constexpr evaluation for __builtin_assume_aligned

Richard noted in the review of r217349 that extra handling of
__builtin_assume_aligned inside of the expression evaluator was needed. He was
right, and this should address the concerns raised, namely:

 1. The offset argument to __builtin_assume_aligned can have side effects, and
    we need to make sure that all arguments are properly evaluated.

 2. If the alignment assumption does not hold, that introduces undefined
    behavior, and undefined behavior cannot appear inside a constexpr.

and hopefully the diagnostics produced are detailed enough to explain what is
going on.

llvm-svn: 218992

1 files changed, 98 insertions, 37 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index c1fc7eee315..74bc31db34f 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -4885,6 +4885,38 @@ bool PointerExprEvaluator::VisitCastExpr(const CastExpr* E) {
   return ExprEvaluatorBaseTy::VisitCastExpr(E);
 }
 
+static CharUnits GetAlignOfType(EvalInfo &Info, QualType T) {
+  // C++ [expr.alignof]p3:
+  //     When alignof is applied to a reference type, the result is the
+  //     alignment of the referenced type.
+  if (const ReferenceType *Ref = T->getAs<ReferenceType>())
+    T = Ref->getPointeeType();
+
+  // __alignof is defined to return the preferred alignment.
+  return Info.Ctx.toCharUnitsFromBits(
+    Info.Ctx.getPreferredTypeAlign(T.getTypePtr()));
+}
+
+static CharUnits GetAlignOfExpr(EvalInfo &Info, const Expr *E) {
+  E = E->IgnoreParens();
+
+  // The kinds of expressions that we have special-case logic here for
+  // should be kept up to date with the special checks for those
+  // expressions in Sema.
+
+  // alignof decl is always accepted, even if it doesn't make sense: we default
+  // to 1 in those cases.
+  if (const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E))
+    return Info.Ctx.getDeclAlign(DRE->getDecl(),
+                                 /*RefAsPointee*/true);
+
+  if (const MemberExpr *ME = dyn_cast<MemberExpr>(E))
+    return Info.Ctx.getDeclAlign(ME->getMemberDecl(),
+                                 /*RefAsPointee*/true);
+
+  return GetAlignOfType(Info, E->getType());
+}
+
 bool PointerExprEvaluator::VisitCallExpr(const CallExpr *E) {
   if (IsStringLiteralCall(E))
     return Success(E);
@@ -4892,7 +4924,71 @@ bool PointerExprEvaluator::VisitCallExpr(const CallExpr *E) {
   switch (E->getBuiltinCallee()) {
   case Builtin::BI__builtin_addressof:
     return EvaluateLValue(E->getArg(0), Result, Info);
+  case Builtin::BI__builtin_assume_aligned: {
+    // We need to be very careful here because: if the pointer does not have the
+    // asserted alignment, then the behavior is undefined, and undefined
+    // behavior is non-constant.
+    if (!EvaluatePointer(E->getArg(0), Result, Info))
+      return false;
+
+    LValue OffsetResult(Result);
+    APSInt Alignment;
+    if (!EvaluateInteger(E->getArg(1), Alignment, Info))
+      return false;
+    CharUnits Align = CharUnits::fromQuantity(getExtValue(Alignment));
+
+    if (E->getNumArgs() > 2) {
+      APSInt Offset;
+      if (!EvaluateInteger(E->getArg(2), Offset, Info))
+        return false;
+
+      int64_t AdditionalOffset = -getExtValue(Offset);
+      OffsetResult.Offset += CharUnits::fromQuantity(AdditionalOffset);
+    }
+
+    // If there is a base object, then it must have the correct alignment.
+    if (OffsetResult.Base) {
+      CharUnits BaseAlignment;
+      if (const ValueDecl *VD =
+          OffsetResult.Base.dyn_cast<const ValueDecl*>()) {
+        BaseAlignment = Info.Ctx.getDeclAlign(VD);
+      } else {
+        BaseAlignment =
+          GetAlignOfExpr(Info, OffsetResult.Base.get<const Expr*>());
+      }
+
+      if (BaseAlignment < Align) {
+        Result.Designator.setInvalid();
+	// FIXME: Quantities here cast to integers because the plural modifier
+	// does not work on APSInts yet.
+        CCEDiag(E->getArg(0),
+                diag::note_constexpr_baa_insufficient_alignment) << 0
+          << (int) BaseAlignment.getQuantity()
+          << (unsigned) getExtValue(Alignment);
+        return false;
+      }
+    }
+
+    // The offset must also have the correct alignment.
+    if (OffsetResult.Offset.RoundUpToAlignment(Align) != OffsetResult.Offset) {
+      Result.Designator.setInvalid();
+      APSInt Offset(64, false);
+      Offset = OffsetResult.Offset.getQuantity();
+
+      if (OffsetResult.Base)
+        CCEDiag(E->getArg(0),
+                diag::note_constexpr_baa_insufficient_alignment) << 1
+          << (int) getExtValue(Offset) << (unsigned) getExtValue(Alignment);
+      else
+        CCEDiag(E->getArg(0),
+                diag::note_constexpr_baa_value_insufficient_alignment)
+          << Offset << (unsigned) getExtValue(Alignment);
 
+      return false;
+    }
+
+    return true;
+  }
   default:
     return ExprEvaluatorBaseTy::VisitCallExpr(E);
   }
@@ -5853,8 +5949,6 @@ public:
   bool VisitSizeOfPackExpr(const SizeOfPackExpr *E);
 
 private:
-  CharUnits GetAlignOfExpr(const Expr *E);
-  CharUnits GetAlignOfType(QualType T);
   static QualType GetObjectType(APValue::LValueBase B);
   bool TryEvaluateBuiltinObjectSize(const CallExpr *E);
   // FIXME: Missing: array subscript of vector, member of vector
@@ -7019,39 +7113,6 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
   return ExprEvaluatorBaseTy::VisitBinaryOperator(E);
 }
 
-CharUnits IntExprEvaluator::GetAlignOfType(QualType T) {
-  // C++ [expr.alignof]p3:
-  //     When alignof is applied to a reference type, the result is the
-  //     alignment of the referenced type.
-  if (const ReferenceType *Ref = T->getAs<ReferenceType>())
-    T = Ref->getPointeeType();
-
-  // __alignof is defined to return the preferred alignment.
-  return Info.Ctx.toCharUnitsFromBits(
-    Info.Ctx.getPreferredTypeAlign(T.getTypePtr()));
-}
-
-CharUnits IntExprEvaluator::GetAlignOfExpr(const Expr *E) {
-  E = E->IgnoreParens();
-
-  // The kinds of expressions that we have special-case logic here for
-  // should be kept up to date with the special checks for those
-  // expressions in Sema.
-
-  // alignof decl is always accepted, even if it doesn't make sense: we default
-  // to 1 in those cases.
-  if (const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E))
-    return Info.Ctx.getDeclAlign(DRE->getDecl(),
-                                 /*RefAsPointee*/true);
-
-  if (const MemberExpr *ME = dyn_cast<MemberExpr>(E))
-    return Info.Ctx.getDeclAlign(ME->getMemberDecl(),
-                                 /*RefAsPointee*/true);
-
-  return GetAlignOfType(E->getType());
-}
-
-
 /// VisitUnaryExprOrTypeTraitExpr - Evaluate a sizeof, alignof or vec_step with
 /// a result as the expression's type.
 bool IntExprEvaluator::VisitUnaryExprOrTypeTraitExpr(
@@ -7059,9 +7120,9 @@ bool IntExprEvaluator::VisitUnaryExprOrTypeTraitExpr(
   switch(E->getKind()) {
   case UETT_AlignOf: {
     if (E->isArgumentType())
-      return Success(GetAlignOfType(E->getArgumentType()), E);
+      return Success(GetAlignOfType(Info, E->getArgumentType()), E);
     else
-      return Success(GetAlignOfExpr(E->getArgumentExpr()), E);
+      return Success(GetAlignOfExpr(Info, E->getArgumentExpr()), E);
   }
 
   case UETT_VecStep: {