler via C++ variables looks like a hack to me, but considering that we already have it in place, I'm okay with the patch.
Conceptually, SYCL-CTS build system separates C++ sources from SYCL sources and uses different compilers for building corresponding source files.
We need to take a closer look at configuring compilers separately.

@frengels, one more note: cross-compilation is not tested by regular CI, so your use case can be broken by future PRs.