[CIR] Add support for using enum constants (llvm#143214)

Although support for declaring enums and using values whose type was an
enum was previously upstreamed, we didn't have support for referencing
the constant values declared in the enum. This change adds that support.

Author: andykaylor

clang/lib/CIR/CodeGen/CIRGenExpr.cpp

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "Address.h"
+#include "CIRGenConstantEmitter.h"
 #include "CIRGenFunction.h"
 #include "CIRGenModule.h"
 #include "CIRGenValue.h"
@@ -1495,3 +1496,57 @@ cir::AllocaOp CIRGenFunction::createTempAlloca(mlir::Type ty,
       emitAlloca(name.str(), ty, loc, CharUnits(), ip, arraySize)
           .getDefiningOp());
 }
+
+/// Try to emit a reference to the given value without producing it as
+/// an l-value.  For many cases, this is just an optimization, but it avoids
+/// us needing to emit global copies of variables if they're named without
+/// triggering a formal use in a context where we can't emit a direct
+/// reference to them, for instance if a block or lambda or a member of a
+/// local class uses a const int variable or constexpr variable from an
+/// enclosing function.
+///
+/// For named members of enums, this is the only way they are emitted.
+CIRGenFunction::ConstantEmission
+CIRGenFunction::tryEmitAsConstant(DeclRefExpr *refExpr) {
+  ValueDecl *value = refExpr->getDecl();
+
+  // There is a lot more to do here, but for now only EnumConstantDecl is
+  // supported.
+  assert(!cir::MissingFeatures::tryEmitAsConstant());
+
+  // The value needs to be an enum constant or a constant variable.
+  if (!isa<EnumConstantDecl>(value))
+    return ConstantEmission();
+
+  Expr::EvalResult result;
+  if (!refExpr->EvaluateAsRValue(result, getContext()))
+    return ConstantEmission();
+
+  QualType resultType = refExpr->getType();
+
+  // As long as we're only handling EnumConstantDecl, there should be no
+  // side-effects.
+  assert(!result.HasSideEffects);
+
+  // Emit as a constant.
+  // FIXME(cir): have emitAbstract build a TypedAttr instead (this requires
+  // somewhat heavy refactoring...)
+  mlir::Attribute c = ConstantEmitter(*this).emitAbstract(
+      refExpr->getLocation(), result.Val, resultType);
+  mlir::TypedAttr cstToEmit = mlir::dyn_cast_if_present<mlir::TypedAttr>(c);
+  assert(cstToEmit && "expected a typed attribute");
+
+  assert(!cir::MissingFeatures::generateDebugInfo());
+
+  return ConstantEmission::forValue(cstToEmit);
+}
+
+mlir::Value CIRGenFunction::emitScalarConstant(
+    const CIRGenFunction::ConstantEmission &constant, Expr *e) {
+  assert(constant && "not a constant");
+  if (constant.isReference()) {
+    cgm.errorNYI(e->getSourceRange(), "emitScalarConstant: reference");
+    return {};
+  }
+  return builder.getConstant(getLoc(e->getSourceRange()), constant.getValue());
+}

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -140,7 +140,9 @@ class ScalarExprEmitter : public StmtVisitor<ScalarExprEmitter, mlir::Value> {
 
   // l-values
   mlir::Value VisitDeclRefExpr(DeclRefExpr *e) {
-    assert(!cir::MissingFeatures::tryEmitAsConstant());
+    if (CIRGenFunction::ConstantEmission constant = cgf.tryEmitAsConstant(e))
+      return cgf.emitScalarConstant(constant, e);
+
     return emitLoadOfLValue(e);
   }
 

clang/lib/CIR/CodeGen/CIRGenFunction.h

@@ -374,6 +374,41 @@ class CIRGenFunction : public CIRGenTypeCache {
   /// that we can just remove the code.
   bool containsLabel(const clang::Stmt *s, bool ignoreCaseStmts = false);
 
+  class ConstantEmission {
+    // Cannot use mlir::TypedAttr directly here because of bit availability.
+    llvm::PointerIntPair<mlir::Attribute, 1, bool> valueAndIsReference;
+    ConstantEmission(mlir::TypedAttr c, bool isReference)
+        : valueAndIsReference(c, isReference) {}
+
+  public:
+    ConstantEmission() {}
+    static ConstantEmission forReference(mlir::TypedAttr c) {
+      return ConstantEmission(c, true);
+    }
+    static ConstantEmission forValue(mlir::TypedAttr c) {
+      return ConstantEmission(c, false);
+    }
+
+    explicit operator bool() const {
+      return valueAndIsReference.getOpaqueValue() != nullptr;
+    }
+
+    bool isReference() const { return valueAndIsReference.getInt(); }
+    LValue getReferenceLValue(CIRGenFunction &cgf, Expr *refExpr) const {
+      assert(isReference());
+      cgf.cgm.errorNYI(refExpr->getSourceRange(),
+                       "ConstantEmission::getReferenceLValue");
+      return {};
+    }
+
+    mlir::TypedAttr getValue() const {
+      assert(!isReference());
+      return mlir::cast<mlir::TypedAttr>(valueAndIsReference.getPointer());
+    }
+  };
+
+  ConstantEmission tryEmitAsConstant(DeclRefExpr *refExpr);
+
   struct AutoVarEmission {
     const clang::VarDecl *Variable;
     /// The address of the alloca for languages with explicit address space
@@ -840,6 +875,8 @@ class CIRGenFunction : public CIRGenTypeCache {
 
   mlir::LogicalResult emitReturnStmt(const clang::ReturnStmt &s);
 
+  mlir::Value emitScalarConstant(const ConstantEmission &constant, Expr *e);
+
   /// Emit a conversion from the specified type to the specified destination
   /// type, both of which are CIR scalar types.
   mlir::Value emitScalarConversion(mlir::Value src, clang::QualType srcType,

clang/test/CIR/CodeGen/enum.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck %s --input-file=%t.cir
+
+enum Numbers {
+  Zero,
+  One,
+  Two,
+  Three
+};
+
+int f() {
+  return Numbers::One;
+}
+
+// CHECK: cir.func{{.*}} @_Z1fv
+// CHECK:    cir.const #cir.int<1> : !u32i