[lldb][windows] add support for out of PATH python.dll resolution #162509

charles-zablit · 2025-10-08T16:22:03Z

This patch adds the LLDB_PYTHON_DLL_RELATIVE_PATH Cmake variable which is the relative path to the directory containing python.dll. The path is relative to the directory containing lldb.exe.

If this variable is set and the resolved path points to an existing directory, we call SetDllDirectoryW to add python.dll to the list of DLL search paths.

This, combined with liblldb.dll being delay loaded, allows to package python.dll with the llvm installer.

llvmbot · 2025-10-08T16:22:44Z

@llvm/pr-subscribers-lldb

Author: Charles Zablit (charles-zablit)

Changes

This patch adds the LLDB_PYTHON_DLL_RELATIVE_PATH Cmake variable which is the relative path to the directory containing python.dll. The path is relative to the directory containing lldb.exe.

If this variable is set and the resolved path points to an existing directory, we call SetDllDirectoryW to add python.dll to the list of DLL search paths.

This, combined with liblldb.dll being delay loaded, allows to package python.dll with the llvm installer.

Full diff: https://github.com/llvm/llvm-project/pull/162509.diff

3 Files Affected:

(modified) lldb/cmake/modules/AddLLDB.cmake (+6)
(modified) lldb/tools/driver/CMakeLists.txt (+4)
(modified) lldb/tools/driver/Driver.cpp (+49)

diff --git a/lldb/cmake/modules/AddLLDB.cmake b/lldb/cmake/modules/AddLLDB.cmake
index 28bf8d816d89a..4d2036e68704f 100644
--- a/lldb/cmake/modules/AddLLDB.cmake
+++ b/lldb/cmake/modules/AddLLDB.cmake
@@ -167,6 +167,12 @@ function(add_lldb_executable name)
   )
 
   target_link_libraries(${name} PRIVATE ${ARG_LINK_LIBS})
+  if(WIN32)
+    list(FIND ARG_LINK_LIBS liblldb LIBLLD_INDEX)
+    if(NOT LIBLLD_INDEX EQUAL -1)
+      target_link_options(${name} PRIVATE "/DELAYLOAD:$<TARGET_FILE_BASE_NAME:liblldb>.dll")
+    endif()
+  endif()
   if(CLANG_LINK_CLANG_DYLIB)
     target_link_libraries(${name} PRIVATE clang-cpp)
   else()
diff --git a/lldb/tools/driver/CMakeLists.txt b/lldb/tools/driver/CMakeLists.txt
index 5c0cac484cc94..941ef0d83385b 100644
--- a/lldb/tools/driver/CMakeLists.txt
+++ b/lldb/tools/driver/CMakeLists.txt
@@ -34,6 +34,10 @@ add_dependencies(lldb
   ${tablegen_deps}
 )
 
+if(DEFINED LLDB_PYTHON_DLL_RELATIVE_PATH)
+  target_compile_definitions(lldb PRIVATE LLDB_PYTHON_DLL_RELATIVE_PATH="${LLDB_PYTHON_DLL_RELATIVE_PATH}")
+endif()
+
 if(LLDB_BUILD_FRAMEWORK)
   # In the build-tree, we know the exact path to the framework directory.
   # The installed framework can be in different locations.
diff --git a/lldb/tools/driver/Driver.cpp b/lldb/tools/driver/Driver.cpp
index 16cc736441b59..df31d649ada48 100644
--- a/lldb/tools/driver/Driver.cpp
+++ b/lldb/tools/driver/Driver.cpp
@@ -22,11 +22,15 @@
 #include "lldb/Host/MainLoop.h"
 #include "lldb/Host/MainLoopBase.h"
 #include "lldb/Utility/Status.h"
+#include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/ConvertUTF.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Signals.h"
+#include "llvm/Support/Windows/WindowsSupport.h"
 #include "llvm/Support/WithColor.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -426,6 +430,47 @@ SBError Driver::ProcessArgs(const opt::InputArgList &args, bool &exiting) {
   return error;
 }
 
+#ifdef _WIN32
+// Returns the full path to the lldb.exe executable
+inline std::wstring GetPathToExecutableW() {
+  // Iterate until we reach the Windows max path length (32,767).
+  std::vector<WCHAR> buffer;
+  buffer.resize(MAX_PATH);
+  while (buffer.size() < 32767) {
+    if (GetModuleFileNameW(NULL, buffer.data(), buffer.size()) < buffer.size())
+      return std::wstring(buffer.begin(), buffer.end());
+    buffer.resize(buffer.size() * 2);
+  }
+  return L"";
+}
+
+// Resolve the full path of the directory defined by
+// LLDB_PYTHON_DLL_RELATIVE_PATH. If it exists, add it to the list of DLL search
+// directories.
+void AddPythonDLLToSearchPath() {
+  std::wstring modulePath = GetPathToExecutableW();
+  if (modulePath.empty()) {
+    WithColor::error() << "Unable to find python: " << GetLastError() << '\n';
+    return;
+  }
+
+  SmallVector<char, MAX_PATH> utf8Path;
+  if (sys::windows::UTF16ToUTF8(modulePath.c_str(), modulePath.length(),
+                                utf8Path))
+    return;
+  sys::path::remove_filename(utf8Path);
+  sys::path::append(utf8Path, LLDB_PYTHON_DLL_RELATIVE_PATH);
+  sys::fs::make_absolute(utf8Path);
+
+  SmallVector<wchar_t, 1> widePath;
+  if (sys::windows::widenPath(utf8Path.data(), widePath))
+    return;
+
+  if (sys::fs::exists(utf8Path))
+    SetDllDirectoryW(widePath.data());
+}
+#endif
+
 std::string EscapeString(std::string arg) {
   std::string::size_type pos = 0;
   while ((pos = arg.find_first_of("\"\\", pos)) != std::string::npos) {
@@ -728,6 +773,10 @@ int main(int argc, char const *argv[]) {
                         "~/Library/Logs/DiagnosticReports/.\n");
 #endif
 
+#ifdef _WIN32
+  AddPythonDLLToSearchPath();
+#endif
+
   // Parse arguments.
   LLDBOptTable T;
   unsigned MissingArgIndex;

lldb/tools/driver/Driver.cpp

bulbazord · 2025-10-08T17:56:50Z

lldb/cmake/modules/AddLLDB.cmake

nit: missing a B -- LIBLLDB_INDEX

Fixed thanks!

bulbazord · 2025-10-08T17:57:10Z

lldb/tools/driver/Driver.cpp

Should this be an ifdef?

It should, I'm surprised that it worked before.

adrian-prantl · 2025-10-08T22:37:48Z

lldb/tools/driver/Driver.cpp

 }

+#ifdef _WIN32
+// Returns the full path to the lldb.exe executable


Suggested change

// Returns the full path to the lldb.exe executable

// Returns the full path to the lldb.exe executable.

adrian-prantl · 2025-10-08T22:39:21Z

lldb/tools/driver/Driver.cpp

+  // Iterate until we reach the Windows max path length (32,767).
+  std::vector<WCHAR> buffer;
+  buffer.resize(MAX_PATH);
+  while (buffer.size() < 32767) {


if MAX_PATH is 32767 isn't this condition always false?

There is a doubling on failure, so this shouldn't be always false.

MAX_PATH is defined in the Windows API as 260. However, a lot of the Windows API functions can return Unicode paths that are up to 32,767 bytes long.

I have updated the comments to explain this better.

adrian-prantl · 2025-10-08T22:41:35Z

lldb/tools/driver/Driver.cpp

+void AddPythonDLLToSearchPath() {
+  std::wstring modulePath = GetPathToExecutableW();
+  if (modulePath.empty()) {
+    WithColor::error() << "Unable to find python: " << GetLastError() << '\n';


We typically with the word "error:" in the error color, and not the entire message.

Fixed by using llvm::errs() and changing the message to start with error:

github-actions · 2025-10-09T09:38:36Z

✅ With the latest revision this PR passed the C/C++ code formatter.

dzhidzhoev · 2025-10-09T16:51:04Z

Please note that this patch has broken lldb-remote-linux-win.

C:\buildbot\as-builder-10\lldb-x-aarch64\llvm-project\lldb\tools\driver\Driver.cpp(465): error C2065: 'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier

https://lab.llvm.org/buildbot/#/builders/197/builds/9689

Could you please take a look at that?

charles-zablit · 2025-10-09T17:30:39Z

Please note that this patch has broken lldb-remote-linux-win.
C:\buildbot\as-builder-10\lldb-x-aarch64\llvm-project\lldb\tools\driver\Driver.cpp(465): error C2065: 'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier
lab.llvm.org/buildbot#/builders/197/builds/9689

Could you please take a look at that?

Thank you for letting me know, I have opened a PR for the fix here:

[lldb][windows] fix undeclared identifier error #162705

This patch fixes the `'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier` error introduced by #162509.

This patch fixes the `'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier` error introduced by llvm/llvm-project#162509.

…62509) This patch adds the `LLDB_PYTHON_DLL_RELATIVE_PATH` Cmake variable which is the relative path to the directory containing `python.dll`. The path is relative to the directory containing `lldb.exe`. If this variable is set and the resolved path points to an existing directory, we call `SetDllDirectoryW` to add `python.dll` to the list of DLL search paths. This, combined with `liblldb.dll` being delay loaded, allows to package `python.dll` with the `llvm` installer.

This patch fixes the `'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier` error introduced by #162509.

…vm#162509) This patch adds the `LLDB_PYTHON_DLL_RELATIVE_PATH` Cmake variable which is the relative path to the directory containing `python.dll`. The path is relative to the directory containing `lldb.exe`. If this variable is set and the resolved path points to an existing directory, we call `SetDllDirectoryW` to add `python.dll` to the list of DLL search paths. This, combined with `liblldb.dll` being delay loaded, allows to package `python.dll` with the `llvm` installer.

mstorsjo · 2025-10-10T11:35:26Z

lldb/cmake/modules/AddLLDB.cmake

+  if(WIN32)
+    list(FIND ARG_LINK_LIBS liblldb LIBLLDB_INDEX)
+    if(NOT LIBLLDB_INDEX EQUAL -1)
+      target_link_options(${name} PRIVATE "/DELAYLOAD:$<TARGET_FILE_BASE_NAME:liblldb>.dll")


Any particular reason for using $<TARGET_FILE_BASE_NAME:liblldb>.dll rather than the more straightforward $<TARGET_FILE_NAME:liblldb>?

This patch broke compilation for mingw targets, as these options are specific to the MSVC style build case - and for that case, the DLL name we'd like to pass here has a lib prefix, i.e. liblldb.dll, not lldb.dll. So instead of trying to manually reapply a prefix and add .dll at the end, it seems more straightforward to me to just use $<TARGET_FILE_NAME:liblldb> which should give the right string in both cases?

This used to be gated behind a if(MSVC) check in the downstream patch: swiftlang@af218c3. I think that with #162831 we will get a complete solution as if LLDB_PYTHON_DLL_RELATIVE_PATH is not set, the mechanism to add to the DLL search path will not even be compiled. If it's set, then it will only work for MSVC and mingw and there will be a warning if it's a different compiler.

which should give the right string in both cases?

I agree, this is much better 👍

mstorsjo · 2025-10-10T11:39:01Z

This, combined with liblldb.dll being delay loaded, allows to package python.dll with the llvm installer.

Just for reference - for llvm-mingw, we've been bundling a python install in our toolchain as well, but we've solved this slightly differently. We've just copied python.dll (but nothing else from the Python install) into the LLVM tool bin directory. That, together with the LLDB_PYTHON_HOME flag, has been enough for finding the bundled python install in a different directory. This solution is kinda neat though, and would allow skipping copying the python.dll.

[lldb][windows] add support for out of PATH python.dll resolution

37cf46e

charles-zablit requested a review from JDevlieghere as a code owner October 8, 2025 16:22

charles-zablit self-assigned this Oct 8, 2025

llvmbot added the lldb label Oct 8, 2025

charles-zablit requested review from adrian-prantl and compnerd October 8, 2025 16:22

charles-zablit mentioned this pull request Oct 8, 2025

[lldb][windows] delay loading liblldb.dll and manually add Python's DLL directory swiftlang/llvm-project#11571

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compnerd approved these changes Oct 8, 2025

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fixup! [lldb][windows] add support for out of PATH python.dll resolution

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bulbazord reviewed Oct 8, 2025

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address comments

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fixup! address comments

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charles-zablit merged commit ec28b95 into llvm:main Oct 9, 2025
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charles-zablit mentioned this pull request Oct 9, 2025

[lldb][windows] fix undeclared identifier error #162705

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[lldb][windows] fix undeclared identifier error (#162705)

d2413d9

This patch fixes the `'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier` error introduced by #162509.

svkeerthy pushed a commit that referenced this pull request Oct 9, 2025

[lldb][windows] fix undeclared identifier error (#162705)

b36ffaa

This patch fixes the `'LLDB_PYTHON_DLL_RELATIVE_PATH': undeclared identifier` error introduced by #162509.

charles-zablit mentioned this pull request Oct 10, 2025

🍒 [lldb][windows] add support for out of PATH python.dll resolution swiftlang/llvm-project#11613

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	// Returns the full path to the lldb.exe executable
	// Returns the full path to the lldb.exe executable.

[lldb][windows] add support for out of PATH python.dll resolution #162509

[lldb][windows] add support for out of PATH python.dll resolution #162509

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