[clang] perform semantic checking in constant context

Summary:
Since the addition of __builtin_is_constant_evaluated the result of an expression can change based on whether it is evaluated in constant context. a lot of semantic checking performs evaluations with out specifying context. which can lead to wrong diagnostics.
for example:
```
constexpr int i0 = (long long)__builtin_is_constant_evaluated() * (1ll << 33); //#1
constexpr int i1 = (long long)!__builtin_is_constant_evaluated() * (1ll << 33); //#2
```
before the patch, #2 was diagnosed incorrectly and #1 wasn't diagnosed.
after the patch #1 is diagnosed as it should and #2 isn't.

Changes:
 - add a flag to Sema to passe in constant context mode.
 - in SemaChecking.cpp calls to Expr::Evaluate* are now done in constant context when they should.
 - in SemaChecking.cpp diagnostics for UB are not checked for in constant context because an error will be emitted by the constant evaluator.
 - in SemaChecking.cpp diagnostics for construct that cannot appear in constant context are not checked for in constant context.
 - in SemaChecking.cpp diagnostics on constant expression are always emitted because constant expression are always evaluated.
 - semantic checking for initialization of constexpr variables is now done in constant context.
 - adapt test that were depending on warning changes.
 - add test.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62009

llvm-svn: 363488

clang/include/clang/AST/Expr.h

@@ -599,7 +599,8 @@ class Expr : public ValueStmt {
   /// which we can fold and convert to a boolean condition using
   /// any crazy technique that we want to, even if the expression has
   /// side-effects.
-  bool EvaluateAsBooleanCondition(bool &Result, const ASTContext &Ctx) const;
+  bool EvaluateAsBooleanCondition(bool &Result, const ASTContext &Ctx,
+                                  bool InConstantContext = false) const;
 
   enum SideEffectsKind {
     SE_NoSideEffects,          ///< Strictly evaluate the expression.
@@ -611,20 +612,21 @@ class Expr : public ValueStmt {
   /// EvaluateAsInt - Return true if this is a constant which we can fold and
   /// convert to an integer, using any crazy technique that we want to.
   bool EvaluateAsInt(EvalResult &Result, const ASTContext &Ctx,
-                     SideEffectsKind AllowSideEffects = SE_NoSideEffects) const;
+                     SideEffectsKind AllowSideEffects = SE_NoSideEffects,
+                     bool InConstantContext = false) const;
 
   /// EvaluateAsFloat - Return true if this is a constant which we can fold and
   /// convert to a floating point value, using any crazy technique that we
   /// want to.
-  bool
-  EvaluateAsFloat(llvm::APFloat &Result, const ASTContext &Ctx,
-                  SideEffectsKind AllowSideEffects = SE_NoSideEffects) const;
+  bool EvaluateAsFloat(llvm::APFloat &Result, const ASTContext &Ctx,
+                       SideEffectsKind AllowSideEffects = SE_NoSideEffects,
+                       bool InConstantContext = false) const;
 
   /// EvaluateAsFloat - Return true if this is a constant which we can fold and
   /// convert to a fixed point value.
-  bool EvaluateAsFixedPoint(
-      EvalResult &Result, const ASTContext &Ctx,
-      SideEffectsKind AllowSideEffects = SE_NoSideEffects) const;
+  bool EvaluateAsFixedPoint(EvalResult &Result, const ASTContext &Ctx,
+                            SideEffectsKind AllowSideEffects = SE_NoSideEffects,
+                            bool InConstantContext = false) const;
 
   /// isEvaluatable - Call EvaluateAsRValue to see if this expression can be
   /// constant folded without side-effects, but discard the result.
@@ -660,7 +662,8 @@ class Expr : public ValueStmt {
 
   /// EvaluateAsLValue - Evaluate an expression to see if we can fold it to an
   /// lvalue with link time known address, with no side-effects.
-  bool EvaluateAsLValue(EvalResult &Result, const ASTContext &Ctx) const;
+  bool EvaluateAsLValue(EvalResult &Result, const ASTContext &Ctx,
+                        bool InConstantContext = false) const;
 
   /// EvaluateAsInitializer - Evaluate an expression as if it were the
   /// initializer of the given declaration. Returns true if the initializer

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -85,6 +85,8 @@ def note_constexpr_past_end_subobject : Note<
   "access array element of|ERROR|"
   "access real component of|access imaginary component of}0 "
   "pointer past the end of object">;
+def note_non_null_attribute_failed : Note<
+  "null passed to a callee that requires a non-null argument">;
 def note_constexpr_null_subobject : Note<
   "cannot %select{access base class of|access derived class of|access field of|"
   "access array element of|perform pointer arithmetic on|"

clang/include/clang/Sema/Sema.h

@@ -797,6 +797,15 @@ class Sema {
     }
   };
 
+  /// Used to change context to isConstantEvaluated without pushing a heavy
+  /// ExpressionEvaluationContextRecord object.
+  bool isConstantEvaluatedOverride;
+
+  bool isConstantEvaluated() {
+    return ExprEvalContexts.back().isConstantEvaluated() ||
+           isConstantEvaluatedOverride;
+  }
+
   /// RAII object to handle the state changes required to synthesize
   /// a function body.
   class SynthesizedFunctionScope {

clang/lib/AST/ExprConstant.cpp

@@ -47,6 +47,7 @@
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/FixedPoint.h"
 #include "clang/Basic/TargetInfo.h"
+#include "llvm/ADT/SmallBitVector.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstring>
@@ -5091,9 +5092,25 @@ typedef SmallVector<APValue, 8> ArgVector;
 }
 
 /// EvaluateArgs - Evaluate the arguments to a function call.
-static bool EvaluateArgs(ArrayRef<const Expr*> Args, ArgVector &ArgValues,
-                         EvalInfo &Info) {
+static bool EvaluateArgs(ArrayRef<const Expr *> Args, ArgVector &ArgValues,
+                         EvalInfo &Info, const FunctionDecl *Callee) {
   bool Success = true;
+  llvm::SmallBitVector ForbiddenNullArgs;
+  if (Callee->hasAttr<NonNullAttr>()) {
+    ForbiddenNullArgs.resize(Args.size());
+    for (const auto *Attr : Callee->specific_attrs<NonNullAttr>()) {
+      if (!Attr->args_size()) {
+        ForbiddenNullArgs.set();
+        break;
+      } else
+        for (auto Idx : Attr->args()) {
+          unsigned ASTIdx = Idx.getASTIndex();
+          if (ASTIdx >= Args.size())
+            continue;
+          ForbiddenNullArgs[ASTIdx] = 1;
+        }
+    }
+  }
   for (ArrayRef<const Expr*>::iterator I = Args.begin(), E = Args.end();
        I != E; ++I) {
     if (!Evaluate(ArgValues[I - Args.begin()], Info, *I)) {
@@ -5102,6 +5119,13 @@ static bool EvaluateArgs(ArrayRef<const Expr*> Args, ArgVector &ArgValues,
       if (!Info.noteFailure())
         return false;
       Success = false;
+    } else if (!ForbiddenNullArgs.empty() &&
+               ForbiddenNullArgs[I - Args.begin()] &&
+               ArgValues[I - Args.begin()].isNullPointer()) {
+      Info.CCEDiag(*I, diag::note_non_null_attribute_failed);
+      if (!Info.noteFailure())
+        return false;
+      Success = false;
     }
   }
   return Success;
@@ -5114,7 +5138,7 @@ static bool HandleFunctionCall(SourceLocation CallLoc,
                                EvalInfo &Info, APValue &Result,
                                const LValue *ResultSlot) {
   ArgVector ArgValues(Args.size());
-  if (!EvaluateArgs(Args, ArgValues, Info))
+  if (!EvaluateArgs(Args, ArgValues, Info, Callee))
     return false;
 
   if (!Info.CheckCallLimit(CallLoc))
@@ -5338,7 +5362,7 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
                                   const CXXConstructorDecl *Definition,
                                   EvalInfo &Info, APValue &Result) {
   ArgVector ArgValues(Args.size());
-  if (!EvaluateArgs(Args, ArgValues, Info))
+  if (!EvaluateArgs(Args, ArgValues, Info, Definition))
     return false;
 
   return HandleConstructorCall(E, This, ArgValues.data(), Definition,
@@ -11855,54 +11879,61 @@ bool Expr::EvaluateAsRValue(EvalResult &Result, const ASTContext &Ctx,
   return ::EvaluateAsRValue(this, Result, Ctx, Info);
 }
 
-bool Expr::EvaluateAsBooleanCondition(bool &Result,
-                                      const ASTContext &Ctx) const {
+bool Expr::EvaluateAsBooleanCondition(bool &Result, const ASTContext &Ctx,
+                                      bool InConstantContext) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
   EvalResult Scratch;
-  return EvaluateAsRValue(Scratch, Ctx) &&
+  return EvaluateAsRValue(Scratch, Ctx, InConstantContext) &&
          HandleConversionToBool(Scratch.Val, Result);
 }
 
 bool Expr::EvaluateAsInt(EvalResult &Result, const ASTContext &Ctx,
-                         SideEffectsKind AllowSideEffects) const {
+                         SideEffectsKind AllowSideEffects,
+                         bool InConstantContext) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
   EvalInfo Info(Ctx, Result, EvalInfo::EM_IgnoreSideEffects);
+  Info.InConstantContext = InConstantContext;
   return ::EvaluateAsInt(this, Result, Ctx, AllowSideEffects, Info);
 }
 
 bool Expr::EvaluateAsFixedPoint(EvalResult &Result, const ASTContext &Ctx,
-                                SideEffectsKind AllowSideEffects) const {
+                                SideEffectsKind AllowSideEffects,
+                                bool InConstantContext) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
   EvalInfo Info(Ctx, Result, EvalInfo::EM_IgnoreSideEffects);
+  Info.InConstantContext = InConstantContext;
   return ::EvaluateAsFixedPoint(this, Result, Ctx, AllowSideEffects, Info);
 }
 
 bool Expr::EvaluateAsFloat(APFloat &Result, const ASTContext &Ctx,
-                           SideEffectsKind AllowSideEffects) const {
+                           SideEffectsKind AllowSideEffects,
+                           bool InConstantContext) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
 
   if (!getType()->isRealFloatingType())
     return false;
 
   EvalResult ExprResult;
-  if (!EvaluateAsRValue(ExprResult, Ctx) || !ExprResult.Val.isFloat() ||
+  if (!EvaluateAsRValue(ExprResult, Ctx, InConstantContext) ||
+      !ExprResult.Val.isFloat() ||
       hasUnacceptableSideEffect(ExprResult, AllowSideEffects))
     return false;
 
   Result = ExprResult.Val.getFloat();
   return true;
 }
 
-bool Expr::EvaluateAsLValue(EvalResult &Result, const ASTContext &Ctx) const {
+bool Expr::EvaluateAsLValue(EvalResult &Result, const ASTContext &Ctx,
+                            bool InConstantContext) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
 
   EvalInfo Info(Ctx, Result, EvalInfo::EM_ConstantFold);
-
+  Info.InConstantContext = InConstantContext;
   LValue LV;
   if (!EvaluateLValue(this, LV, Info) || Result.HasSideEffects ||
       !CheckLValueConstantExpression(Info, getExprLoc(),
@@ -12685,7 +12716,7 @@ bool Expr::isPotentialConstantExprUnevaluated(Expr *E,
   // Fabricate a call stack frame to give the arguments a plausible cover story.
   ArrayRef<const Expr*> Args;
   ArgVector ArgValues(0);
-  bool Success = EvaluateArgs(Args, ArgValues, Info);
+  bool Success = EvaluateArgs(Args, ArgValues, Info, FD);
   (void)Success;
   assert(Success &&
          "Failed to set up arguments for potential constant evaluation");

clang/lib/Sema/Sema.cpp

@@ -158,6 +158,7 @@ Sema::Sema(Preprocessor &pp, ASTContext &ctxt, ASTConsumer &consumer,
       ThreadSafetyDeclCache(nullptr), VarDataSharingAttributesStack(nullptr),
       CurScope(nullptr), Ident_super(nullptr), Ident___float128(nullptr) {
   TUScope = nullptr;
+  isConstantEvaluatedOverride = false;
 
   LoadedExternalKnownNamespaces = false;
   for (unsigned I = 0; I != NSAPI::NumNSNumberLiteralMethods; ++I)