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[libclc] Add an option to build SPIR-V targets with the LLVM backend #151347

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Merged
merged 1 commit into from
Aug 1, 2025
+44 −22
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[libclc] Add an option to build SPIR-V targets with the LLVM backend #151347

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41 changes: 25 additions & 16 deletions libclc/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,10 @@ set( LIBCLC_TARGETS_TO_BUILD "all"

option( ENABLE_RUNTIME_SUBNORMAL "Enable runtime linking of subnormal support." OFF )

option(
LIBCLC_USE_SPIRV_BACKEND "Build SPIR-V targets with the SPIR-V backend." OFF
)

# Top level target used to build all Libclc libraries.
add_custom_target( libclc ALL )

Expand Down Expand Up @@ -115,14 +119,17 @@ foreach( tool IN ITEMS clang opt llvm-as llvm-link )
endif()
endforeach()

# llvm-spirv is an optional dependency, used to build spirv-* targets.
# It may be provided in-tree or externally.
if( TARGET llvm-spirv )
get_host_tool_path( llvm-spirv LLVM_SPIRV llvm-spirv_exe llvm-spirv_target )
else()
find_program( LLVM_SPIRV llvm-spirv HINTS ${LLVM_TOOLS_BINARY_DIR} )
set( llvm-spirv_exe "${LLVM_SPIRV}" )
set( llvm-spirv_target )
if( NOT LIBCLC_USE_SPIRV_BACKEND )
# llvm-spirv is an optional dependency, used to build spirv-* targets when
# the SPIR-V backend hasn't been requested. It may be provided in-tree or
# externally.
if( TARGET llvm-spirv )
get_host_tool_path( llvm-spirv LLVM_SPIRV llvm-spirv_exe llvm-spirv_target )
else()
find_program( LLVM_SPIRV llvm-spirv HINTS ${LLVM_TOOLS_BINARY_DIR} )
set( llvm-spirv_exe "${LLVM_SPIRV}" )
set( llvm-spirv_target )
endif()
endif()

# List of all targets. Note that some are added dynamically below.
Expand All @@ -138,22 +145,24 @@ set( LIBCLC_TARGETS_ALL
nvptx64--nvidiacl
)

# mesa3d environment is only available since LLVM 4.0
# The mesa3d environment is only available since LLVM 4.0
if( LLVM_PACKAGE_VERSION VERSION_GREATER_EQUAL 4.0.0 )
list( APPEND LIBCLC_TARGETS_ALL amdgcn-mesa-mesa3d )
endif()

# spirv-mesa3d and spirv64-mesa3d targets can only be built with the (optional)
# llvm-spirv external tool.
if( llvm-spirv_exe )
list( APPEND LIBCLC_TARGETS_ALL spirv-mesa3d- spirv64-mesa3d- )
# The spirv-mesa3d and spirv64-mesa3d targets are optional and can be built
# with either the LLVM SPIR-V backend or the external llvm-spirv tool.
if( LIBCLC_USE_SPIRV_BACKEND OR llvm-spirv_exe )
list( APPEND LIBCLC_TARGETS_ALL spirv-mesa3d- spirv64-mesa3d- )
endif()

# Verify that the user hasn't requested mesa3d targets without an available
# llvm-spirv tool.
if( "spirv-mesa3d-" IN_LIST LIBCLC_TARGETS_TO_BUILD OR "spirv64-mesa3d-" IN_LIST LIBCLC_TARGETS_TO_BUILD )
if( NOT llvm-spirv_exe )
message( FATAL_ERROR "SPIR-V targets requested, but spirv-tools is not installed" )
if( spirv-mesa3d- IN_LIST LIBCLC_TARGETS_TO_BUILD
OR spirv64-mesa3d- IN_LIST LIBCLC_TARGETS_TO_BUILD )
if( NOT LIBCLC_USE_SPIRV_BACKEND AND NOT llvm-spirv_exe )
message( FATAL_ERROR "SPIR-V targets requested, but spirv-tools is not "
"installed and the SPIR-V backend has not been requested." )
endif()
endif()

Expand Down
25 changes: 19 additions & 6 deletions libclc/cmake/modules/AddLibclc.cmake
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Original file line number Diff line number Diff line change
Expand Up @@ -164,7 +164,9 @@ function(get_libclc_device_info)
list( GET TRIPLE 0 ARCH )

# Some targets don't have a specific device architecture to target
if( ARG_DEVICE STREQUAL none OR ARCH STREQUAL spirv OR ARCH STREQUAL spirv64 )
if( ARG_DEVICE STREQUAL none
OR ((ARCH STREQUAL spirv OR ARCH STREQUAL spirv64)
AND NOT LIBCLC_USE_SPIRV_BACKEND) )
set( cpu )
set( arch_suffix "${ARG_TRIPLE}" )
else()
Expand All @@ -182,7 +184,11 @@ function(get_libclc_device_info)

# Some libclc targets are not real clang triples: return their canonical
# triples.
if( ARCH STREQUAL spirv OR ARCH STREQUAL clspv )
if( ARCH STREQUAL spirv AND LIBCLC_USE_SPIRV_BACKEND )
set( ARG_TRIPLE "spirv32--" )
elseif( ARCH STREQUAL spirv64 AND LIBCLC_USE_SPIRV_BACKEND )
set( ARG_TRIPLE "spirv64--" )
elseif( ARCH STREQUAL spirv OR ARCH STREQUAL clspv )
set( ARG_TRIPLE "spir--" )
elseif( ARCH STREQUAL spirv64 OR ARCH STREQUAL clspv64 )
set( ARG_TRIPLE "spir64--" )
Expand Down Expand Up @@ -363,10 +369,17 @@ function(add_libclc_builtin_set)
if( ARG_ARCH STREQUAL spirv OR ARG_ARCH STREQUAL spirv64 )
set( obj_suffix ${ARG_ARCH_SUFFIX}.spv )
set( libclc_builtins_lib ${LIBCLC_OUTPUT_LIBRARY_DIR}/${obj_suffix} )
add_custom_command( OUTPUT ${libclc_builtins_lib}
COMMAND ${llvm-spirv_exe} ${spvflags} -o ${libclc_builtins_lib} ${builtins_link_lib}
DEPENDS ${llvm-spirv_target} ${builtins_link_lib} ${builtins_link_lib_tgt}
)
if ( LIBCLC_USE_SPIRV_BACKEND )
add_custom_command( OUTPUT ${libclc_builtins_lib}
COMMAND ${clang_exe} --target=${ARG_TRIPLE} -x ir -o ${libclc_builtins_lib} ${builtins_link_lib}
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should -c be added to clang command line? It avoids spirv-link step.

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Good question! I'm not sure, to be honest. I tried using -c and it generates a slightly larger file (6900 vs 6792). It seems to take a few fewer seconds to do so, which is an upside. I haven't looked into what exactly it's doing.

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@wenju-he wenju-he Aug 1, 2025 •
edited
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perhaps spirv-link does optimization, but obviously there is only one file to link. LGTM as the final size is smaller, so this is not meant to be a blocker.

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sorry, not meant to be

DEPENDS ${clang_target} ${builtins_link_lib} ${builtins_link_lib_tgt}
)
else()
add_custom_command( OUTPUT ${libclc_builtins_lib}
COMMAND ${llvm-spirv_exe} ${spvflags} -o ${libclc_builtins_lib} ${builtins_link_lib}
DEPENDS ${llvm-spirv_target} ${builtins_link_lib} ${builtins_link_lib_tgt}
)
endif()
else()
# Non-SPIR-V targets add an extra step to optimize the bytecode
set( builtins_opt_lib_tgt builtins.opt.${ARG_ARCH_SUFFIX} )
Expand Down
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