Index: llvm/CMakeLists.txt
===================================================================
--- llvm/CMakeLists.txt
+++ llvm/CMakeLists.txt
@@ -423,7 +423,7 @@
 
 option(LLVM_ENABLE_TERMINFO "Use terminfo database if available." ON)
 
-set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, or FORCE_ON")
+set(LLVM_ENABLE_LIBXML2 "ON" CACHE STRING "Use libxml2 if available. Can be ON, OFF, FORCE_ON, or HERMETIC")
 
 option(LLVM_ENABLE_LIBEDIT "Use libedit if available." ON)
 
Index: llvm/cmake/config-ix.cmake
===================================================================
--- llvm/cmake/config-ix.cmake
+++ llvm/cmake/config-ix.cmake
@@ -137,7 +137,10 @@
 endif()
 
 if(LLVM_ENABLE_LIBXML2)
-  if(LLVM_ENABLE_LIBXML2 STREQUAL FORCE_ON)
+  if(LLVM_ENABLE_LIBXML2 STREQUAL HERMETIC)
+    include(HermeticLibXML2)
+    set(LLVM_LIBXML2_IS_HERMETIC ON)
+  elseif(LLVM_ENABLE_LIBXML2 STREQUAL FORCE_ON)
     find_package(LibXml2 REQUIRED)
   elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
     find_package(LibXml2)
Index: llvm/cmake/modules/HermeticLibXML2.cmake
===================================================================
--- /dev/null
+++ llvm/cmake/modules/HermeticLibXML2.cmake
@@ -0,0 +1,74 @@
+include(ExternalProject)
+
+# Once we require CMake 3.24+, we could use FetchContent_Declare() and
+# FetchContent_MakeAvailable() to intercept the normal find_package() call.
+
+set(HERMETIC_LIBXML2_INSTALL_DIR "${CMAKE_CURRENT_BINARY_DIR}/libxml2-install")
+
+# Set variables that `find_package(LibXml2)` would set.
+set(LibXml2_FOUND ON)
+set(LIBXML2_INCLUDE_DIRS "${HERMETIC_LIBXML2_INSTALL_DIR}/include/libxml2")
+if (WIN32)
+  set(LIBXML2_LIBRARY "${HERMETIC_LIBXML2_INSTALL_DIR}/lib/libxml2s.lib")
+else ()
+  set(LIBXML2_LIBRARY "${HERMETIC_LIBXML2_INSTALL_DIR}/lib/libxml2.a")
+endif ()
+set(LIBXML2_LIBRARIES "${LIBXML2_LIBRARY}")
+set(LIBXML2_DEFINITIONS "LIBXML_STATIC")
+
+# Disable everything except WITH_TREE and WITH_OUTPUT, both needed by LLVM's
+# WindowsManifestMerger.
+# Also enable WITH_THREADS, else libxml doesn't compile on Linux.
+ExternalProject_Add(LibXML2
+  GIT_REPOSITORY    https://gitlab.gnome.org/GNOME/libxml2
+  GIT_TAG           v2.9.12
+  GIT_SHALLOW       ON
+  SOURCE_DIR ${CMAKE_CURRENT_BINARY_DIR}/libxml2-src
+  BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/libxml2-bin
+  INSTALL_DIR ${HERMETIC_LIBXML2_INSTALL_DIR}
+
+  # ExternalProject_Add() happens in a sub-build. Let the outer build know
+  # that the subbuild will create $LIBXML2_LIBRARY.
+  BUILD_BYPRODUCTS ${LIBXML2_LIBRARY}
+
+  CMAKE_ARGS
+    -DBUILD_SHARED_LIBS=OFF
+    -DCMAKE_INSTALL_PREFIX=${HERMETIC_LIBXML2_INSTALL_DIR}
+
+    -DLIBXML2_WITH_C14N=OFF
+    -DLIBXML2_WITH_CATALOG=OFF
+    -DLIBXML2_WITH_DEBUG=OFF
+    -DLIBXML2_WITH_DOCB=OFF
+    -DLIBXML2_WITH_FTP=OFF
+    -DLIBXML2_WITH_HTML=OFF
+    -DLIBXML2_WITH_HTTP=OFF
+    -DLIBXML2_WITH_ICONV=OFF
+    -DLIBXML2_WITH_ICU=OFF
+    -DLIBXML2_WITH_ISO8859X=OFF
+    -DLIBXML2_WITH_LEGACY=OFF
+    -DLIBXML2_WITH_LZMA=OFF
+    -DLIBXML2_WITH_MEM_DEBUG=OFF
+    -DLIBXML2_WITH_MODULES=OFF
+    -DLIBXML2_WITH_OUTPUT=ON
+    -DLIBXML2_WITH_PATTERN=OFF
+    -DLIBXML2_WITH_PROGRAMS=OFF
+    -DLIBXML2_WITH_PUSH=OFF
+    -DLIBXML2_WITH_PYTHON=OFF
+    -DLIBXML2_WITH_READER=OFF
+    -DLIBXML2_WITH_REGEXPS=OFF
+    -DLIBXML2_WITH_RUN_DEBUG=OFF
+    -DLIBXML2_WITH_SAX1=OFF
+    -DLIBXML2_WITH_SCHEMAS=OFF
+    -DLIBXML2_WITH_SCHEMATRON=OFF
+    -DLIBXML2_WITH_TESTS=OFF
+    -DLIBXML2_WITH_THREADS=ON
+    -DLIBXML2_WITH_THREAD_ALLOC=OFF
+    -DLIBXML2_WITH_TREE=ON
+    -DLIBXML2_WITH_VALID=OFF
+    -DLIBXML2_WITH_WRITER=OFF
+    -DLIBXML2_WITH_XINCLUDE=OFF
+    -DLIBXML2_WITH_XPATH=OFF
+    -DLIBXML2_WITH_XPTR=OFF
+    -DLIBXML2_WITH_ZLIB=OFF
+  )
+
Index: llvm/lib/WindowsManifest/CMakeLists.txt
===================================================================
--- llvm/lib/WindowsManifest/CMakeLists.txt
+++ llvm/lib/WindowsManifest/CMakeLists.txt
@@ -1,7 +1,8 @@
 include(GetLibraryName)
 
 if(LLVM_ENABLE_LIBXML2)
-  set(imported_libs LibXml2::LibXml2)
+  set(imported_libs ${LIBXML2_LIBRARIES})
+  include_directories(BEFORE ${LIBXML2_INCLUDE_DIRS})
 endif()
 
 add_llvm_component_library(LLVMWindowsManifest
@@ -18,17 +19,14 @@
   Support
   )
 
+if (LLVM_ENABLE_LIBXML2 AND LLVM_LIBXML2_IS_HERMETIC)
+  # Force hermetic LibXML2's install step to run.
+  add_dependencies(LLVMWindowsManifest LibXML2)
+endif()
+
 # This block is only needed for llvm-config. When we deprecate llvm-config and
 # move to using CMake export, this block can be removed.
 if(LLVM_ENABLE_LIBXML2)
-  # CMAKE_BUILD_TYPE is only meaningful to single-configuration generators.
-  if(CMAKE_BUILD_TYPE)
-    string(TOUPPER ${CMAKE_BUILD_TYPE} build_type)
-    get_property(libxml2_library TARGET LibXml2::LibXml2 PROPERTY LOCATION_${build_type})
-  endif()
-  if(NOT libxml2_library)
-    get_property(libxml2_library TARGET LibXml2::LibXml2 PROPERTY LOCATION)
-  endif()
-  get_library_name(${libxml2_library} libxml2_library)
-  set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS ${libxml2_library})
+  get_library_name(${LIBXML2_LIBRARY} libxml2_library_name)
+  set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS ${libxml2_library_name})
 endif()
Index: llvm/utils/release/build_llvm_release.bat
===================================================================
--- llvm/utils/release/build_llvm_release.bat
+++ llvm/utils/release/build_llvm_release.bat
@@ -82,6 +82,7 @@
   -DLLVM_ENABLE_ASSERTIONS=OFF ^
   -DLLVM_INSTALL_TOOLCHAIN_ONLY=ON ^
   -DLLVM_BUILD_LLVM_C_DYLIB=ON ^
+  -DLLVM_ENABLE_LIBXML2=HERMETIC ^
   -DCMAKE_INSTALL_UCRT_LIBRARIES=ON ^
   -DPython3_FIND_REGISTRY=NEVER ^
   -DPACKAGE_VERSION=%package_version% ^
@@ -89,19 +90,12 @@
   -DLLDB_EMBED_PYTHON_HOME=OFF ^
   -DLLDB_TEST_COMPILER=%cd%\build32_stage0\bin\clang.exe ^
   -DCMAKE_CL_SHOWINCLUDES_PREFIX="Note: including file: " ^
-  -DLLVM_ENABLE_LIBXML2=FORCE_ON ^
-  -DLLDB_ENABLE_LIBXML2=OFF ^
-  -DCMAKE_C_FLAGS="-DLIBXML_STATIC" ^
-  -DCMAKE_CXX_FLAGS="-DLIBXML_STATIC" ^
   -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;lld;compiler-rt;lldb;openmp"
 
 REM TODO: Run the "check-all" tests.
 
 set OLDPATH=%PATH%
 
-curl -O https://gitlab.gnome.org/GNOME/libxml2/-/archive/v2.9.12/libxml2-v2.9.12.tar.gz || exit /b
-tar zxf libxml2-v2.9.12.tar.gz
-
 set "VSCMD_START_DIR=%CD%"
 call "%vsdevcmd%" -arch=x86
 set PATH=%python32_dir%;%PATH%
@@ -109,16 +103,7 @@
 set CXX=
 mkdir build32_stage0
 cd build32_stage0
-
-mkdir libxmlbuild
-cd libxmlbuild
-cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install -DBUILD_SHARED_LIBS=OFF -DLIBXML2_WITH_C14N=OFF -DLIBXML2_WITH_CATALOG=OFF -DLIBXML2_WITH_DEBUG=OFF -DLIBXML2_WITH_DOCB=OFF -DLIBXML2_WITH_FTP=OFF -DLIBXML2_WITH_HTML=OFF -DLIBXML2_WITH_HTTP=OFF -DLIBXML2_WITH_ICONV=OFF -DLIBXML2_WITH_ICU=OFF -DLIBXML2_WITH_ISO8859X=OFF -DLIBXML2_WITH_LEGACY=OFF -DLIBXML2_WITH_LZMA=OFF -DLIBXML2_WITH_MEM_DEBUG=OFF -DLIBXML2_WITH_MODULES=OFF -DLIBXML2_WITH_OUTPUT=ON -DLIBXML2_WITH_PATTERN=OFF -DLIBXML2_WITH_PROGRAMS=OFF -DLIBXML2_WITH_PUSH=OFF -DLIBXML2_WITH_PYTHON=OFF -DLIBXML2_WITH_READER=OFF -DLIBXML2_WITH_REGEXPS=OFF -DLIBXML2_WITH_RUN_DEBUG=OFF -DLIBXML2_WITH_SAX1=OFF -DLIBXML2_WITH_SCHEMAS=OFF -DLIBXML2_WITH_SCHEMATRON=OFF -DLIBXML2_WITH_TESTS=OFF -DLIBXML2_WITH_THREADS=ON -DLIBXML2_WITH_THREAD_ALLOC=OFF -DLIBXML2_WITH_TREE=ON -DLIBXML2_WITH_VALID=OFF -DLIBXML2_WITH_WRITER=OFF -DLIBXML2_WITH_XINCLUDE=OFF -DLIBXML2_WITH_XPATH=OFF -DLIBXML2_WITH_XPTR=OFF -DLIBXML2_WITH_ZLIB=OFF ../../libxml2-v2.9.12 || exit /b
-ninja install || exit /b
-set libxmldir=%cd%\install
-set "libxmldir=%libxmldir:\=/%"
-cd ..
-
-cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python32_dir% -DPython3_ROOT_DIR=%python32_dir% -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib ..\llvm-project\llvm || exit /b
+cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python32_dir% -DPython3_ROOT_DIR=%python32_dir% ..\llvm-project\llvm || exit /b
 
 ninja || ninja || ninja || exit /b
 REM ninja check-llvm || ninja check-llvm || ninja check-llvm || exit /b
@@ -132,7 +117,7 @@
 cd build32
 set CC=..\build32_stage0\bin\clang-cl
 set CXX=..\build32_stage0\bin\clang-cl
-cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python32_dir% -DPython3_ROOT_DIR=%python32_dir% -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib ..\llvm-project\llvm || exit /b
+cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python32_dir% -DPython3_ROOT_DIR=%python32_dir% ..\llvm-project\llvm || exit /b
 ninja || ninja || ninja || exit /b
 REM ninja check-llvm || ninja check-llvm || ninja check-llvm || exit /b
 REM ninja check-clang || ninja check-clang || ninja check-clang || exit /b
@@ -150,16 +135,7 @@
 set CXX=
 mkdir build64_stage0
 cd build64_stage0
-
-mkdir libxmlbuild
-cd libxmlbuild
-cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=install -DBUILD_SHARED_LIBS=OFF -DLIBXML2_WITH_C14N=OFF -DLIBXML2_WITH_CATALOG=OFF -DLIBXML2_WITH_DEBUG=OFF -DLIBXML2_WITH_DOCB=OFF -DLIBXML2_WITH_FTP=OFF -DLIBXML2_WITH_HTML=OFF -DLIBXML2_WITH_HTTP=OFF -DLIBXML2_WITH_ICONV=OFF -DLIBXML2_WITH_ICU=OFF -DLIBXML2_WITH_ISO8859X=OFF -DLIBXML2_WITH_LEGACY=OFF -DLIBXML2_WITH_LZMA=OFF -DLIBXML2_WITH_MEM_DEBUG=OFF -DLIBXML2_WITH_MODULES=OFF -DLIBXML2_WITH_OUTPUT=ON -DLIBXML2_WITH_PATTERN=OFF -DLIBXML2_WITH_PROGRAMS=OFF -DLIBXML2_WITH_PUSH=OFF -DLIBXML2_WITH_PYTHON=OFF -DLIBXML2_WITH_READER=OFF -DLIBXML2_WITH_REGEXPS=OFF -DLIBXML2_WITH_RUN_DEBUG=OFF -DLIBXML2_WITH_SAX1=OFF -DLIBXML2_WITH_SCHEMAS=OFF -DLIBXML2_WITH_SCHEMATRON=OFF -DLIBXML2_WITH_TESTS=OFF -DLIBXML2_WITH_THREADS=ON -DLIBXML2_WITH_THREAD_ALLOC=OFF -DLIBXML2_WITH_TREE=ON -DLIBXML2_WITH_VALID=OFF -DLIBXML2_WITH_WRITER=OFF -DLIBXML2_WITH_XINCLUDE=OFF -DLIBXML2_WITH_XPATH=OFF -DLIBXML2_WITH_XPTR=OFF -DLIBXML2_WITH_ZLIB=OFF ../../libxml2-v2.9.12 || exit /b
-ninja install || exit /b
-set libxmldir=%cd%\install
-set "libxmldir=%libxmldir:\=/%"
-cd ..
-
-cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python64_dir% -DPython3_ROOT_DIR=%python64_dir% -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib ..\llvm-project\llvm || exit /b
+cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python64_dir% -DPython3_ROOT_DIR=%python64_dir% ..\llvm-project\llvm || exit /b
 ninja || ninja || ninja || exit /b
 ninja check-llvm || ninja check-llvm || ninja check-llvm || exit /b
 ninja check-clang || ninja check-clang || ninja check-clang || exit /b
@@ -173,7 +149,7 @@
 cd build64
 set CC=..\build64_stage0\bin\clang-cl
 set CXX=..\build64_stage0\bin\clang-cl
-cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python64_dir% -DPython3_ROOT_DIR=%python64_dir% -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib ..\llvm-project\llvm || exit /b
+cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python64_dir% -DPython3_ROOT_DIR=%python64_dir% ..\llvm-project\llvm || exit /b
 ninja || ninja || ninja || exit /b
 ninja check-llvm || ninja check-llvm || ninja check-llvm || exit /b
 ninja check-clang || ninja check-clang || ninja check-clang || exit /b