[SYCL][E2E] Stop CMAKE_CXX_FLAGS cache variable passing to clang++ (intel#20039)

Before intel#9075 the value of CMAKE_CXX_FLAGS was passed to SYCL E2E
compilations. In intel#9075 the variable was explicitly unset in
non-standalone (in-tree) builds of the tests. Unsetting a normal
variable however exposes the cmake cache variable of the same name.

I believe this was not the intent, rather the intent was to not pass any
CMAKE_CXX_FLAGS to the E2E tests in non-standalone builds. It is weird
to pass the user's CMAKE_CXX_FLAGS to the E2E tests, as these flags are
for the SYCL toolchain, not tests.

Passing `CMAKE_CXX_FLAGS` to the E2E tests can cause problems, for
example a test might wish to override options such as setting
`-ffp-model=fast`, but this fails with a warning/error if
`CMAKE_CXX_FLAGS` contains `-ffp-model=precise`: `error: overriding
'-ffp-model=precise' option with '-ffp-model=fast'
[-Werror,-Woverriding-option]`.

While the error could be worked around in the test by appending
`-Wno-overriding-option` to the test flags, I don't believe this is the
right solution.

It is possible that the user is running on a system where some flags
have to be passed to clang++ to make it produce working executables. For
this reason, allow setting `SYCL_E2E_CLANG_CXX_FLAGS` by the user if
needed in in-tree builds, but do not pass `CMAKE_CXX_FLAGS` by default.

Contains a drive-by fix to make sure `-Werror` is passed to clang++ even
when the c++ flags are overriden by the lit parameter `--param
cxx_flags=...` (it can still be disabled using `-Wno-error` explicitly).

Author: Maetveis

sycl/test-e2e/CMakeLists.txt

@@ -39,10 +39,11 @@ endif() # Standalone.
 
 if(SYCL_TEST_E2E_STANDALONE)
   set(SYCL_CXX_COMPILER ${CMAKE_CXX_COMPILER})
+  set(SYCL_E2E_CLANG_CXX_FLAGS ${CMAKE_CXX_FLAGS})
 else()
   set(SYCL_CXX_COMPILER "${LLVM_BINARY_DIR}/bin/clang++")
-  # Don't want options used for building sycl-toolchain.
-  unset(CMAKE_CXX_FLAGS)
+  set(SYCL_E2E_CLANG_CXX_FLAGS "" CACHE STRING
+      "Flags passed to clang++ when building SYCL end-to-end tests")
 endif() # Standalone.
 
 find_package(Threads REQUIRED)
@@ -65,25 +66,6 @@ if(NOT SYCL_TEST_E2E_TARGETS)
   set(SYCL_TEST_E2E_TARGETS "all")
 endif()
 
-if(MSVC AND NOT SYCL_TEST_E2E_STANDALONE)
-  # We're trying to pass MSVC flags to Clang, which doesn't work by default
-  separate_arguments(cxx_flags NATIVE_COMMAND "${CMAKE_CXX_FLAGS}")
-  foreach(flag IN ITEMS ${cxx_flags})
-    # Skip certain flags that only exists for MSVC
-    if("${flag}" STREQUAL "/EHsc")
-      continue()
-    endif()
-    # Change the way compiler definitions are passed in
-    string(REGEX REPLACE "^/D" "-D" clang_flag "${flag}")
-    list(APPEND SYCL_E2E_CLANG_CXX_FLAGS ${clang_flag})
-  endforeach()
-  string (REPLACE ";" " " SYCL_E2E_CLANG_CXX_FLAGS "${SYCL_E2E_CLANG_CXX_FLAGS}")
-else()
-  set(SYCL_E2E_CLANG_CXX_FLAGS ${CMAKE_CXX_FLAGS})
-endif()
-
-set(SYCL_E2E_CLANG_CXX_FLAGS "${SYCL_E2E_CLANG_CXX_FLAGS} -Werror")
-
 if(NOT DEFINED CUDA_LIBS_DIR AND NOT DEFINED CUDA_INCLUDE)
   find_package(CUDAToolkit)
 

sycl/test-e2e/lit.cfg.py

@@ -1147,7 +1147,7 @@ def get_sycl_ls_verbose(sycl_device, env):
     config.substitutions.append(("%clangxx", " true "))
     config.substitutions.append(("%clang", " true "))
 else:
-    clangxx = " " + config.dpcpp_compiler + " "
+    clangxx = " " + config.dpcpp_compiler + " -Werror "
     if "preview-mode" in config.available_features:
         # Technically, `-fpreview-breaking-changes` is reported as unused option
         # if used without `-fsycl`. However, we have far less tests compiling