Frontends

Possible compilation work-flows:

Source language	compilation work-flows	comparison	comments
SPIR-V (binary/text)	SPIR-V front-end	fast	requires SPRV-LLVM Translator or SPIRV-LLVM
LLVM IR binary	LLVM library front-end llvm-spirv + SPIR-V front-end	fast fast	least error-prone only for SPIRV-LLVM
LLVM IR text	llvm-as + either front-end		not implemented
OpenCL C	clang (with PCH) + LLVM library front-end clang (with PCH) + llvm-spirv + SPIR-V front-end clang + llvm-link (vc4cl std-lib module) + LLVM library front-end	default fast fast	slow on RPi with default clang fast on RPi with default clang

Compilation work-flows for version of clang:

"Default" clang

• SPIR-V: not supported
• LLVM IR: LLVM library front-end
• OpenCL C: clang + llvm-link + LLVM front-end

SPIRV-LLVM

• SPIR-V: SPIR-V front-end
• LLVM IR: LLVM library front-end or llvm-spirv + SPIR-V front-end
• OpenCL C: clang (with PCH) + llvm-spirv + SPIR-V front-end or clang (with PCH) + LLVM library front-end

Frontend	SPIR-V reader	LLVM library
Input formats	SPIR-V text/bin	LLVM IR text/bin
Speed	fast	faster
Supported LLVMs	SPIRV	Standard / SPIRV
Runtime deps	SPIRV-LLVM Translator / SPIRV-LLVM CLang	any CLang, matching LLVM library
Development deps	SPIRV-Headers (provided)	LLVM headers
Configuration	always enabled	LLVMLIB_FRONTEND set, LLVM_CONFIG_PATH found
Supports linking	yes	yes

SPIR-V reader

• Requires additional pre-compilation step to convert LLVM IR (generated from OpenCL C source-code) to SPIR-V
• Supports linking of SPIR-V modules via the SPIRV-Tools project (either in-process by setting the SPIRV_FRONTEND CMake flag or by using the spirv-link executable)

LLVM library

• Uses the LLVM library to parse the LLVM IR module
• Only supports one LLVM version at a time, depending on against which library the front-end is linked against
• Supports linking of LLVM IR modules via llvm-link tool provided by LLVM
• Supports linking against static and shared LLVM libraries. For static linkage, the LLVM library does not need to be available at run-time
• The location the LLVM library and headers as well as the type of library (shared, static) to include is automatically obtained from the llvm-config tool found at LLVM_CONFIG_PATH. If LLVM_CONFIG_PATH is not set, the CMake find_package mechanism is used to retrieve the LLVM configuration.