vendor/clang/clang-trunk-r303197

1 files changed, 51 insertions, 3 deletions

diff --git a/lib/Sema/SemaStmt.cpp b/lib/Sema/SemaStmt.cpp
index 5d7eada28717..33a8f9c4afa3 100644
--- a/lib/Sema/SemaStmt.cpp
+++ b/lib/Sema/SemaStmt.cpp
@@ -2268,9 +2268,57 @@ Sema::BuildCXXForRangeStmt(SourceLocation ForLoc, SourceLocation CoawaitLoc,
         BoundExpr = IntegerLiteral::Create(
             Context, CAT->getSize(), Context.getPointerDiffType(), RangeLoc);
       else if (const VariableArrayType *VAT =
-               dyn_cast<VariableArrayType>(UnqAT))
-        BoundExpr = VAT->getSizeExpr();
-      else {
+               dyn_cast<VariableArrayType>(UnqAT)) {
+        // For a variably modified type we can't just use the expression within
+        // the array bounds, since we don't want that to be re-evaluated here.
+        // Rather, we need to determine what it was when the array was first
+        // created - so we resort to using sizeof(vla)/sizeof(element).
+        // For e.g.
+        //  void f(int b) { 
+        //    int vla[b];
+        //    b = -1;   <-- This should not affect the num of iterations below
+        //    for (int &c : vla) { .. }
+        //  }
+
+        // FIXME: This results in codegen generating IR that recalculates the
+        // run-time number of elements (as opposed to just using the IR Value
+        // that corresponds to the run-time value of each bound that was
+        // generated when the array was created.) If this proves too embarassing
+        // even for unoptimized IR, consider passing a magic-value/cookie to
+        // codegen that then knows to simply use that initial llvm::Value (that
+        // corresponds to the bound at time of array creation) within
+        // getelementptr.  But be prepared to pay the price of increasing a
+        // customized form of coupling between the two components - which  could
+        // be hard to maintain as the codebase evolves.
+
+        ExprResult SizeOfVLAExprR = ActOnUnaryExprOrTypeTraitExpr(
+            EndVar->getLocation(), UETT_SizeOf,
+            /*isType=*/true,
+            CreateParsedType(VAT->desugar(), Context.getTrivialTypeSourceInfo(
+                                                 VAT->desugar(), RangeLoc))
+                .getAsOpaquePtr(),
+            EndVar->getSourceRange());
+        if (SizeOfVLAExprR.isInvalid())
+          return StmtError();
+        
+        ExprResult SizeOfEachElementExprR = ActOnUnaryExprOrTypeTraitExpr(
+            EndVar->getLocation(), UETT_SizeOf,
+            /*isType=*/true,
+            CreateParsedType(VAT->desugar(),
+                             Context.getTrivialTypeSourceInfo(
+                                 VAT->getElementType(), RangeLoc))
+                .getAsOpaquePtr(),
+            EndVar->getSourceRange());
+        if (SizeOfEachElementExprR.isInvalid())
+          return StmtError();
+
+        BoundExpr =
+            ActOnBinOp(S, EndVar->getLocation(), tok::slash,
+                       SizeOfVLAExprR.get(), SizeOfEachElementExprR.get());
+        if (BoundExpr.isInvalid())
+          return StmtError();
+        
+      } else {
         // Can't be a DependentSizedArrayType or an IncompleteArrayType since
         // UnqAT is not incomplete and Range is not type-dependent.
         llvm_unreachable("Unexpected array type in for-range");