[llvm] [CodeGen] Expose the extensibility of PassConfig to plugins (PR #139059)

[via llvm-commits]

https://github.com/Tcc100 created https://github.com/llvm/llvm-project/pull/139059

This PR exposes the backend pass config to plugins via a callback. Plugin authors can register a callback that is being triggered before the target backend adds their passes to the pipeline. In the callback they then get access to the `TargetMachine`, the `PassManager`, and the `TargetPassConfig`. This allows plugins to call `TargetPassConfig::insertPass`, which is honored in the subsequent `addPass` of the main backend. We implemented this using the legacy pass manager as the backend is still using the old pass manager.

The following example shows how plugin authors can use the callback. Since its a callback that is not doing anything without anybody registering it, there shouldn't be any potential harm to the compiler unless a plugin is present.

```cpp
__attribute__((constructor)) static void initCodeGenPlugin() {
    initializeCodeGenTestPass(*PassRegistry::getPassRegistry());

    TargetMachine::registerTargetPassConfigCallback([](auto &TM, auto &PM, auto *TPC) {
        TPC->insertPass(&GCLoweringID, &CodeGenTest::ID);
    });
}
```

>From ee72ef991ff97622a947dd8c4fc929446cacd3f1 Mon Sep 17 00:00:00 2001
From: T <T>
Date: Thu, 8 May 2025 11:56:00 +0200
Subject: [PATCH] [CodeGen] Expose the extensibility of PassConfig to plugins

---
 llvm/include/llvm/Target/TargetMachine.h      | 14 ++++++++++++++
 llvm/lib/CodeGen/CodeGenTargetMachineImpl.cpp |  3 +++
 llvm/lib/Target/TargetMachine.cpp             |  3 +++
 3 files changed, 20 insertions(+)

diff --git a/llvm/include/llvm/Target/TargetMachine.h b/llvm/include/llvm/Target/TargetMachine.h
index 906926729ed74..bcc1ce29b8282 100644
--- a/llvm/include/llvm/Target/TargetMachine.h
+++ b/llvm/include/llvm/Target/TargetMachine.h
@@ -20,10 +20,12 @@
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/PGOOptions.h"
 #include "llvm/Target/CGPassBuilderOption.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/TargetParser/Triple.h"
+#include <functional>
 #include <optional>
 #include <string>
 #include <utility>
@@ -72,6 +74,10 @@ namespace yaml {
 struct MachineFunctionInfo;
 } // namespace yaml
 
+class TargetMachine;
+using PassConfigCallback =
+    std::function<void(TargetMachine &, PassManagerBase &, TargetPassConfig *)>;
+
 //===----------------------------------------------------------------------===//
 ///
 /// Primary interface to the complete machine description for the target
@@ -119,6 +125,9 @@ class TargetMachine {
   std::optional<PGOOptions> PGOOption;
 
 public:
+  static ManagedStatic<SmallVector<PassConfigCallback, 1>>
+      TargetPassConfigCallbacks;
+
   mutable TargetOptions Options;
 
   TargetMachine(const TargetMachine &) = delete;
@@ -518,6 +527,11 @@ class TargetMachine {
 
   // MachineRegisterInfo callback function
   virtual void registerMachineRegisterInfoCallback(MachineFunction &MF) const {}
+
+  // TargetPassConfig callback function
+  static void registerTargetPassConfigCallback(const PassConfigCallback &C) {
+    TargetPassConfigCallbacks->push_back(C);
+  }
 };
 
 } // end namespace llvm
diff --git a/llvm/lib/CodeGen/CodeGenTargetMachineImpl.cpp b/llvm/lib/CodeGen/CodeGenTargetMachineImpl.cpp
index 4a3503a2da7db..336f1db776036 100644
--- a/llvm/lib/CodeGen/CodeGenTargetMachineImpl.cpp
+++ b/llvm/lib/CodeGen/CodeGenTargetMachineImpl.cpp
@@ -119,6 +119,9 @@ addPassesToGenerateCode(CodeGenTargetMachineImpl &TM, PassManagerBase &PM,
   PM.add(PassConfig);
   PM.add(&MMIWP);
 
+  for (auto& C : *TargetMachine::TargetPassConfigCallbacks)
+    C(TM, PM, PassConfig);
+
   if (PassConfig->addISelPasses())
     return nullptr;
   PassConfig->addMachinePasses();
diff --git a/llvm/lib/Target/TargetMachine.cpp b/llvm/lib/Target/TargetMachine.cpp
index 69b6e26e602f6..c43e2ba00f733 100644
--- a/llvm/lib/Target/TargetMachine.cpp
+++ b/llvm/lib/Target/TargetMachine.cpp
@@ -332,3 +332,6 @@ std::pair<int, int> TargetMachine::parseBinutilsVersion(StringRef Version) {
     Version.consumeInteger(10, Ret.second);
   return Ret;
 }
+
+// TargetPassConfig callbacks
+ManagedStatic<SmallVector<PassConfigCallback, 1>> TargetMachine::TargetPassConfigCallbacks{};