[libc-commits] [libc] e39233d - [libc] Add baremetal minor archive support. (#203015)

[via libc-commits]

Author: Simi Pallipurath
Date: 2026-10-08T10:04:25+01:00
New Revision: e39233d7f36c19c73cf6bd89e62dca8e8632e7b6

URL: https://github.com/llvm/llvm-project/commit/e39233d7f36c19c73cf6bd89e62dca8e8632e7b6
DIFF: https://github.com/llvm/llvm-project/commit/e39233d7f36c19c73cf6bd89e62dca8e8632e7b6.diff

LOG: [libc] Add baremetal minor archive support. (#203015)

This adds support for building a minor LLVM libc archive on baremetal
targets, such as libllvmstring.a, from only the entrypoints under a
requested libc subdirectory.

When ENABLE_LLVM_LIBC_MINOR_VARIANT is enabled, the libc archive build
now:
1. Requires LLVM_LIBC_SUBARCHIVE_NAME to be set.
3. Selects only entrypoints matching that subarchive name, for example
libc.src.<name>.*.
4. Builds and installs an archive using the requested name.
5. Avoids installing startup objects for minor archive builds.

This lets a minor variant provide only one category of libc routines
instead of duplicating a full libc.a.

Assisted-by: codex, reviewed and cross checked, also tested with
ATfE(Arm Toolchain for Embedded), by me.

Added: 
    

Modified: 
    libc/CMakeLists.txt
    libc/lib/CMakeLists.txt
    libc/startup/baremetal/CMakeLists.txt

Removed: 
    


################################################################################
diff  --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
index 86ddcd2df264f..a5e3cd20f5a8e 100644
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -270,6 +270,12 @@ option(LLVM_LIBC_ENABLE_LINTING "Enables linting of libc source files" OFF)
 option(LLVM_LIBC_ALL_HEADERS "Outputs all functions in header files, regardless of whether they are enabled on this target" OFF)
 option(LLVM_LIBC_ENABLE_EXPERIMENTAL_ENTRYPOINTS
   "Enable entrypoints with known-incomplete implementations (off by default)" OFF)
+option(LLVM_LIBC_ENABLE_MINOR_VARIANT
+  "Build LLVM libc minor variant archive" OFF)
+set(LLVM_LIBC_SUBARCHIVE_NAME "" CACHE STRING
+  "Target and archive name for the LLVM libc minor variant")
+set(LLVM_LIBC_SUBARCHIVE_ENTRYPOINTS "" CACHE STRING
+  "Semicolon-separated entrypoint targets for the LLVM libc minor variant")
 
 option(LIBC_CONFIG_PATH "The path to user provided folder that configures the build for the target system." OFF)
 

diff  --git a/libc/lib/CMakeLists.txt b/libc/lib/CMakeLists.txt
index 0d8e7424e6247..b244a8f766756 100644
--- a/libc/lib/CMakeLists.txt
+++ b/libc/lib/CMakeLists.txt
@@ -1,11 +1,33 @@
+set(libc_subarchive_entrypoints "")
+if(LLVM_LIBC_ENABLE_MINOR_VARIANT)
+  if(NOT LLVM_LIBC_SUBARCHIVE_NAME)
+    message(FATAL_ERROR
+      "LLVM_LIBC_ENABLE_MINOR_VARIANT requires LLVM_LIBC_SUBARCHIVE_NAME to be set.")
+  endif()
+  if(NOT LLVM_LIBC_SUBARCHIVE_ENTRYPOINTS)
+    message(FATAL_ERROR
+      "LLVM_LIBC_ENABLE_MINOR_VARIANT requires "
+      "LLVM_LIBC_SUBARCHIVE_ENTRYPOINTS to be set.")
+  endif()
+endif()
+
 set(libc_archive_targets "")
 set(libc_archive_names "")
 set(libc_archive_entrypoint_lists "")
-if(LLVM_LIBC_FULL_BUILD)
+if(LLVM_LIBC_ENABLE_MINOR_VARIANT)
+  foreach(entrypoint IN LISTS LLVM_LIBC_SUBARCHIVE_ENTRYPOINTS)
+    if(NOT entrypoint IN_LIST TARGET_LLVMLIBC_ENTRYPOINTS)
+      message(FATAL_ERROR
+        "LLVM_LIBC_SUBARCHIVE_ENTRYPOINTS contains '${entrypoint}', which is "
+        "not enabled by the target's entrypoints.txt configuration.")
+    endif()
+    list(APPEND libc_subarchive_entrypoints "${entrypoint}")
+  endforeach()
+elseif(LLVM_LIBC_FULL_BUILD)
   list(APPEND libc_archive_names c m mvec)
   list(APPEND libc_archive_targets libc libm libmvec)
   list(APPEND libc_archive_entrypoint_lists
-    TARGET_LIBC_ENTRYPOINTS TARGET_LIBM_ENTRYPOINTS TARGET_LIBMVEC_ENTRYPOINTS)
+       TARGET_LIBC_ENTRYPOINTS TARGET_LIBM_ENTRYPOINTS TARGET_LIBMVEC_ENTRYPOINTS)
 else()
   list(APPEND libc_archive_names llvmlibc)
   list(APPEND libc_archive_targets libc)
@@ -56,6 +78,26 @@ foreach(archive IN ZIP_LISTS
   endif()
 endforeach()
 
+if(libc_subarchive_entrypoints)
+  add_entrypoint_library(
+    ${LLVM_LIBC_SUBARCHIVE_NAME}
+    DEPENDS
+      ${libc_subarchive_entrypoints}
+  )
+  set_target_properties(
+    ${LLVM_LIBC_SUBARCHIVE_NAME}
+    PROPERTIES
+      ARCHIVE_OUTPUT_NAME llvm${LLVM_LIBC_SUBARCHIVE_NAME}
+  )
+  if(LLVM_LIBC_FULL_BUILD)
+    target_link_libraries(${LLVM_LIBC_SUBARCHIVE_NAME} PUBLIC libc-headers)
+    if(TARGET libc-startup)
+      add_dependencies(${LLVM_LIBC_SUBARCHIVE_NAME} libc-startup)
+    endif()
+  endif()
+  list(APPEND added_archive_targets ${LLVM_LIBC_SUBARCHIVE_NAME})
+endif()
+
 install(
   TARGETS ${added_archive_targets}
   ARCHIVE DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}

diff  --git a/libc/startup/baremetal/CMakeLists.txt b/libc/startup/baremetal/CMakeLists.txt
index 2b46b79054426..d186b65688c80 100644
--- a/libc/startup/baremetal/CMakeLists.txt
+++ b/libc/startup/baremetal/CMakeLists.txt
@@ -69,7 +69,9 @@ add_custom_target(libc-startup)
 
 set(fq_target_name libc.startup.baremetal.${LIBC_TARGET_ARCHITECTURE}.crt1)
 add_dependencies(libc-startup ${fq_target_name})
-install(FILES $<TARGET_OBJECTS:${fq_target_name}>
-  DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
-  RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>
-  COMPONENT libc)
+if(NOT LLVM_LIBC_ENABLE_MINOR_VARIANT)
+  install(FILES $<TARGET_OBJECTS:${fq_target_name}>
+          DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
+          RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>
+          COMPONENT libc)
+endif()