Make __builtin_object_size use the EM_IgnoreSideEffects evaluation mode.

[jyknight]

Summary

And, since EM_OffsetFold is now unused, remove it.

While builtin_object_size intends to ignore the presence of
side-effects in its argument, the EM_OffsetFold mode was NOT
configured to ignore side-effects. Rather it was effectively identical
to EM_ConstantFold -- its explanatory comment
notwithstanding.

However, currently, keepEvaluatingAfterSideEffect() is not always
honored -- sometimes evaluation continues despite it returning
false. Therefore, since the b_o_s code was only checking the return
value from evaluation, and not additionally checking the
HasSideEffects flag, side-effects _were_ in many cases actually being
ignored.

This change is a prerequisite cleanup towards fixing that issue.

[rsmith]

Thanks, looks like this has been largely redundant since r294800 removed the only interesting thing that EM_OffsetFold did.

[george.burgess.iv]

Thanks!

lib/AST/ExprConstant.cpp

enum EvaluationMode {
       /// Evaluate as a potential constant expression. Keep going if we hit a
       /// construct that we can't evaluate yet (because we don't yet know the
       /// value of something) but stop if we hit something that could never be
       /// a constant expression. Some expressions can be retried in the
       /// optimizer if we don't constant fold them here, but in an unevaluated
       /// context we try to fold them immediately since the optimizer never
       /// gets a chance to look at it.
       EM_PotentialConstantExpressionUnevaluated,
-
-      /// Evaluate as a constant expression. In certain scenarios, if:
-      /// - we find a MemberExpr with a base that can't be evaluated, or
-      /// - we find a variable initialized with a call to a function that has
-      ///   the alloc_size attribute on it
-      /// then we may consider evaluation to have succeeded.
-      ///
-      /// In either case, the LValue returned shall have an invalid base; in the
-      /// former, the base will be the invalid MemberExpr, in the latter, the
-      /// base will be either the alloc_size CallExpr or a CastExpr wrapping
-      /// said CallExpr.
-      EM_OffsetFold,
     } EvalMode;
 
     /// Are we checking whether the expression is a potential constant
     /// expression?
     bool checkingPotentialConstantExpression() const {
       return EvalMode == EM_PotentialConstantExpression ||
              EvalMode == EM_PotentialConstantExpressionUnevaluated;
     }
OptionalDiagnostic Diag(SourceLocation Loc, diag::kind DiagId,
             if (!HasFoldFailureDiagnostic)
               break;
             // We've already failed to fold something. Keep that diagnostic.
             LLVM_FALLTHROUGH;
           case EM_ConstantExpression:
           case EM_PotentialConstantExpression:
           case EM_ConstantExpressionUnevaluated:
           case EM_PotentialConstantExpressionUnevaluated:
-          case EM_OffsetFold:
             HasActiveDiagnostic = false;
             return OptionalDiagnostic();
           }
         }
 
         unsigned CallStackNotes = CallStackDepth - 1;
         unsigned Limit = Ctx.getDiagnostics().getConstexprBacktraceLimit();
         if (Limit)
bool keepEvaluatingAfterSideEffect() {
       case EM_PotentialConstantExpressionUnevaluated:
       case EM_EvaluateForOverflow:
       case EM_IgnoreSideEffects:
         return true;
 
       case EM_ConstantExpression:
       case EM_ConstantExpressionUnevaluated:
       case EM_ConstantFold:
-      case EM_OffsetFold:
         return false;
       }
       llvm_unreachable("Missed EvalMode case");
     }
 
     /// Note that we have had a side-effect, and determine whether we should
     /// keep evaluating.
     bool noteSideEffect() {
       EvalStatus.HasSideEffects = true;
       return keepEvaluatingAfterSideEffect();
     }
 
     /// Should we continue evaluation after encountering undefined behavior?
     bool keepEvaluatingAfterUndefinedBehavior() {
       switch (EvalMode) {
       case EM_EvaluateForOverflow:
       case EM_IgnoreSideEffects:
       case EM_ConstantFold:
-      case EM_OffsetFold:
         return true;
 
       case EM_PotentialConstantExpression:
       case EM_PotentialConstantExpressionUnevaluated:
       case EM_ConstantExpression:
       case EM_ConstantExpressionUnevaluated:
         return false;
       }
bool keepEvaluatingAfterFailure() {
       case EM_PotentialConstantExpressionUnevaluated:
       case EM_EvaluateForOverflow:
         return true;
 
       case EM_ConstantExpression:
       case EM_ConstantExpressionUnevaluated:
       case EM_ConstantFold:
       case EM_IgnoreSideEffects:
-      case EM_OffsetFold:
         return false;
       }
       llvm_unreachable("Missed EvalMode case");
     }
 
     /// Notes that we failed to evaluate an expression that other expressions
     /// directly depend on, and determine if we should keep evaluating. This
     /// should only be called if we actually intend to keep evaluating.
struct FoldConstant {
     ~FoldConstant() {
       if (Enabled && HadNoPriorDiags && !Info.EvalStatus.Diag->empty() &&
           !Info.EvalStatus.HasSideEffects)
         Info.EvalStatus.Diag->clear();
       Info.EvalMode = OldMode;
     }
   };
 
-  /// RAII object used to treat the current evaluation as the correct pointer
-  /// offset fold for the current EvalMode
-  struct FoldOffsetRAII {
+  /// RAII object used to set the current evaluation mode to ignore
+  /// side-effects.
+  struct IgnoreSideEffectsRAII {
     EvalInfo &Info;
     EvalInfo::EvaluationMode OldMode;
-    explicit FoldOffsetRAII(EvalInfo &Info)
+    explicit IgnoreSideEffectsRAII(EvalInfo &Info)
         : Info(Info), OldMode(Info.EvalMode) {
       if (!Info.checkingPotentialConstantExpression())
-        Info.EvalMode = EvalInfo::EM_OffsetFold;
+        Info.EvalMode = EvalInfo::EM_IgnoreSideEffects;
     }
 
-    ~FoldOffsetRAII() { Info.EvalMode = OldMode; }
+    ~IgnoreSideEffectsRAII() { Info.EvalMode = OldMode; }
   };
 
   /// RAII object used to optionally suppress diagnostics and side-effects from
   /// a speculative evaluation.
   class SpeculativeEvaluationRAII {
     EvalInfo *Info = nullptr;
     Expr::EvalStatus OldStatus;
     bool OldIsSpeculativelyEvaluating;
static bool tryEvaluateBuiltinObjectSize(const Expr *E, unsigned Type,
                                          EvalInfo &Info, uint64_t &Size) {
   // Determine the denoted object.
   LValue LVal;
   {
     // The operand of __builtin_object_size is never evaluated for side-effects.
     // If there are any, but we can determine the pointed-to object anyway, then
     // ignore the side-effects.
     SpeculativeEvaluationRAII SpeculativeEval(Info);
-    FoldOffsetRAII Fold(Info);
+    IgnoreSideEffectsRAII Fold(Info);
 
     if (E->isGLValue()) {
       // It's possible for us to be given GLValues if we're called via
       // Expr::tryEvaluateObjectSize.
       APValue RVal;
       if (!EvaluateAsRValue(Info, E, RVal))
         return false;
       LVal.setFrom(Info.Ctx, RVal);
case Builtin::BI__builtin_object_size: {
     // Expression had no side effects, but we couldn't statically determine the
     // size of the referenced object.
     switch (Info.EvalMode) {
     case EvalInfo::EM_ConstantExpression:
     case EvalInfo::EM_PotentialConstantExpression:
     case EvalInfo::EM_ConstantFold:
     case EvalInfo::EM_EvaluateForOverflow:
     case EvalInfo::EM_IgnoreSideEffects:
-    case EvalInfo::EM_OffsetFold:
       // Leave it to IR generation.
       return Error(E);
     case EvalInfo::EM_ConstantExpressionUnevaluated:
     case EvalInfo::EM_PotentialConstantExpressionUnevaluated:
       // Reduce it to a constant now.
       return Success((Type & 2) ? 0 : -1, E);
     }