[llvm-branch-commits] [llvm] release/23.x: [cmake] FindLibXml2: keep user-provided LIBXML2_DEFINITIONS (#221294) (PR #222008)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/222008

Backport cb180d50a81674b467887eab836f563607ae89e8

Requested by: @rvandermeulen

>From f573022f7c45ad993924b525171a53521fd1d25e Mon Sep 17 00:00:00 2001
From: Ryan VanderMeulen <ryanvm at gmail.com>
Date: Tue, 8 Sep 2026 09:59:03 -0400
Subject: [PATCH] [cmake] FindLibXml2: keep user-provided LIBXML2_DEFINITIONS
 (#221294)

LLVM's `FindLibXml2.cmake` (added in #166867) unconditionally does
`set(LIBXML2_DEFINITIONS ${PC_LIBXML_CFLAGS_OTHER})` and puts
`PC_LIBXML_CFLAGS_OTHER` on the imported targets. CMake's own
`FindLibXml2` module only takes pkg-config's flags when the library it
found is the one pkg-config describes; otherwise it keeps whatever
`LIBXML2_DEFINITIONS` the user passed and exposes it through
`INTERFACE_COMPILE_OPTIONS`.

This matters when cross-compiling against a static libxml2 on Windows.
Passing `-DLIBXML2_DEFINITIONS=-DLIBXML_STATIC
-DLIBXML2_LIBRARIES=.../libxml2s.lib` worked with LLVM 22 (CMake's
module): the define reached both the `xmlReadMemory` configure check and
the consumers of `LibXml2::LibXml2`. With LLVM 23 it is silently
dropped, `xmlexports.h` declares the API `__declspec(dllimport)`, the
check fails to link against the static library and
`LLVM_ENABLE_LIBXML2=FORCE_ON` aborts with `Failed to configure
libxml2`. (pkg-config on the Linux host finds the host's libxml2, so
`PC_LIBXML_CFLAGS_OTHER` is empty and unrelated to the library actually
being used.)

Mirror CMake's behaviour: keep the user's definitions unless pkg-config
found the same library, and attach them to both `LibXml2::LibXml2` and
`LibXml2::LibXml2Static`.

Seen while building the Firefox clang 23 toolchains (x86_64 and aarch64
`-pc-windows-msvc`, cross-compiled from Linux with
`LLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON`).

---
**AI tool use disclosure** (per the [LLVM AI Tool Use
Policy](https://llvm.org/docs/AIToolPolicy.html)): this change was
developed with Claude Code assisting in the analysis and drafting. The
root cause was established against real 23.1.0 binaries; this cmake
change itself has so far only been verified to apply to 23.1.0 and main
and by code reading, while Firefox's CI validated the equivalent
workaround of passing -DLIBXML_STATIC via the compiler flags. The author
reviewed the change and is accountable for and able to answer questions
about it. Commits carry an `Assisted-by:` trailer.

(cherry picked from commit cb180d50a81674b467887eab836f563607ae89e8)
---
 llvm/cmake/modules/FindLibXml2.cmake | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/llvm/cmake/modules/FindLibXml2.cmake b/llvm/cmake/modules/FindLibXml2.cmake
index cbfafc7ecf577..f3f761606260d 100644
--- a/llvm/cmake/modules/FindLibXml2.cmake
+++ b/llvm/cmake/modules/FindLibXml2.cmake
@@ -59,6 +59,16 @@ if(NOT PC_LIBXML_VERSION AND LIBXML2_INCLUDE_DIR AND
   unset(_libxml2_version_str)
 endif()
 
+# Only take pkg-config's flags if it describes the library we found; otherwise
+# keep any user-provided LIBXML2_DEFINITIONS, like CMake's own module does.
+unset(LIBXML2_DEFINITIONS)
+foreach(libxml2_pc_lib_dir IN LISTS PC_LIBXML_LIBDIR PC_LIBXML_LIBRARY_DIRS)
+  if(LIBXML2_LIBRARY MATCHES "^${libxml2_pc_lib_dir}")
+    set(LIBXML2_DEFINITIONS ${PC_LIBXML_CFLAGS_OTHER})
+    break()
+  endif()
+endforeach()
+
 find_package_handle_standard_args(LibXml2
   REQUIRED_VARS LIBXML2_LIBRARY LIBXML2_INCLUDE_DIR
   VERSION_VAR PC_LIBXML_VERSION
@@ -69,14 +79,14 @@ if(LibXml2_FOUND)
     add_library(LibXml2::LibXml2 UNKNOWN IMPORTED)
     set_target_properties(LibXml2::LibXml2 PROPERTIES
         INTERFACE_INCLUDE_DIRECTORIES "${LIBXML2_INCLUDE_DIR}"
-        INTERFACE_COMPILE_OPTIONS "${PC_LIBXML_CFLAGS_OTHER}"
+        INTERFACE_COMPILE_OPTIONS "${LIBXML2_DEFINITIONS}"
         IMPORTED_LOCATION "${LIBXML2_LIBRARY}")
   endif()
   if(LIBXML2_STATIC_LIBRARY AND NOT TARGET LibXml2::LibXml2Static)
     add_library(LibXml2::LibXml2Static STATIC IMPORTED)
     set_target_properties(LibXml2::LibXml2Static PROPERTIES
         INTERFACE_INCLUDE_DIRECTORIES "${LIBXML2_INCLUDE_DIR}"
-        INTERFACE_COMPILE_OPTIONS "${PC_LIBXML_CFLAGS_OTHER}"
+        INTERFACE_COMPILE_OPTIONS "${LIBXML2_DEFINITIONS}"
         IMPORTED_LOCATION "${LIBXML2_STATIC_LIBRARY}")
     # Static libraries need their transitive dependencies for linking.
     set(LIBXML2_STATIC_DEPS)
@@ -94,6 +104,5 @@ endif()
 
 set(LIBXML2_INCLUDE_DIRS ${LIBXML2_INCLUDE_DIR})
 set(LIBXML2_LIBRARIES ${LIBXML2_LIBRARY})
-set(LIBXML2_DEFINITIONS ${PC_LIBXML_CFLAGS_OTHER})
 
 mark_as_advanced(LIBXML2_INCLUDE_DIR LIBXML2_LIBRARY LIBXML2_STATIC_LIBRARY)