[CIR] Remove return !cir.void from IR and textual representation (#1249)

C/C++ functions returning void had an explicit !cir.void return type
while not
having any returned value, which was breaking a lot of MLIR invariants
when the
CIR dialect is used in a greater context, for example with the inliner.

Now, a C/C++ function returning void has no return type and no return
values,
which does not break the MLIR invariant about the same number of return
types
and returned values.

This change does not keeps the same parsing/pretty-printed syntax as
before for
compatibility like in #1203 because
it
requires some new features from the MLIR parser infrastructure itself,
which is
not great.

This uses an optional type for function return type.

The default MLIR parser for optional parameters requires an optional
anchor we
do not have in the syntax, so use a custom FuncType parser to handle the
optional
return type.

clang/include/clang/CIR/Dialect/IR/CIROps.td

@@ -3474,8 +3474,6 @@ def FuncOp : CIR_Op<"func", [
     /// Returns the results types that the callable region produces when
     /// executed.
     llvm::ArrayRef<mlir::Type> getCallableResults() {
-      if (::llvm::isa<cir::VoidType>(getFunctionType().getReturnType()))
-        return {};
       return getFunctionType().getReturnTypes();
     }
 
@@ -3492,10 +3490,15 @@ def FuncOp : CIR_Op<"func", [
     }
 
     /// Returns the argument types of this function.
-    llvm::ArrayRef<mlir::Type> getArgumentTypes() { return getFunctionType().getInputs(); }
+    llvm::ArrayRef<mlir::Type> getArgumentTypes() {
+       return getFunctionType().getInputs();
+    }
 
-    /// Returns the result types of this function.
-    llvm::ArrayRef<mlir::Type> getResultTypes() { return getFunctionType().getReturnTypes(); }
+    /// Returns 0 or 1 result type of this function (0 in the case of a function
+    /// returing void)
+    llvm::ArrayRef<mlir::Type> getResultTypes() {
+       return getFunctionType().getReturnTypes();
+    }
 
     /// Hook for OpTrait::FunctionOpInterfaceTrait, called after verifying that
     /// the 'type' attribute is present and checks if it holds a function type.

clang/include/clang/CIR/Dialect/IR/CIRTypes.td

@@ -372,29 +372,38 @@ def CIR_VectorType : CIR_Type<"Vector", "vector",
 def CIR_FuncType : CIR_Type<"Func", "func"> {
   let summary = "CIR function type";
   let description = [{
-    The `!cir.func` is a function type. It consists of a single return type, a
-    list of parameter types and can optionally be variadic.
+    The `!cir.func` is a function type. It consists of an optional return type,
+    a list of parameter types and can optionally be variadic.
 
     Example:
 
     ```mlir
+    !cir.func<()>
     !cir.func<!bool ()>
+    !cir.func<(!s8i, !s8i)>
     !cir.func<!s32i (!s8i, !s8i)>
     !cir.func<!s32i (!s32i, ...)>
     ```
   }];
 
-  let parameters = (ins ArrayRefParameter<"mlir::Type">:$inputs, "mlir::Type":$returnType,
+  let parameters = (ins ArrayRefParameter<"mlir::Type">:$inputs,
+                        "mlir::Type":$optionalReturnType,
                         "bool":$varArg);
+  // Use a custom parser to handle the optional return and argument types
+  // without an optional anchor.
   let assemblyFormat = [{
-    `<` $returnType ` ` `(` custom<FuncTypeArgs>($inputs, $varArg) `>`
+    `<` custom<FuncType>($optionalReturnType, $inputs, $varArg) `>`
   }];
 
   let builders = [
+    // Construct with an actual return type or explicit !cir.void
     TypeBuilderWithInferredContext<(ins
       "llvm::ArrayRef<mlir::Type>":$inputs, "mlir::Type":$returnType,
       CArg<"bool", "false">:$isVarArg), [{
-      return $_get(returnType.getContext(), inputs, returnType, isVarArg);
+      return $_get(returnType.getContext(), inputs,
+                       mlir::isa<cir::VoidType>(returnType) ? nullptr
+                                                            : returnType,
+                       isVarArg);
     }]>
   ];
 
@@ -408,11 +417,15 @@ def CIR_FuncType : CIR_Type<"Func", "func"> {
     /// Returns the number of arguments to the function.
     unsigned getNumInputs() const { return getInputs().size(); }
 
+    /// Returns the result type of the function as an actual return type or
+    /// explicit !cir.void
+    mlir::Type getReturnType() const;
+
     /// Returns the result type of the function as an ArrayRef, enabling better
     /// integration with generic MLIR utilities.
     llvm::ArrayRef<mlir::Type> getReturnTypes() const;
 
-    /// Returns whether the function is returns void.
+    /// Returns whether the function returns void.
     bool isVoid() const;
 
     /// Returns a clone of this function type with the given argument

clang/lib/CIR/CodeGen/CIRGenTypes.cpp

@@ -271,7 +271,7 @@ mlir::Type CIRGenTypes::convertFunctionTypeInternal(QualType QFT) {
   assert(QFT.isCanonical());
   const Type *Ty = QFT.getTypePtr();
   const FunctionType *FT = cast<FunctionType>(QFT.getTypePtr());
-  // First, check whether we can build the full fucntion type. If the function
+  // First, check whether we can build the full function type. If the function
   // type depends on an incomplete type (e.g. a struct or enum), we cannot lower
   // the function type.
   assert(isFuncTypeConvertible(FT) && "NYI");

clang/lib/CIR/Dialect/IR/CIRDialect.cpp

@@ -2490,13 +2490,8 @@ void cir::FuncOp::print(OpAsmPrinter &p) {
   p.printSymbolName(getSymName());
   auto fnType = getFunctionType();
   llvm::SmallVector<Type, 1> resultTypes;
-  if (!fnType.isVoid())
-    function_interface_impl::printFunctionSignature(
-        p, *this, fnType.getInputs(), fnType.isVarArg(),
-        fnType.getReturnTypes());
-  else
-    function_interface_impl::printFunctionSignature(
-        p, *this, fnType.getInputs(), fnType.isVarArg(), {});
+  function_interface_impl::printFunctionSignature(
+      p, *this, fnType.getInputs(), fnType.isVarArg(), fnType.getReturnTypes());
 
   if (mlir::ArrayAttr annotations = getAnnotationsAttr()) {
     p << ' ';
@@ -2565,6 +2560,11 @@ LogicalResult cir::FuncOp::verifyType() {
   if (!getNoProto() && type.isVarArg() && type.getNumInputs() == 0)
     return emitError()
            << "prototyped function must have at least one non-variadic input";
+  if (auto rt = type.getReturnTypes();
+      !rt.empty() && mlir::isa<cir::VoidType>(rt.front()))
+    return emitOpError("The return type for a function returning void should "
+                       "be empty instead of an explicit !cir.void");
+
   return success();
 }
 

clang/lib/CIR/Dialect/IR/CIRTypes.cpp

@@ -33,6 +33,7 @@
 #include "llvm/ADT/TypeSwitch.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
+#include <cassert>
 #include <optional>
 
 using cir::MissingFeatures;
@@ -41,12 +42,13 @@ using cir::MissingFeatures;
 // CIR Custom Parser/Printer Signatures
 //===----------------------------------------------------------------------===//
 
-static mlir::ParseResult
-parseFuncTypeArgs(mlir::AsmParser &p, llvm::SmallVector<mlir::Type> &params,
-                  bool &isVarArg);
-static void printFuncTypeArgs(mlir::AsmPrinter &p,
-                              mlir::ArrayRef<mlir::Type> params, bool isVarArg);
+static mlir::ParseResult parseFuncType(mlir::AsmParser &p,
+                                       mlir::Type &optionalReturnTypes,
+                                       llvm::SmallVector<mlir::Type> &params,
+                                       bool &isVarArg);
 
+static void printFuncType(mlir::AsmPrinter &p, mlir::Type optionalReturnTypes,
+                          mlir::ArrayRef<mlir::Type> params, bool isVarArg);
 static mlir::ParseResult parsePointerAddrSpace(mlir::AsmParser &p,
                                                mlir::Attribute &addrSpaceAttr);
 static void printPointerAddrSpace(mlir::AsmPrinter &p,
@@ -913,9 +915,38 @@ FuncType FuncType::clone(TypeRange inputs, TypeRange results) const {
   return get(llvm::to_vector(inputs), results[0], isVarArg());
 }
 
-mlir::ParseResult parseFuncTypeArgs(mlir::AsmParser &p,
-                                    llvm::SmallVector<mlir::Type> &params,
-                                    bool &isVarArg) {
+// A special parser is needed for function returning void to handle the missing
+// type.
+static mlir::ParseResult parseFuncTypeReturn(mlir::AsmParser &p,
+                                             mlir::Type &optionalReturnType) {
+  if (succeeded(p.parseOptionalLParen())) {
+    // If we have already a '(', the function has no return type
+    optionalReturnType = {};
+    return mlir::success();
+  }
+  mlir::Type type;
+  if (p.parseType(type))
+    return mlir::failure();
+  if (isa<cir::VoidType>(type))
+    // An explicit !cir.void means also no return type.
+    optionalReturnType = {};
+  else
+    // Otherwise use the actual type.
+    optionalReturnType = type;
+  return p.parseLParen();
+}
+
+// A special pretty-printer for function returning or not a result.
+static void printFuncTypeReturn(mlir::AsmPrinter &p,
+                                mlir::Type optionalReturnType) {
+  if (optionalReturnType)
+    p << optionalReturnType << ' ';
+  p << '(';
+}
+
+static mlir::ParseResult
+parseFuncTypeArgs(mlir::AsmParser &p, llvm::SmallVector<mlir::Type> &params,
+                  bool &isVarArg) {
   isVarArg = false;
   // `(` `)`
   if (succeeded(p.parseOptionalRParen()))
@@ -945,8 +976,9 @@ mlir::ParseResult parseFuncTypeArgs(mlir::AsmParser &p,
   return p.parseRParen();
 }
 
-void printFuncTypeArgs(mlir::AsmPrinter &p, mlir::ArrayRef<mlir::Type> params,
-                       bool isVarArg) {
+static void printFuncTypeArgs(mlir::AsmPrinter &p,
+                              mlir::ArrayRef<mlir::Type> params,
+                              bool isVarArg) {
   llvm::interleaveComma(params, p,
                         [&p](mlir::Type type) { p.printType(type); });
   if (isVarArg) {
@@ -957,11 +989,49 @@ void printFuncTypeArgs(mlir::AsmPrinter &p, mlir::ArrayRef<mlir::Type> params,
   p << ')';
 }
 
+// Use a custom parser to handle the optional return and argument types without
+// an optional anchor.
+static mlir::ParseResult parseFuncType(mlir::AsmParser &p,
+                                       mlir::Type &optionalReturnTypes,
+                                       llvm::SmallVector<mlir::Type> &params,
+                                       bool &isVarArg) {
+  if (failed(parseFuncTypeReturn(p, optionalReturnTypes)))
+    return failure();
+  return parseFuncTypeArgs(p, params, isVarArg);
+}
+
+static void printFuncType(mlir::AsmPrinter &p, mlir::Type optionalReturnTypes,
+                          mlir::ArrayRef<mlir::Type> params, bool isVarArg) {
+  printFuncTypeReturn(p, optionalReturnTypes);
+  printFuncTypeArgs(p, params, isVarArg);
+}
+
+// Return the actual return type or an explicit !cir.void if the function does
+// not return anything
+mlir::Type FuncType::getReturnType() const {
+  if (isVoid())
+    return cir::VoidType::get(getContext());
+  return static_cast<detail::FuncTypeStorage *>(getImpl())->optionalReturnType;
+}
+
+/// Returns the result type of the function as an ArrayRef, enabling better
+/// integration with generic MLIR utilities.
 llvm::ArrayRef<mlir::Type> FuncType::getReturnTypes() const {
-  return static_cast<detail::FuncTypeStorage *>(getImpl())->returnType;
+  if (isVoid())
+    return {};
+  return static_cast<detail::FuncTypeStorage *>(getImpl())->optionalReturnType;
 }
 
-bool FuncType::isVoid() const { return mlir::isa<VoidType>(getReturnType()); }
+// Whether the function returns void
+bool FuncType::isVoid() const {
+  auto rt =
+      static_cast<detail::FuncTypeStorage *>(getImpl())->optionalReturnType;
+  assert(!rt ||
+         !mlir::isa<cir::VoidType>(rt) &&
+             "The return type for a function returning void should be empty "
+             "instead of a real !cir.void");
+  return !rt;
+}
 
 //===----------------------------------------------------------------------===//
 // MethodType Definitions

clang/lib/CIR/Dialect/Transforms/TargetLowering/LowerTypes.cpp

@@ -109,7 +109,7 @@ FuncType LowerTypes::getFunctionType(const LowerFunctionInfo &FI) {
     }
   }
 
-  return FuncType::get(getMLIRContext(), ArgTypes, resultType, FI.isVariadic());
+  return FuncType::get(ArgTypes, resultType, FI.isVariadic());
 }
 
 /// Convert a CIR type to its ABI-specific default form.

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

@@ -55,10 +55,10 @@ int foo(Data* d) {
     return f(d);
 }
 
-// CIR:  cir.func private {{@.*test.*}}() -> !cir.ptr<!cir.func<!void ()>>
+// CIR:  cir.func private {{@.*test.*}}() -> !cir.ptr<!cir.func<()>>
 // CIR:  cir.func {{@.*bar.*}}()
-// CIR:    [[RET:%.*]] = cir.call {{@.*test.*}}() : () -> !cir.ptr<!cir.func<!void ()>>
-// CIR:    cir.call [[RET]]() : (!cir.ptr<!cir.func<!void ()>>) -> ()
+// CIR:    [[RET:%.*]] = cir.call {{@.*test.*}}() : () -> !cir.ptr<!cir.func<()>>
+// CIR:    cir.call [[RET]]() : (!cir.ptr<!cir.func<()>>) -> ()
 // CIR:    cir.return
 
 // LLVM: declare ptr {{@.*test.*}}()

clang/test/CIR/CodeGen/gnu-extension.c

@@ -15,5 +15,5 @@ void bar(void) {
 }
 
 //CHECK:  cir.func @bar()
-//CHECK:    {{.*}} = cir.get_global @bar : !cir.ptr<!cir.func<!void ()>>
+//CHECK:    {{.*}} = cir.get_global @bar : !cir.ptr<!cir.func<()>>
 //CHECK:    cir.return

clang/test/CIR/CodeGen/member-init-struct.cpp

@@ -34,7 +34,7 @@ C a, b(x), c(0, 2);
 // CHECK:   %[[VAL_8:.*]] = cir.get_member %[[VAL_2]][2] {name = "d"} : !cir.ptr<!ty_C> -> !cir.ptr<!cir.array<!s32i x 10>>
 // CHECK:   %[[VAL_9:.*]] = cir.const {{.*}} : !cir.array<!s32i x 10>
 // CHECK:   cir.store %[[VAL_9]], %[[VAL_8]] : !cir.array<!s32i x 10>, !cir.ptr<!cir.array<!s32i x 10>>
-// CHECK:   %[[VAL_10:.*]] = cir.get_member %[[VAL_2]][4] {name = "e"} : !cir.ptr<!ty_C> -> !cir.ptr<!cir.method<!cir.func<!void ()> in !ty_C>>
-// CHECK:   %[[VAL_11:.*]] = cir.const #cir.method<null> : !cir.method<!cir.func<!void ()> in !ty_C>
-// CHECK:   cir.store %[[VAL_11]], %[[VAL_10]] : !cir.method<!cir.func<!void ()> in !ty_C>, !cir.ptr<!cir.method<!cir.func<!void ()> in !ty_C>>
-// CHECK:   cir.return
+// CHECK:   %[[VAL_10:.*]] = cir.get_member %[[VAL_2]][4] {name = "e"} : !cir.ptr<!ty_C> -> !cir.ptr<!cir.method<!cir.func<()> in !ty_C>>
+// CHECK:   %[[VAL_11:.*]] = cir.const #cir.method<null> : !cir.method<!cir.func<()> in !ty_C>
+// CHECK:   cir.store %[[VAL_11]], %[[VAL_10]] : !cir.method<!cir.func<()> in !ty_C>, !cir.ptr<!cir.method<!cir.func<()> in !ty_C>>
+// CHECK:   cir.return

clang/test/CIR/CodeGen/multi-vtable.cpp

@@ -74,9 +74,9 @@ int main() {
 
 // CIR: cir.func @main() -> !s32i extra(#fn_attr) {
 
-// CIR:   %{{[0-9]+}} = cir.vtable.address_point( %{{[0-9]+}} : !cir.ptr<!cir.ptr<!cir.func<!void (!cir.ptr<!ty_Mother>)>>>, vtable_index = 0, address_point_index = 0) : !cir.ptr<!cir.ptr<!cir.func<!void (!cir.ptr<!ty_Mother>)>>>
+// CIR:   %{{[0-9]+}} = cir.vtable.address_point( %{{[0-9]+}} : !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!ty_Mother>)>>>, vtable_index = 0, address_point_index = 0) : !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!ty_Mother>)>>>
 
-// CIR:   %{{[0-9]+}} = cir.vtable.address_point( %{{[0-9]+}} : !cir.ptr<!cir.ptr<!cir.func<!void (!cir.ptr<!ty_Child>)>>>, vtable_index = 0, address_point_index = 0) : !cir.ptr<!cir.ptr<!cir.func<!void (!cir.ptr<!ty_Child>)>>>
+// CIR:   %{{[0-9]+}} = cir.vtable.address_point( %{{[0-9]+}} : !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!ty_Child>)>>>, vtable_index = 0, address_point_index = 0) : !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!ty_Child>)>>>
 
 // CIR: }
 

clang/test/CIR/CodeGen/no-proto-fun-ptr.c

@@ -7,9 +7,9 @@ void check_noproto_ptr() {
 }
 
 // CHECK:  cir.func no_proto @check_noproto_ptr()
-// CHECK:    [[ALLOC:%.*]] = cir.alloca !cir.ptr<!cir.func<!void ()>>, !cir.ptr<!cir.ptr<!cir.func<!void ()>>>, ["fun", init] {alignment = 8 : i64}
-// CHECK:    [[GGO:%.*]] = cir.get_global @empty : !cir.ptr<!cir.func<!void ()>>
-// CHECK:    cir.store [[GGO]], [[ALLOC]] : !cir.ptr<!cir.func<!void ()>>, !cir.ptr<!cir.ptr<!cir.func<!void ()>>>
+// CHECK:    [[ALLOC:%.*]] = cir.alloca !cir.ptr<!cir.func<()>>, !cir.ptr<!cir.ptr<!cir.func<()>>>, ["fun", init] {alignment = 8 : i64}
+// CHECK:    [[GGO:%.*]] = cir.get_global @empty : !cir.ptr<!cir.func<()>>
+// CHECK:    cir.store [[GGO]], [[ALLOC]] : !cir.ptr<!cir.func<()>>, !cir.ptr<!cir.ptr<!cir.func<()>>>
 // CHECK:    cir.return
 
 void empty(void) {}
@@ -20,8 +20,8 @@ void buz() {
 }
 
 // CHECK:  cir.func no_proto @buz()
-// CHECK:    [[FNPTR_ALLOC:%.*]] = cir.alloca !cir.ptr<!cir.func<!void (...)>>, !cir.ptr<!cir.ptr<!cir.func<!void (...)>>>, ["func"] {alignment = 8 : i64}
-// CHECK:    [[FNPTR:%.*]] = cir.load deref [[FNPTR_ALLOC]] : !cir.ptr<!cir.ptr<!cir.func<!void (...)>>>, !cir.ptr<!cir.func<!void (...)>>
-// CHECK:    [[CAST:%.*]] = cir.cast(bitcast, %1 : !cir.ptr<!cir.func<!void (...)>>), !cir.ptr<!cir.func<!void ()>>
-// CHECK:    cir.call [[CAST]]() : (!cir.ptr<!cir.func<!void ()>>) -> ()
+// CHECK:    [[FNPTR_ALLOC:%.*]] = cir.alloca !cir.ptr<!cir.func<(...)>>, !cir.ptr<!cir.ptr<!cir.func<(...)>>>, ["func"] {alignment = 8 : i64}
+// CHECK:    [[FNPTR:%.*]] = cir.load deref [[FNPTR_ALLOC]] : !cir.ptr<!cir.ptr<!cir.func<(...)>>>, !cir.ptr<!cir.func<(...)>>
+// CHECK:    [[CAST:%.*]] = cir.cast(bitcast, %1 : !cir.ptr<!cir.func<(...)>>), !cir.ptr<!cir.func<()>>
+// CHECK:    cir.call [[CAST]]() : (!cir.ptr<!cir.func<()>>) -> ()
 // CHECK:    cir.return

clang/test/CIR/CodeGen/pointer-arith-ext.c

@@ -50,9 +50,9 @@ void *f4_1(void *a, int b) { return (a -= b); }
 
 FP f5(FP a, int b) { return a + b; }
 // CIR-LABEL: f5
-// CIR: %[[PTR:.*]] = cir.load {{.*}} : !cir.ptr<!cir.ptr<!cir.func<!void ()>>>, !cir.ptr<!cir.func<!void ()>>
+// CIR: %[[PTR:.*]] = cir.load {{.*}} : !cir.ptr<!cir.ptr<!cir.func<()>>>, !cir.ptr<!cir.func<()>>
 // CIR: %[[STRIDE:.*]] = cir.load {{.*}} : !cir.ptr<!s32i>, !s32i
-// CIR: cir.ptr_stride(%[[PTR]] : !cir.ptr<!cir.func<!void ()>>, %[[STRIDE]] : !s32i)
+// CIR: cir.ptr_stride(%[[PTR]] : !cir.ptr<!cir.func<()>>, %[[STRIDE]] : !s32i)
 
 // LLVM-LABEL: f5
 // LLVM: %[[PTR:.*]] = load ptr, ptr {{.*}}, align 8
@@ -67,10 +67,10 @@ FP f6_1(int a, FP b) { return (a += b); }
 
 FP f7(FP a, int b) { return a - b; }
 // CIR-LABEL: f7
-// CIR: %[[PTR:.*]] = cir.load {{.*}} : !cir.ptr<!cir.ptr<!cir.func<!void ()>>>, !cir.ptr<!cir.func<!void ()>>
+// CIR: %[[PTR:.*]] = cir.load {{.*}} : !cir.ptr<!cir.ptr<!cir.func<()>>>, !cir.ptr<!cir.func<()>>
 // CIR: %[[STRIDE:.*]] = cir.load {{.*}} : !cir.ptr<!s32i>, !s32i
 // CIR: %[[SUB:.*]] = cir.unary(minus, %[[STRIDE]]) : !s32i, !s32i
-// CIR: cir.ptr_stride(%[[PTR]] : !cir.ptr<!cir.func<!void ()>>, %[[SUB]] : !s32i)
+// CIR: cir.ptr_stride(%[[PTR]] : !cir.ptr<!cir.func<()>>, %[[SUB]] : !s32i)
 
 // LLVM-LABEL: f7
 // LLVM: %[[PTR:.*]] = load ptr, ptr {{.*}}, align 8