[clang] [llvm] [RFC][LLVM][Clang] Add LLVM plugin hook for back-ends (PR #170846)
Alexis Engelke via llvm-commits
llvm-commits at lists.llvm.org
Fri Dec 5 04:34:13 PST 2025
https://github.com/aengelke created https://github.com/llvm/llvm-project/pull/170846
Add a mechanism to permit plugins running code between optimizations and
the back-end passes. Implement this through the LLVM plug-in mechanism
to make permit plugins to be written independently of the front-end.
The primary motivation for this point is TPDE-LLVM, which substitutes
the LLVM back-end (optionally falling back to it for unsupported IR). We
have been distributing a Clang patch; but requiring a custom-build
toolchain is impracticable for many users.
---
This is an alternative to #165257 based on feedback there.
TODOs/questions:
- The default initializer in PassPluginInfo causes warnings
(-Wreturn-type-c-linkage). Not default-initializing causes warnings
for all code, as that needs to be changed. Maybe we can just disable
the warning?
- Implement plugin support in llc and add tests.
>From 89e9b4a5863e957971a3febc95862c1d5fe43f28 Mon Sep 17 00:00:00 2001
From: Alexis Engelke <engelke at in.tum.de>
Date: Fri, 5 Dec 2025 12:33:55 +0000
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.5-bogner
---
clang/lib/CodeGen/BackendUtil.cpp | 33 +++++++++++++++++++--------
llvm/include/llvm/Passes/PassPlugin.h | 31 +++++++++++++++++++++----
llvm/lib/Passes/PassPlugin.cpp | 5 ----
3 files changed, 49 insertions(+), 20 deletions(-)
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 97bc063ad34e5..188ea36d44523 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -144,6 +144,7 @@ class EmitAssemblyHelper {
const LangOptions &LangOpts;
llvm::Module *TheModule;
IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS;
+ llvm::SmallVector<llvm::PassPlugin> Plugins;
std::unique_ptr<raw_pwrite_stream> OS;
@@ -973,16 +974,9 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
}
#endif
}
- // Attempt to load pass plugins and register their callbacks with PB.
- for (auto &PluginFN : CodeGenOpts.PassPlugins) {
- auto PassPlugin = PassPlugin::Load(PluginFN);
- if (PassPlugin) {
- PassPlugin->registerPassBuilderCallbacks(PB);
- } else {
- Diags.Report(diag::err_fe_unable_to_load_plugin)
- << PluginFN << toString(PassPlugin.takeError());
- }
- }
+ // Register plugin callbacks with PB.
+ for (auto &Plugin : Plugins)
+ Plugin.registerPassBuilderCallbacks(PB);
for (const auto &PassCallback : CodeGenOpts.PassBuilderCallbacks)
PassCallback(PB);
#define HANDLE_EXTENSION(Ext) \
@@ -1211,6 +1205,14 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
void EmitAssemblyHelper::RunCodegenPipeline(
BackendAction Action, std::unique_ptr<raw_pwrite_stream> &OS,
std::unique_ptr<llvm::ToolOutputFile> &DwoOS) {
+ // Invoke pre-codegen callback from plugin, which might want to take over the
+ // entire code generation itself.
+ for (auto &Plugin : Plugins) {
+ CodeGenFileType CGFT = getCodeGenFileType(Action);
+ if (Plugin.invokePreCodeGenCallback(*TheModule, *TM, CGFT, *OS))
+ return;
+ }
+
// We still use the legacy PM to run the codegen pipeline since the new PM
// does not work with the codegen pipeline.
// FIXME: make the new PM work with the codegen pipeline.
@@ -1274,6 +1276,17 @@ void EmitAssemblyHelper::emitAssembly(BackendAction Action,
// Before executing passes, print the final values of the LLVM options.
cl::PrintOptionValues();
+ // Attempt to load pass plugins.
+ for (auto &PluginFN : CodeGenOpts.PassPlugins) {
+ auto PassPlugin = PassPlugin::Load(PluginFN);
+ if (PassPlugin) {
+ Plugins.push_back(std::move(*PassPlugin));
+ } else {
+ Diags.Report(diag::err_fe_unable_to_load_plugin)
+ << PluginFN << toString(PassPlugin.takeError());
+ }
+ }
+
std::unique_ptr<llvm::ToolOutputFile> ThinLinkOS, DwoOS;
RunOptimizationPipeline(Action, OS, ThinLinkOS, BC);
RunCodegenPipeline(Action, OS, DwoOS);
diff --git a/llvm/include/llvm/Passes/PassPlugin.h b/llvm/include/llvm/Passes/PassPlugin.h
index 947504bc207a7..9ca0b4c29ed96 100644
--- a/llvm/include/llvm/Passes/PassPlugin.h
+++ b/llvm/include/llvm/Passes/PassPlugin.h
@@ -14,6 +14,7 @@
#define LLVM_PASSES_PASSPLUGIN_H
#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/CodeGen.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/DynamicLibrary.h"
#include "llvm/Support/Error.h"
@@ -21,7 +22,9 @@
#include <string>
namespace llvm {
+class Module;
class PassBuilder;
+class TargetMachine;
/// \macro LLVM_PLUGIN_API_VERSION
/// Identifies the API version understood by this plugin.
@@ -30,14 +33,15 @@ class PassBuilder;
/// against that of the plugin. A mismatch is an error. The supported version
/// will be incremented for ABI-breaking changes to the \c PassPluginLibraryInfo
/// struct, i.e. when callbacks are added, removed, or reordered.
-#define LLVM_PLUGIN_API_VERSION 1
+#define LLVM_PLUGIN_API_VERSION 2
extern "C" {
/// Information about the plugin required to load its passes
///
/// This struct defines the core interface for pass plugins and is supposed to
-/// be filled out by plugin implementors. LLVM-side users of a plugin are
-/// expected to use the \c PassPlugin class below to interface with it.
+/// be filled out by plugin implementors. Unused function pointers can be set to
+/// nullptr. LLVM-side users of a plugin are expected to use the \c PassPlugin
+/// class below to interface with it.
struct PassPluginLibraryInfo {
/// The API version understood by this plugin, usually \c
/// LLVM_PLUGIN_API_VERSION
@@ -49,7 +53,14 @@ struct PassPluginLibraryInfo {
/// The callback for registering plugin passes with a \c PassBuilder
/// instance
- void (*RegisterPassBuilderCallbacks)(PassBuilder &);
+ void (*RegisterPassBuilderCallbacks)(PassBuilder &) = nullptr;
+
+ /// Callback called before running the back-end passes on the module. The
+ /// callback can generate code itself by writing the expected output to OS and
+ /// returning true to prevent the default pipeline and further plugin
+ /// callbacks from running.
+ bool (*PreCodeGenCallback)(Module &, TargetMachine &, CodeGenFileType,
+ raw_pwrite_stream &OS) = nullptr;
};
}
@@ -80,7 +91,17 @@ class PassPlugin {
/// Invoke the PassBuilder callback registration
void registerPassBuilderCallbacks(PassBuilder &PB) const {
- Info.RegisterPassBuilderCallbacks(PB);
+ if (Info.RegisterPassBuilderCallbacks)
+ Info.RegisterPassBuilderCallbacks(PB);
+ }
+
+ /// Invoke the pre-codegen callback.
+ bool invokePreCodeGenCallback(Module &M, TargetMachine &TM,
+ CodeGenFileType CGFT,
+ raw_pwrite_stream &OS) const {
+ if (Info.PreCodeGenCallback)
+ return Info.PreCodeGenCallback(M, TM, CGFT, OS);
+ return false;
}
private:
diff --git a/llvm/lib/Passes/PassPlugin.cpp b/llvm/lib/Passes/PassPlugin.cpp
index 6182cbbb1fddd..201f5eef080c3 100644
--- a/llvm/lib/Passes/PassPlugin.cpp
+++ b/llvm/lib/Passes/PassPlugin.cpp
@@ -45,10 +45,5 @@ Expected<PassPlugin> PassPlugin::Load(const std::string &Filename) {
Twine(LLVM_PLUGIN_API_VERSION) + ".",
inconvertibleErrorCode());
- if (!P.Info.RegisterPassBuilderCallbacks)
- return make_error<StringError>(Twine("Empty entry callback in plugin '") +
- Filename + "'.'",
- inconvertibleErrorCode());
-
return P;
}
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