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

[Simi Pallipurath via libc-commits]

https://github.com/simpal01 created https://github.com/llvm/llvm-project/pull/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 a baremetal target.
2. 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 baremetal 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, by me.

>From 3cfc6e639428dc9421d6b33046a02e395b49754b Mon Sep 17 00:00:00 2001
From: Simi Pallipurath <simi.pallipurath at arm.com>
Date: Wed, 10 Jun 2026 13:10:15 +0100
Subject: [PATCH] [libc] Add baremetal minor archive support.

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 a baremetal target.
2. 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 baremetal 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,
by me.
---
 libc/lib/CMakeLists.txt               | 62 +++++++++++++++++++++++----
 libc/startup/baremetal/CMakeLists.txt | 10 +++--
 2 files changed, 60 insertions(+), 12 deletions(-)

diff --git a/libc/lib/CMakeLists.txt b/libc/lib/CMakeLists.txt
index 08026e937be94..fa057c1c50137 100644
--- a/libc/lib/CMakeLists.txt
+++ b/libc/lib/CMakeLists.txt
@@ -1,15 +1,40 @@
+set(libc_subarchive_entrypoints "")
+if(ENABLE_LLVM_LIBC_MINOR_VARIANT)
+  if(NOT LIBC_TARGET_OS_IS_BAREMETAL)
+    message(FATAL_ERROR
+            "ENABLE_LLVM_LIBC_MINOR_VARIANT is only supported on baremetal targets.")
+  endif()
+  if(NOT LLVM_LIBC_SUBARCHIVE_NAME)
+    message(FATAL_ERROR
+            "ENABLE_LLVM_LIBC_MINOR_VARIANT requires LLVM_LIBC_SUBARCHIVE_NAME to be set.")
+  endif()
+endif()
+
 set(libc_archive_targets "")
 set(libc_archive_names "")
 set(libc_archive_entrypoint_lists "")
-if(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)
+if(NOT ENABLE_LLVM_LIBC_MINOR_VARIANT)
+  if(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)
+  else()
+    list(APPEND libc_archive_names llvmlibc)
+    list(APPEND libc_archive_targets libc)
+    list(APPEND libc_archive_entrypoint_lists TARGET_LLVMLIBC_ENTRYPOINTS)
+  endif()
 else()
-  list(APPEND libc_archive_names llvmlibc)
-  list(APPEND libc_archive_targets libc)
-  list(APPEND libc_archive_entrypoint_lists TARGET_LLVMLIBC_ENTRYPOINTS)
+  foreach(entrypoint IN LISTS TARGET_LLVMLIBC_ENTRYPOINTS)
+    if(entrypoint MATCHES "^libc\\.src\\.${LLVM_LIBC_SUBARCHIVE_NAME}\\.")
+      list(APPEND libc_subarchive_entrypoints ${entrypoint})
+    endif()
+  endforeach()
+  if(NOT libc_subarchive_entrypoints)
+    message(FATAL_ERROR
+            "ENABLE_LLVM_LIBC_MINOR_VARIANT requires matching entrypoints for "
+            "'${LLVM_LIBC_SUBARCHIVE_NAME}'.")
+  endif()
 endif()
 
 set(added_archive_targets "")
@@ -56,6 +81,27 @@ 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(
+    ${LLVM_LIBC_SUBARCHIVE_NAME}
+    PROPERTIES
+      PREFIX ""
+      ARCHIVE_OUTPUT_NAME llvm${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 e361000f788ea..0e50126b81bf2 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 ENABLE_LLVM_LIBC_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()