[Clang] Add attribute for consteval builtin functions (#91894)

Summary:
Builtins with the new `Consteval` attribute will also be marked
`Constexpr` and will only be available in C++20 mode where `consteval`
makes sense.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251624

Author: MitalAshok

clang/include/clang/Basic/Builtins.def

@@ -101,3 +101,4 @@
 //                      M_0, ..., M_k as payload
 //  z -> this is a function in (possibly-versioned) namespace std
 //  E -> this function can be constant evaluated by Clang frontend
+//  G -> this is a C++20 consteval function

clang/include/clang/Basic/Builtins.h

@@ -280,6 +280,11 @@ class Context {
     return strchr(getRecord(ID).Attributes, 'E') != nullptr;
   }
 
+  /// Returns true if this is an immediate (consteval) function
+  bool isImmediate(unsigned ID) const {
+    return strchr(getRecord(ID).Attributes, 'G') != nullptr;
+  }
+
 private:
   const Info &getRecord(unsigned ID) const;
 

clang/include/clang/Basic/BuiltinsBase.td

@@ -70,6 +70,8 @@ class VScanfFormat<int I> : IndexedAttribute<"S", I>;
 
 // Builtin can be constant evaluated
 def Constexpr : Attribute<"E">;
+// Builtin is immediate and must be constant evaluated. Implies Constexpr, and will only be supported in C++20 mode.
+def Consteval : Attribute<"EG">;
 
 // Builtin kinds
 // =============

clang/lib/Basic/Builtins.cpp

@@ -119,6 +119,9 @@ static bool builtinIsSupported(const Builtin::Info &BuiltinInfo,
   /* CPlusPlus Unsupported */
   if (!LangOpts.CPlusPlus && BuiltinInfo.Langs == CXX_LANG)
     return false;
+  /* consteval Unsupported */
+  if (!LangOpts.CPlusPlus20 && strchr(BuiltinInfo.Attributes, 'G') != nullptr)
+    return false;
   return true;
 }
 

clang/lib/Sema/SemaDecl.cpp

@@ -2294,10 +2294,17 @@ FunctionDecl *Sema::CreateBuiltin(IdentifierInfo *II, QualType Type,
     Parent = CLinkageDecl;
   }
 
-  FunctionDecl *New = FunctionDecl::Create(Context, Parent, Loc, Loc, II, Type,
-                                           /*TInfo=*/nullptr, SC_Extern,
-                                           getCurFPFeatures().isFPConstrained(),
-                                           false, Type->isFunctionProtoType());
+  ConstexprSpecKind ConstexprKind = ConstexprSpecKind::Unspecified;
+  if (Context.BuiltinInfo.isImmediate(ID)) {
+    assert(getLangOpts().CPlusPlus20 &&
+           "consteval builtins should only be available in C++20 mode");
+    ConstexprKind = ConstexprSpecKind::Consteval;
+  }
+
+  FunctionDecl *New = FunctionDecl::Create(
+      Context, Parent, Loc, Loc, II, Type, /*TInfo=*/nullptr, SC_Extern,
+      getCurFPFeatures().isFPConstrained(), /*isInlineSpecified=*/false,
+      Type->isFunctionProtoType(), ConstexprKind);
   New->setImplicit();
   New->addAttr(BuiltinAttr::CreateImplicit(Context, ID));
 

clang/lib/Sema/SemaExpr.cpp

@@ -6770,8 +6770,12 @@ ExprResult Sema::BuildResolvedCallExpr(Expr *Fn, NamedDecl *NDecl,
   }
 
   // Bail out early if calling a builtin with custom type checking.
-  if (BuiltinID && Context.BuiltinInfo.hasCustomTypechecking(BuiltinID))
-    return CheckBuiltinFunctionCall(FDecl, BuiltinID, TheCall);
+  if (BuiltinID && Context.BuiltinInfo.hasCustomTypechecking(BuiltinID)) {
+    ExprResult E = CheckBuiltinFunctionCall(FDecl, BuiltinID, TheCall);
+    if (!E.isInvalid() && Context.BuiltinInfo.isImmediate(BuiltinID))
+      E = CheckForImmediateInvocation(E, FDecl);
+    return E;
+  }
 
   if (getLangOpts().CUDA) {
     if (Config) {