[SYCL][LIBCLC] Allow custom tools location when building libclc

Author: jchlanda

libclc/CMakeLists.txt

@@ -116,12 +116,54 @@ execute_process( COMMAND ${LLVM_CONFIG} "--bindir"
 # These were not properly reported in early LLVM and we don't need them
 list( APPEND LLVM_CXX_FLAGS -fno-rtti -fno-exceptions )
 
+# Use custom toolchain to build libclc, this can be useful when dealing with
+# debug builds, that do not need libclc bitcode to be built using debug tools.
+if ( EXISTS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR} )
+  message( WARNING "Using custom LLVM tools to build libclc: "
+    "${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR}, "
+    " make sure that the tools are up to date." )
+
+  # find_program needs the variable to be cleared in order to perform a search.
+  # Make sure that the cached entries are cleared as well.
+  unset( LLVM_CLANG CACHE )
+  unset( LLVM_AS CACHE )
+  unset( LLVM_LINK CACHE )
+  unset( LLVM_OPT CACHE )
+  unset( LLVM_SPIRV CACHE )
+  unset( LIBCLC_REMANGLER CACHE )
+  unset( LLVM_CLANG )
+  unset( LLVM_AS )
+  unset( LLVM_LINK )
+  unset( LLVM_OPT )
+  unset( LLVM_SPIRV )
+  unset( LIBCLC_REMANGLER )
+  find_program( LLVM_CLANG clang PATHS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR}
+    NO_DEFAULT_PATH )
+  find_program( LLVM_AS llvm-as PATHS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR}
+    NO_DEFAULT_PATH )
+  find_program( LLVM_LINK llvm-link PATHS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR}
+    NO_DEFAULT_PATH )
+  find_program( LLVM_OPT opt PATHS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR}
+    NO_DEFAULT_PATH )
+  find_program( LLVM_SPIRV llvm-spirv
+    PATHS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH )
+  find_program( LIBCLC_REMANGLER libclc-remangler
+    PATHS ${LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH )
+  if( NOT LLVM_CLANG OR NOT LLVM_OPT OR NOT LLVM_AS OR NOT LLVM_LINK
+      OR NOT LIBCLC_REMANGLER )
+    message( WARNING
+      "Custom toolchain incomplete, will try the default location." )
+  endif()
+endif()
+
 # Print LLVM variables
 message( "LLVM libdir: ${LLVM_LIBRARY_DIR}" )
 message( "LLVM bindir: ${LLVM_TOOLS_BINARY_DIR}" )
 message( "LLVM cxx flags: ${LLVM_CXX_FLAGS}" )
 message( "" )
 
+# It's OK to call find program again, if the variables have been set in the
+# custom location clause, find_program returns immediately.
 find_program( LLVM_CLANG clang PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH )
 find_program( LLVM_AS llvm-as PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH )
 find_program( LLVM_LINK llvm-link PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH )

sycl/doc/GetStartedGuide.md

@@ -377,6 +377,21 @@ control which revision of Unified Runtime should be used when building DPC++:
 * `SYCL_PI_UR_SOURCE_DIR` is a variable used to specify the path to the Unified
   Runtime repository when `SYCL_PI_UR_USE_FETCH_CONTENT` is set of `OFF`.
 
+### Build DPC++ libclc with a custom toolchain
+
+libclc is an implementation of the OpenCL required libraries, as described in
+the [OpenCL C specification](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_C.html),
+additionally providing definitions of SPIR-V builtins. It is built to
+target-specific bitcode, that is linked against SYCL binaries. By default, the
+built system uses the SYCL toolchain currently being built to create libclc
+bitcode. This can be suboptimal in case of debug builds, in which case debug
+tools are used to build non-debug libclc bitcode (the notion of debug builds
+doesn't really apply to libclc), resulting in very long compilation time. In
+order to specify a directory containing custom toolchain users can set:
+`LIBCLC_CUSTOM_LLVM_TOOLS_BINARY_DIR` variable. Care is required, as the
+changes to the local SYCL tree might not be reflected in the custom location
+during the build time.
+
 ### Deployment
 
 TODO: add instructions how to deploy built DPC++ toolchain.