[MLIR] Mark debug.callbacks as inactive functions. (#706)

**Context:** `debug.callbacks` will always be inactive functions when
taking the gradient of them.

**Description of the Change:** Change `pyregistry` to
`inactive_callback` and mark all `inactive_callback`s as inactive.

**Benefits:** Inactive callbacks are inactive.

Notes:

* Future PRs will make active_callbacks through specialization and will
call inactive_callback as a primitive.
* Future PR will re-enable callbacks in gradients.

[sc-60496]

doc/changelog.md

@@ -83,6 +83,10 @@
 
 <h3>Improvements</h3>
 
+* `debug.callbacks` are marked as inactive. This means `debug.callbacks` will not be considered
+  as active for the computation of gradients.
+  [(#706)](https://github.com/PennyLaneAI/catalyst/pull/706)
+
 * Added support for IsingZZ gate in Catalyst frontend. Previously, the IsingZZ gate would be
   decomposed into a CNOT and RZ gates. However, this is not needed as the PennyLane-Lightning
   simulator supports this gate.

frontend/catalyst/compiler.py

@@ -213,6 +213,7 @@ def run_writing_command(command: List[str], compile_options: Optional[CompileOpt
         "reconcile-unrealized-casts",
         "gep-inbounds",
         "add-exception-handling",
+        "register-inactive-callback",
     ],
 )
 

mlir/include/Catalyst/Transforms/Passes.h

@@ -28,6 +28,7 @@ std::unique_ptr<mlir::Pass> createHloCustomCallLoweringPass();
 std::unique_ptr<mlir::Pass> createQnodeToAsyncLoweringPass();
 std::unique_ptr<mlir::Pass> createAddExceptionHandlingPass();
 std::unique_ptr<mlir::Pass> createGEPInboundsPass();
+std::unique_ptr<mlir::Pass> createRegisterInactiveCallbackPass();
 
 void registerAllCatalystPasses();
 

mlir/include/Catalyst/Transforms/Passes.td

@@ -130,4 +130,15 @@ def GEPInboundsPass : Pass<"gep-inbounds"> {
     let constructor = "catalyst::GEPInboundsPass()";
 }
 
+def RegisterInactiveCallbackPass : Pass<"register-inactive-callback", "ModuleOp"> {
+    let summary = "Register `inactive_callback` as inactive with Enzyme";
+
+    let dependentDialects = [
+        "mlir::LLVM::LLVMDialect"
+    ];
+
+    let constructor = "catalyst::createRegisterInactiveCallbackPass()";
+
+}
+
 #endif // CATALYST_PASSES

mlir/include/Gradient/Transforms/EnzymeConstants.h

@@ -0,0 +1,28 @@
+// Copyright 2024 Xanadu Quantum Technologies Inc.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#pragma once
+
+namespace catalyst {
+namespace gradient {
+
+static constexpr const char *enzyme_autodiff_func_name = "__enzyme_autodiff";
+static constexpr const char *enzyme_allocation_key = "__enzyme_allocation_like";
+static constexpr const char *enzyme_custom_gradient_key = "__enzyme_register_gradient_";
+static constexpr const char *enzyme_like_free_key = "__enzyme_function_like_free";
+static constexpr const char *enzyme_const_key = "enzyme_const";
+static constexpr const char *enzyme_dupnoneed_key = "enzyme_dupnoneed";
+static constexpr const char *enzyme_inactivefn_key = "__enzyme_inactivefn";
+
+}
+}

mlir/lib/Catalyst/Transforms/CMakeLists.txt

@@ -16,6 +16,7 @@ file(GLOB SRC
     AsyncUtils.cpp
     GEPInboundsPatterns.cpp
     GEPInboundsPass.cpp
+    RegisterInactiveCallbackPass.cpp
 )
 
 get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)

mlir/lib/Catalyst/Transforms/RegisterAllPasses.cpp

@@ -40,4 +40,5 @@ void catalyst::registerAllCatalystPasses()
     mlir::registerPass(catalyst::createGEPInboundsPass);
     mlir::registerPass(catalyst::createRemoveChainedSelfInversePass);
     mlir::registerPass(catalyst::createAnnotateFunctionPass);
+    mlir::registerPass(catalyst::createRegisterInactiveCallbackPass);
 }

mlir/lib/Catalyst/Transforms/RegisterInactiveCallbackPass.cpp

@@ -0,0 +1,67 @@
+// Copyright 2024 Xanadu Quantum Technologies Inc.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+
+#include "Catalyst/Transforms/Passes.h"
+#include "Catalyst/Transforms/Patterns.h"
+#include "Gradient/Transforms/EnzymeConstants.h"
+
+using namespace mlir;
+
+namespace catalyst {
+
+#define GEN_PASS_DEF_REGISTERINACTIVECALLBACKPASS
+#define GEN_PASS_DECL_REGISTERINACTIVECALLBACKPASS
+#include "Catalyst/Transforms/Passes.h.inc"
+
+struct RegisterInactiveCallbackPass
+    : impl::RegisterInactiveCallbackPassBase<RegisterInactiveCallbackPass> {
+    using RegisterInactiveCallbackPassBase::RegisterInactiveCallbackPassBase;
+    void runOnOperation() final
+    {
+        auto mod = getOperation();
+        StringRef inactive_callbackFnName = "inactive_callback";
+        auto fnDecl = mod.lookupSymbol<LLVM::LLVMFuncOp>(inactive_callbackFnName);
+        if (!fnDecl) {
+            return;
+        }
+        MLIRContext *context = &getContext();
+        auto builder = OpBuilder(context);
+        builder.setInsertionPointToStart(mod.getBody());
+        auto ptrTy = LLVM::LLVMPointerType::get(context);
+        auto arrTy = LLVM::LLVMArrayType::get(ptrTy, 1);
+        auto loc = mod.getLoc();
+        auto isConstant = false;
+        auto linkage = LLVM::Linkage::External;
+        auto key = catalyst::gradient::enzyme_inactivefn_key;
+        auto glb = builder.create<LLVM::GlobalOp>(loc, arrTy, isConstant, linkage, key, nullptr);
+        // Create a block and push it to the global
+        Block *block = new Block();
+        glb.getInitializerRegion().push_back(block);
+        builder.setInsertionPointToStart(block);
+        auto undef = builder.create<LLVM::UndefOp>(glb.getLoc(), arrTy);
+        auto fnSym = SymbolRefAttr::get(context, inactive_callbackFnName);
+        auto fnPtr = builder.create<LLVM::AddressOfOp>(glb.getLoc(), ptrTy, fnSym);
+        auto filledInArray = builder.create<LLVM::InsertValueOp>(glb.getLoc(), undef, fnPtr, 0);
+        builder.create<LLVM::ReturnOp>(glb.getLoc(), filledInArray);
+    }
+};
+
+std::unique_ptr<Pass> createRegisterInactiveCallbackPass()
+{
+    return std::make_unique<RegisterInactiveCallbackPass>();
+}
+} // namespace catalyst

mlir/lib/Catalyst/Transforms/catalyst_to_llvm.cpp

@@ -444,7 +444,7 @@ struct PythonCallOpPattern : public OpConversionPattern<PythonCallOp> {
 
         bool isVarArg = true;
         LLVM::LLVMFuncOp customCallFnOp = mlir::LLVM::lookupOrCreateFn(
-            mod, "pyregistry", {/*args=*/i64, i64, i64}, /*ret_type=*/voidType, isVarArg);
+            mod, "inactive_callback", {/*args=*/i64, i64, i64}, /*ret_type=*/voidType, isVarArg);
         customCallFnOp.setPrivate();
         rewriter.restoreInsertionPoint(point);
 

mlir/lib/Gradient/Transforms/ConversionPatterns.cpp

@@ -34,6 +34,7 @@
 
 #include "Catalyst/Utils/CallGraph.h"
 #include "Gradient/IR/GradientOps.h"
+#include "Gradient/Transforms/EnzymeConstants.h"
 #include "Gradient/Transforms/Patterns.h"
 #include "Gradient/Utils/DestinationPassingStyle.h"
 #include "Gradient/Utils/EinsumLinalgGeneric.h"
@@ -149,13 +150,6 @@ struct EnzymeMemRefInterfaceOptions {
     bool dupNoNeed = false;
 };
 
-static constexpr const char *enzyme_autodiff_func_name = "__enzyme_autodiff";
-static constexpr const char *enzyme_allocation_key = "__enzyme_allocation_like";
-static constexpr const char *enzyme_custom_gradient_key = "__enzyme_register_gradient_";
-static constexpr const char *enzyme_like_free_key = "__enzyme_function_like_free";
-static constexpr const char *enzyme_const_key = "enzyme_const";
-static constexpr const char *enzyme_dupnoneed_key = "enzyme_dupnoneed";
-
 /// Enzyme custom gradients appear to exhibit better stability when they are registered for
 /// functions where MemRefs are passed via wrapped pointers (!llvm.ptr<struct(ptr, ptr, i64, ...)>)
 /// rather than having their fields unpacked. This function automatically transforms MemRef

mlir/test/Catalyst/ConversionTest.mlir

@@ -117,7 +117,7 @@ func.func @python_call () {
     // CHECK: [[identifier:%.+]] = llvm.mlir.constant(0 : i64)
     // CHECK: [[argcount:%.+]] = llvm.mlir.constant(0 : i64)
     // CHECK: [[rescount:%.+]] = llvm.mlir.constant(0 : i64)
-    // CHECK: llvm.call @pyregistry([[identifier]], [[argcount]], [[rescount]])
+    // CHECK: llvm.call @inactive_callback([[identifier]], [[argcount]], [[rescount]])
     catalyst.pycallback() { identifier = 0} : () -> ()
     return
 }

mlir/test/Catalyst/RegisterInactive.mlir

@@ -0,0 +1,56 @@
+// Copyright 2024 Xanadu Quantum Technologies Inc.
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// RUN: quantum-opt %s --register-inactive-callback --split-input-file --verify-diagnostics | FileCheck %s
+
+// This test just makes sure that we can
+// run the compiler with the option
+//
+//   --register-inactive-callback
+//
+// and that if there are no callbacks present
+// it doesn't change anything
+
+// CHECK-LABEL: @test0
+module @test0 {
+  // CHECK-NOT: llvm.mlir.global external @__enzyme_inactivefn
+  // CHECK-LABEL: @foo
+  func.func @foo() {
+    return
+  }
+  // CHECK-NOT: llvm.mlir.global external @__enzyme_inactivefn
+}
+
+// -----
+
+// This test checks the invariant that after the transformation
+// the attribute has been removed.
+
+// CHECK-LABEL: @test1
+module @test1 {
+
+  // CHECK: llvm.mlir.global external @__enzyme_inactivefn
+  // CHECK: [[undef:%.+]] = llvm.mlir.undef
+  // CHECK: [[ptr:%.+]] = llvm.mlir.addressof @inactive_callback
+  // CHECK: [[retval:%.+]] = llvm.insertvalue [[ptr]], [[undef]][0]
+  // CHECK: llvm.return [[retval]]
+
+  llvm.func @inactive_callback(i64, i64, i64, ...)
+  llvm.func @wrapper() {
+    %0 = llvm.mlir.constant(139935726668624 : i64) : i64
+    %1 = llvm.mlir.constant(0 : i64) : i64
+    llvm.call @inactive_callback(%0, %1, %1) vararg(!llvm.func<void (i64, i64, i64, ...)>) : (i64, i64, i64) -> ()
+    llvm.return
+  }
+}

runtime/lib/capi/ExecutionContext.hpp

@@ -33,7 +33,7 @@ extern void callbackCall(int64_t, int64_t, int64_t, va_list);
 
 namespace Catalyst::Runtime {
 
-extern "C" void pyregistry(int64_t identifier, int64_t argc, int64_t retc, ...);
+extern "C" void inactive_callback(int64_t identifier, int64_t argc, int64_t retc, ...);
 
 class MemoryManager final {
   private:

runtime/lib/capi/RuntimeCAPI.cpp

@@ -108,7 +108,7 @@ extern "C" {
 using namespace Catalyst::Runtime;
 using timer = catalyst::utils::Timer;
 
-void pyregistry(int64_t identifier, int64_t argc, int64_t retc, ...)
+void inactive_callback(int64_t identifier, int64_t argc, int64_t retc, ...)
 {
     // We need to guard calls to callback.
     // These are implemented in Python.