[CIR][CIRGen] Add codegen for stdarg builtins

Implement the necessary codegen to emit the va_start, va_end, va_arg,
and va_copy builtins as custom CIR instructions.

ghstack-source-id: d641b62
Pull Request resolved: #94

Author: sitio-couto

clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp

@@ -19,6 +19,7 @@
 
 // TODO(cir): we shouldn't need this but we currently reuse intrinsic IDs for
 // convenience.
+#include "clang/CIR/Dialect/IR/CIRTypes.h"
 #include "llvm/IR/Intrinsics.h"
 
 #include "clang/AST/GlobalDecl.h"
@@ -334,6 +335,26 @@ RValue CIRGenFunction::buildBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     llvm_unreachable("NYI");
     break;
 
+  // C stdarg builtins.
+  case Builtin::BI__builtin_stdarg_start:
+  case Builtin::BI__builtin_va_start:
+  case Builtin::BI__va_start: {
+    auto vaList = buildScalarExpr(E->getArg(0));
+    builder.create<mlir::cir::VAStartOp>(vaList.getLoc(), vaList);
+    return {};
+  }
+  case Builtin::BI__builtin_va_end: {
+    auto vaList = buildVAListRef(E->getArg(0)).getPointer();
+    builder.create<mlir::cir::VAEndOp>(vaList.getLoc(), vaList);
+    return {};
+  }
+  case Builtin::BI__builtin_va_copy: {
+    auto dstPtr = buildVAListRef(E->getArg(0)).getPointer();
+    auto srcPtr = buildVAListRef(E->getArg(1)).getPointer();
+    builder.create<mlir::cir::VACopyOp>(dstPtr.getLoc(), dstPtr, srcPtr);
+    return {};
+  }
+
   // C++ std:: builtins.
   case Builtin::BImove:
   case Builtin::BImove_if_noexcept:

clang/lib/CIR/CodeGen/CIRGenCall.cpp

@@ -1131,3 +1131,15 @@ RValue CallArg::getRValue(CIRGenFunction &CGF, mlir::Location loc) const {
   IsUsed = true;
   return RValue::getAggregate(Copy.getAddress());
 }
+
+/* VarArg handling */
+
+// FIXME(cir): This completely abstracts away the ABI with a generic CIR Op. We
+// need to decide how to handle va_arg target-specific codegen.
+mlir::Value CIRGenFunction::buildVAArg(VAArgExpr *VE, Address &VAListAddr) {
+  assert(!VE->isMicrosoftABI() && "NYI");
+  auto loc = CGM.getLoc(VE->getExprLoc());
+  auto type = ConvertType(VE->getType());
+  auto vaList = buildVAListRef(VE->getSubExpr()).getPointer();
+  return builder.create<mlir::cir::VAArgOp>(loc, type, vaList);
+}

clang/lib/CIR/CodeGen/CIRGenExpr.cpp

@@ -663,6 +663,10 @@ Address CIRGenFunction::buildPointerWithAlignment(const Expr *E,
     case CK_LValueToRValue:
       break;
 
+    // Array-to-pointer decay. TODO(cir): BaseInfo and TBAAInfo.
+    case CK_ArrayToPointerDecay:
+      return buildArrayToPointerDecay(CE->getSubExpr());
+
     case CK_UncheckedDerivedToBase:
     case CK_DerivedToBase: {
       // TODO: Support accesses to members of base classes in TBAA. For now, we

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -551,7 +551,7 @@ class ScalarExprEmitter : public StmtVisitor<ScalarExprEmitter, mlir::Value> {
   mlir::Value
   VisitAbstractConditionalOperator(const AbstractConditionalOperator *E);
   mlir::Value VisitChooseExpr(ChooseExpr *E) { llvm_unreachable("NYI"); }
-  mlir::Value VisitVAArgExpr(VAArgExpr *E) { llvm_unreachable("NYI"); }
+  mlir::Value VisitVAArgExpr(VAArgExpr *VE);
   mlir::Value VisitObjCStringLiteral(const ObjCStringLiteral *E) {
     llvm_unreachable("NYI");
   }
@@ -1945,3 +1945,15 @@ mlir::Value ScalarExprEmitter::VisitBinLOr(const clang::BinaryOperator *E) {
 
   return Builder.createZExtOrBitCast(ResOp.getLoc(), ResOp, ResTy);
 }
+
+mlir::Value ScalarExprEmitter::VisitVAArgExpr(VAArgExpr *VE) {
+  QualType Ty = VE->getType();
+
+  if (Ty->isVariablyModifiedType())
+    assert(!UnimplementedFeature::variablyModifiedTypeEmission() && "NYI");
+
+  Address ArgValue = Address::invalid();
+  mlir::Value Val = CGF.buildVAArg(VE, ArgValue);
+
+  return Val;
+}

clang/lib/CIR/CodeGen/CIRGenFunction.cpp

@@ -1246,3 +1246,9 @@ void CIRGenFunction::buildDeclRefExprDbgValue(const DeclRefExpr *E,
                                               const APValue &Init) {
   assert(!UnimplementedFeature::generateDebugInfo());
 }
+
+Address CIRGenFunction::buildVAListRef(const Expr* E) {
+  if (getContext().getBuiltinVaListType()->isArrayType())
+    return buildPointerWithAlignment(E);
+  return buildLValue(E).getAddress();
+}

clang/lib/CIR/CodeGen/CIRGenFunction.h

@@ -803,6 +803,28 @@ class CIRGenFunction : public CIRGenTypeCache {
   RValue convertTempToRValue(Address addr, clang::QualType type,
                              clang::SourceLocation Loc);
 
+  // Build a "reference" to a va_list; this is either the address or the value
+  // of the expression, depending on how va_list is defined.
+  Address buildVAListRef(const Expr *E);
+
+  /// Emits a call to an LLVM variable-argument intrinsic, either
+  /// \c llvm.va_start or \c llvm.va_end.
+  /// \param ArgValue A reference to the \c va_list as emitted by either
+  /// \c EmitVAListRef or \c EmitMSVAListRef.
+  /// \param IsStart If \c true, emits a call to \c llvm.va_start; otherwise,
+  /// calls \c llvm.va_end.
+  mlir::cir::CallOp buildVAStartEnd(mlir::Value ArgValue, bool IsStart);
+
+  /// Generate code to get an argument from the passed in pointer
+  /// and update it accordingly.
+  /// \param VE The \c VAArgExpr for which to generate code.
+  /// \param VAListAddr Receives a reference to the \c va_list as emitted by
+  /// either \c EmitVAListRef or \c EmitMSVAListRef.
+  /// \returns SSA value with the argument.
+  // FIXME: We should be able to get rid of this method and use the va_arg
+  // instruction in LLVM instead once it works well enough.
+  mlir::Value buildVAArg(VAArgExpr *VE, Address &VAListAddr);
+
   /// Given an expression that represents a value lvalue, this method emits the
   /// address of the lvalue, then loads the result as an rvalue, returning the
   /// rvalue.

clang/lib/CIR/CodeGen/UnimplementedFeatureGuarding.h

@@ -62,6 +62,9 @@ struct UnimplementedFeature {
   // Coroutines
   static bool unhandledException() { return false; }
 
+  // Missing Emissions
+  static bool variablyModifiedTypeEmission() { return false; }
+
   // Clang early struct optimizations
   static bool shouldUseBZeroPlusStoresToInitialize() { return false; }
   static bool shouldUseMemSetToInitialize() { return false; }

clang/test/CIR/CodeGen/variadics.c

@@ -3,8 +3,29 @@
 // RUN: %clang_cc1 -x c++ -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir-enable -emit-cir %s -o %t.cir
 // RUN: FileCheck --input-file=%t.cir %s
 
-int average(int count, ...);
-// CHECK: cir.func private @{{.*}}average{{.*}}(!s32i, ...) -> !s32i
+#include <stdarg.h>
+// CHECK: [[VALISTTYPE:!.+va_list_.+]] = !cir.struct<"struct.__va_list_tag"
+
+int average(int count, ...) {
+// CHECK: cir.func @{{.*}}average{{.*}}(%arg0: !s32i loc({{.+}}), ...) -> !s32i
+    va_list args, args_copy;
+    va_start(args, count);
+    // CHECK: cir.vastart %{{[0-9]+}} : !cir.ptr<[[VALISTTYPE]]>
+
+    va_copy(args_copy, args);
+    // CHECK: cir.vacopy %{{[0-9]+}} to %{{[0-9]+}} : !cir.ptr<[[VALISTTYPE]]>, !cir.ptr<[[VALISTTYPE]]>
+
+    int sum = 0;
+    for(int i = 0; i < count; i++) {
+        sum += va_arg(args, int);
+        // CHECK: %{{[0-9]+}} = cir.vaarg %{{[0-9]+}} : (!cir.ptr<[[VALISTTYPE]]>) -> !s32i
+    }
+
+    va_end(args);
+    // CHECK: cir.vaend %{{[0-9]+}} : !cir.ptr<[[VALISTTYPE]]>
+
+    return count > 0 ? sum / count : 0;
+}
 
 int test(void) {
   return average(5, 1, 2, 3, 4, 5);