Reinstate r359059, reverted in r359361, with a fix to properly prevent

us emitting the operand of __builtin_constant_p if it has side-effects.

Original commit message:

Fix interactions between __builtin_constant_p and constexpr to match
current trunk GCC.

GCC permits information from outside the operand of
__builtin_constant_p (but in the same constant evaluation context) to be
used within that operand; clang now does so too. A few other minor
deviations from GCC's behavior showed up in my testing and are also
fixed (matching GCC):
  * Clang now supports nullptr_t as the argument type for
    __builtin_constant_p
    * Clang now returns true from __builtin_constant_p if called with a
    null pointer
    * Clang now returns true from __builtin_constant_p if called with an
    integer cast to pointer type

llvm-svn: 359367

3 files changed, 59 insertions, 39 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index a30e83c6328..24b28971af3 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -7801,19 +7801,33 @@ EvaluateBuiltinClassifyType(const CallExpr *E, const LangOptions &LangOpts) {
 }
 
 /// EvaluateBuiltinConstantPForLValue - Determine the result of
-/// __builtin_constant_p when applied to the given lvalue.
+/// __builtin_constant_p when applied to the given pointer.
 ///
-/// An lvalue is only "constant" if it is a pointer or reference to the first
-/// character of a string literal.
-template<typename LValue>
-static bool EvaluateBuiltinConstantPForLValue(const LValue &LV) {
-  const Expr *E = LV.getLValueBase().template dyn_cast<const Expr*>();
-  return E && isa<StringLiteral>(E) && LV.getLValueOffset().isZero();
+/// A pointer is only "constant" if it is null (or a pointer cast to integer)
+/// or it points to the first character of a string literal.
+static bool EvaluateBuiltinConstantPForLValue(const APValue &LV) {
+  APValue::LValueBase Base = LV.getLValueBase();
+  if (Base.isNull()) {
+    // A null base is acceptable.
+    return true;
+  } else if (const Expr *E = Base.dyn_cast<const Expr *>()) {
+    if (!isa<StringLiteral>(E))
+      return false;
+    return LV.getLValueOffset().isZero();
+  } else {
+    // Any other base is not constant enough for GCC.
+    return false;
+  }
 }
 
 /// EvaluateBuiltinConstantP - Evaluate __builtin_constant_p as similarly to
 /// GCC as we can manage.
-static bool EvaluateBuiltinConstantP(ASTContext &Ctx, const Expr *Arg) {
+static bool EvaluateBuiltinConstantP(EvalInfo &Info, const Expr *Arg) {
+  // Constant-folding is always enabled for the operand of __builtin_constant_p
+  // (even when the enclosing evaluation context otherwise requires a strict
+  // language-specific constant expression).
+  FoldConstant Fold(Info, true);
+
   QualType ArgType = Arg->getType();
 
   // __builtin_constant_p always has one operand. The rules which gcc follows
@@ -7821,34 +7835,30 @@ static bool EvaluateBuiltinConstantP(ASTContext &Ctx, const Expr *Arg) {
   //
   //  - If the operand is of integral, floating, complex or enumeration type,
   //    and can be folded to a known value of that type, it returns 1.
-  //  - If the operand and can be folded to a pointer to the first character
-  //    of a string literal (or such a pointer cast to an integral type), it
-  //    returns 1.
+  //  - If the operand can be folded to a pointer to the first character
+  //    of a string literal (or such a pointer cast to an integral type)
+  //    or to a null pointer or an integer cast to a pointer, it returns 1.
   //
   // Otherwise, it returns 0.
   //
   // FIXME: GCC also intends to return 1 for literals of aggregate types, but
-  // its support for this does not currently work.
-  if (ArgType->isIntegralOrEnumerationType()) {
-    Expr::EvalResult Result;
-    if (!Arg->EvaluateAsRValue(Result, Ctx) || Result.HasSideEffects)
+  // its support for this did not work prior to GCC 9 and is not yet well
+  // understood.
+  if (ArgType->isIntegralOrEnumerationType() || ArgType->isFloatingType() ||
+      ArgType->isAnyComplexType() || ArgType->isPointerType() ||
+      ArgType->isNullPtrType()) {
+    APValue V;
+    if (!::EvaluateAsRValue(Info, Arg, V)) {
+      Fold.keepDiagnostics();
       return false;
+    }
 
-    APValue &V = Result.Val;
-    if (V.getKind() == APValue::Int)
-      return true;
+    // For a pointer (possibly cast to integer), there are special rules.
     if (V.getKind() == APValue::LValue)
       return EvaluateBuiltinConstantPForLValue(V);
-  } else if (ArgType->isFloatingType() || ArgType->isAnyComplexType()) {
-    return Arg->isEvaluatable(Ctx);
-  } else if (ArgType->isPointerType() || Arg->isGLValue()) {
-    LValue LV;
-    Expr::EvalStatus Status;
-    EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantFold);
-    if ((Arg->isGLValue() ? EvaluateLValue(Arg, LV, Info)
-                          : EvaluatePointer(Arg, LV, Info)) &&
-        !Status.HasSideEffects)
-      return EvaluateBuiltinConstantPForLValue(LV);
+
+    // Otherwise, any constant value is good enough.
+    return V.getKind() != APValue::Uninitialized;
   }
 
   // Anything else isn't considered to be sufficiently constant.
@@ -8258,18 +8268,15 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
   }
 
   case Builtin::BI__builtin_constant_p: {
-    auto Arg = E->getArg(0);
-    if (EvaluateBuiltinConstantP(Info.Ctx, Arg))
+    const Expr *Arg = E->getArg(0);
+    if (EvaluateBuiltinConstantP(Info, Arg))
       return Success(true, E);
-    auto ArgTy = Arg->IgnoreImplicit()->getType();
-    if (!Info.InConstantContext && !Arg->HasSideEffects(Info.Ctx) &&
-        !ArgTy->isAggregateType() && !ArgTy->isPointerType()) {
-      // We can delay calculation of __builtin_constant_p until after
-      // inlining. Note: This diagnostic won't be shown to the user.
+    else if (Info.InConstantContext)
+      return Success(false, E);
+    else {
       Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
       return false;
     }
-    return Success(false, E);
   }
 
   case Builtin::BI__builtin_is_constant_evaluated:
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index bc5f6cfe43e..5abb62c560b 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -2027,8 +2027,17 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
 
     const Expr *Arg = E->getArg(0);
     QualType ArgType = Arg->getType();
-    if (!hasScalarEvaluationKind(ArgType) || ArgType->isFunctionType())
-      // We can only reason about scalar types.
+    // FIXME: The allowance for Obj-C pointers and block pointers is historical
+    // and likely a mistake.
+    if (!ArgType->isIntegralOrEnumerationType() && !ArgType->isFloatingType() &&
+        !ArgType->isObjCObjectPointerType() && !ArgType->isBlockPointerType())
+      // Per the GCC documentation, only numeric constants are recognized after
+      // inlining.
+      return RValue::get(ConstantInt::get(ResultType, 0));
+
+    if (Arg->HasSideEffects(getContext()))
+      // The argument is unevaluated, so be conservative if it might have
+      // side-effects.
       return RValue::get(ConstantInt::get(ResultType, 0));
 
     Value *ArgValue = EmitScalarExpr(Arg);
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index de061e62aad..4c2d1dc768d 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -1199,10 +1199,14 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     if (checkArgCount(*this, TheCall, 1)) return true;
     TheCall->setType(Context.IntTy);
     break;
-  case Builtin::BI__builtin_constant_p:
+  case Builtin::BI__builtin_constant_p: {
     if (checkArgCount(*this, TheCall, 1)) return true;
+    ExprResult Arg = DefaultFunctionArrayLvalueConversion(TheCall->getArg(0));
+    if (Arg.isInvalid()) return true;
+    TheCall->setArg(0, Arg.get());
     TheCall->setType(Context.IntTy);
     break;
+  }
   case Builtin::BI__builtin_launder:
     return SemaBuiltinLaunder(*this, TheCall);
   case Builtin::BI__sync_fetch_and_add: