[CIR][CIRGen][Lowering] supports functions pointers (llvm#316)

This PR adds a support of the function pointers in CIR.

From the implementation point of view, we emit an address of a function
as a `GlobalViewAttr`.

clang/lib/CIR/CodeGen/CIRGenExprConst.cpp

@@ -1155,8 +1155,14 @@ ConstantLValueEmitter::tryEmitBase(const APValue::LValueBase &base) {
     if (D->hasAttr<WeakRefAttr>())
       llvm_unreachable("emit pointer base for weakref is NYI");
 
-    if (auto *FD = dyn_cast<FunctionDecl>(D))
-      llvm_unreachable("emit pointer base for fun decl is NYI");
+    if (auto *FD = dyn_cast<FunctionDecl>(D)) {
+      auto fop = CGM.GetAddrOfFunction(FD);
+      auto builder = CGM.getBuilder();
+      auto ctxt = builder.getContext();
+      return mlir::cir::GlobalViewAttr::get(
+          builder.getPointerTo(fop.getFunctionType()),
+          mlir::FlatSymbolRefAttr::get(ctxt, fop.getSymNameAttr()));
+    }
 
     if (auto *VD = dyn_cast<VarDecl>(D)) {
       // We can never refer to a variable with local storage.

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -236,6 +236,9 @@ mlir::Value lowerCirAttrAsValue(mlir::Operation *parentOp,
   } else if (auto cirSymbol = dyn_cast<mlir::cir::GlobalOp>(sourceSymbol)) {
     sourceType = converter->convertType(cirSymbol.getSymType());
     symName = cirSymbol.getSymName();
+  } else if (auto llvmFun = dyn_cast<mlir::LLVM::LLVMFuncOp>(sourceSymbol)) {
+    sourceType = llvmFun.getFunctionType();
+    symName = llvmFun.getSymName();
   } else {
     llvm_unreachable("Unexpected GlobalOp type");
   }

clang/test/CIR/CodeGen/fun-ptr.c

@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o -  | FileCheck %s -check-prefix=CIR
+// RUN: %clang_cc1 -x c++ -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o -  | FileCheck %s -check-prefix=CIR
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o - | FileCheck %s -check-prefix=LLVM
+// RUN: %clang_cc1 -x c++ -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o - | FileCheck %s -check-prefix=LLVM
+// XFAIL: *
+
+typedef struct {
+    int a;
+    int b;
+} Data;
+
+typedef int (*fun_t)(Data* d);
+
+int extract_a(Data* d) {
+    return d->a;
+}
+
+// CIR: cir.func {{@.*foo.*}}(%arg0: !cir.ptr<!ty_22Data22>
+// CIR:   [[TMP0:%.*]] = cir.alloca !cir.ptr<!ty_22Data22>, cir.ptr <!cir.ptr<!ty_22Data22>>, ["d", init]
+// CIR:   [[TMP1:%.*]] = cir.alloca !s32i, cir.ptr <!s32i>, ["__retval"]
+// CIR:   [[TMP2:%.*]] = cir.alloca !cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>, cir.ptr <!cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>>, ["f", init]
+// CIR:   cir.store %arg0, [[TMP0]] : !cir.ptr<!ty_22Data22>, cir.ptr <!cir.ptr<!ty_22Data22>>
+// CIR:   [[TMP3:%.*]] = cir.const(#cir.ptr<null> : !cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>) : !cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>
+// CIR:   cir.store [[TMP3]], [[TMP2]] : !cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>, cir.ptr <!cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>>
+// CIR:   [[TMP4:%.*]] = cir.get_global {{@.*extract_a.*}} : cir.ptr <!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>
+// CIR:   cir.store [[TMP4]], [[TMP2]] : !cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>, cir.ptr <!cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>>
+// CIR:   [[TMP5:%.*]] = cir.load [[TMP2]] : cir.ptr <!cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>>, !cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>
+// CIR:   [[TMP6:%.*]] = cir.load [[TMP0]] : cir.ptr <!cir.ptr<!ty_22Data22>>, !cir.ptr<!ty_22Data22>
+// CIR:   [[TMP7:%.*]] = cir.call [[TMP5]]([[TMP6]]) : (!cir.ptr<!cir.func<!s32i (!cir.ptr<!ty_22Data22>)>>, !cir.ptr<!ty_22Data22>) -> !s32i
+// CIR:   cir.store [[TMP7]], [[TMP1]] : !s32i, cir.ptr <!s32i>
+
+// LLVM: define i32 {{@.*foo.*}}(ptr %0)
+// LLVM:   [[TMP1:%.*]] = alloca ptr, i64 1
+// LLVM:   [[TMP2:%.*]] = alloca i32, i64 1
+// LLVM:   [[TMP3:%.*]] = alloca ptr, i64 1
+// LLVM:   store ptr %0, ptr [[TMP1]]
+// LLVM:   store ptr null, ptr [[TMP3]]
+// LLVM:   store ptr {{@.*extract_a.*}}, ptr [[TMP3]]
+// LLVM:   [[TMP4:%.*]] = load ptr, ptr [[TMP3]]
+// LLVM:   [[TMP5:%.*]] = load ptr, ptr [[TMP1]]
+// LLVM:   [[TMP6:%.*]] = call i32 [[TMP4]](ptr [[TMP5]])
+// LLVM:   store i32 [[TMP6]], ptr [[TMP2]]
+int foo(Data* d) {
+    fun_t f = 0;
+    f = extract_a;
+    return f(d);
+}