[llvm] [Support] Add option to use Windows vendored ICU (PR #186371)

[via llvm-commits]

https://github.com/paperchalice updated https://github.com/llvm/llvm-project/pull/186371

>From 373f2e2b36e93ef3b52058cd6314efaedb8f66ed Mon Sep 17 00:00:00 2001
From: PaperChalice <liujunchang97 at outlook.com>
Date: Fri, 13 Mar 2026 19:02:47 +0800
Subject: [PATCH] [Support] Add option to use Windows vendored ICU

---
 llvm/CMakeLists.txt                     |  9 +++++++++
 llvm/cmake/config-ix.cmake              | 21 +++++++++++++++------
 llvm/include/llvm/Config/config.h.cmake |  3 +++
 llvm/lib/Support/CMakeLists.txt         | 18 ++++++++++++++----
 llvm/lib/Support/TextEncoding.cpp       |  4 ++++
 5 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 420774b629b8b..23c4601bdeea4 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -614,6 +614,15 @@ option(LLVM_ENABLE_THREADS "Use threads if available." ON)
 
 set(LLVM_ENABLE_ICU "OFF" CACHE STRING "Use ICU for text encoding conversion support if available. Can be ON, OFF, or FORCE_ON")
 
+# After Windows 10 1903, icu.lib is available.
+if(WIN32 AND CMAKE_SYSTEM_VERSION VERSION_GREATER_EQUAL "10.0.18362")
+  cmake_dependent_option(LLVM_ENABLE_WINDOWS_ICU "Use Windows vendored ICU if possible" OFF LLVM_ENABLE_ICU OFF)
+
+  # test
+  set(LLVM_ENABLE_ICU ON)
+  set(LLVM_ENABLE_WINDOWS_ICU ON)
+endif()
+
 set(LLVM_ENABLE_ICONV "OFF" CACHE STRING "Use iconv for text encoding conversion support if available. Can be ON, OFF, or FORCE_ON")
 
 set(LLVM_ENABLE_ZLIB "ON" CACHE STRING "Use zlib for compression/decompression if available. Can be ON, OFF, or FORCE_ON")
diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
index a12eaf76d15df..7a5631b1ae3b5 100644
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -313,15 +313,24 @@ endif()
 if(LLVM_ENABLE_ICU AND NOT(LLVM_ENABLE_ICONV STREQUAL FORCE_ON))
   set(LIBRARY_SUFFIXES ${CMAKE_FIND_LIBRARY_SUFFIXES})
   set(CMAKE_FIND_LIBRARY_SUFFIXES "${CMAKE_SHARED_LIBRARY_SUFFIX}")
-  if (LLVM_ENABLE_ICU STREQUAL FORCE_ON)
-    find_package(ICU REQUIRED COMPONENTS uc i18n)
-    if (NOT ICU_FOUND)
-      message(FATAL_ERROR "Failed to configure ICU, but LLVM_ENABLE_ICU is FORCE_ON")
+  set(HAVE_WINDOWS_ICU OFF)
+  if(LLVM_ENABLE_WINDOWS_ICU AND WIN32 AND CMAKE_SYSTEM_VERSION VERSION_GREATER_EQUAL "10.0.18362")
+    message(STATUS "Use Windows vendored ICU")
+    set(HAVE_WINDOWS_ICU ON)
+  endif()
+  if(NOT HAVE_WINDOWS_ICU)
+    if (LLVM_ENABLE_ICU STREQUAL FORCE_ON)
+      find_package(ICU REQUIRED COMPONENTS uc i18n)
+      if (NOT ICU_FOUND)
+        message(FATAL_ERROR "Failed to configure ICU, but LLVM_ENABLE_ICU is FORCE_ON")
+      endif()
+    else()
+      find_package(ICU COMPONENTS uc i18n)
     endif()
+    set(HAVE_ICU ${ICU_FOUND})
   else()
-    find_package(ICU COMPONENTS uc i18n)
+    set(HAVE_ICU ON)
   endif()
-  set(HAVE_ICU ${ICU_FOUND})
   set(CMAKE_FIND_LIBRARY_SUFFIXES ${LIBRARY_SUFFIXES})
 endif()
 
diff --git a/llvm/include/llvm/Config/config.h.cmake b/llvm/include/llvm/Config/config.h.cmake
index ce83de8e4cba9..94fde52de11b3 100644
--- a/llvm/include/llvm/Config/config.h.cmake
+++ b/llvm/include/llvm/Config/config.h.cmake
@@ -242,6 +242,9 @@
 /* Define if ICU library is available */
 #cmakedefine01 HAVE_ICU
 
+/* Define if Windows vendored ICU is available */
+#cmakedefine01 HAVE_WINDOWS_ICU
+
 /* Define if iconv library is available */
 #cmakedefine01 HAVE_ICONV
 
diff --git a/llvm/lib/Support/CMakeLists.txt b/llvm/lib/Support/CMakeLists.txt
index 380be042df69d..392502b0d6dd0 100644
--- a/llvm/lib/Support/CMakeLists.txt
+++ b/llvm/lib/Support/CMakeLists.txt
@@ -42,6 +42,9 @@ if( WIN32 )
   # advapi32 required for CryptAcquireContextW in lib/Support/Windows/Path.inc.
   # ntdll required for RtlGetLastNtStatus in lib/Support/ErrorHandling.cpp.
   set(system_libs ${system_libs} psapi shell32 ole32 uuid advapi32 ws2_32 ntdll)
+  if( HAVE_WINDOWS_ICU )
+    list(APPEND system_libs icu)
+  endif()
 elseif( CMAKE_HOST_UNIX )
   if( HAVE_LIBRT )
     set(system_libs ${system_libs} rt)
@@ -334,10 +337,17 @@ add_llvm_component_library(LLVMSupport
 
 # Link ICU library if it is an external library.
 if(ICU_FOUND)
-  target_link_libraries(LLVMSupport
-  PRIVATE
-  ${ICU_LIBRARIES}
-  )
+  if(HAVE_WINDOWS_ICU)
+    target_link_libraries(LLVMSupport
+    PRIVATE
+    icu
+    )
+  else()
+    target_link_libraries(LLVMSupport
+    PRIVATE
+    ${ICU_LIBRARIES}
+    )
+  endif()
 endif()
 
 set(llvm_system_libs ${system_libs})
diff --git a/llvm/lib/Support/TextEncoding.cpp b/llvm/lib/Support/TextEncoding.cpp
index 453af6f7287bc..d36f02c1300b9 100644
--- a/llvm/lib/Support/TextEncoding.cpp
+++ b/llvm/lib/Support/TextEncoding.cpp
@@ -20,7 +20,11 @@
 #include <system_error>
 
 #if HAVE_ICU
+#if HAVE_WINDOWS_ICU
+#include <icu.h>
+#else
 #include <unicode/ucnv.h>
+#endif
 #elif HAVE_ICONV
 #include <iconv.h>
 #endif