Implementing C99 partial re-initialization behavior (DR-253)

Based on previous discussion on the mailing list, clang currently lacks support
for C99 partial re-initialization behavior:
Reference: http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-April/029188.html
Reference: http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_253.htm

This patch attempts to fix this problem.

Given the following code snippet,

struct P1 { char x[6]; };
struct LP1 { struct P1 p1; };

struct LP1 l = { .p1 = { "foo" }, .p1.x[2] = 'x' };
// this example is adapted from the example for "struct fred x[]" in DR-253;
// currently clang produces in l: { "\0\0x" },
//   whereas gcc 4.8 produces { "fox" };
// with this fix, clang will also produce: { "fox" };


Differential Review: http://reviews.llvm.org/D5789

llvm-svn: 239446

1 files changed, 19 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGExprCXX.cpp b/clang/lib/CodeGen/CGExprCXX.cpp
index 13dfbb38816..b3353ba0db8 100644
--- a/clang/lib/CodeGen/CGExprCXX.cpp
+++ b/clang/lib/CodeGen/CGExprCXX.cpp
@@ -958,6 +958,25 @@ void CodeGenFunction::EmitNewArrayInitializer(
     if (ILE->getNumInits() == 0 && TryMemsetInitialization())
       return;
 
+  // If we have a struct whose every field is value-initialized, we can
+  // usually use memset.
+  if (auto *ILE = dyn_cast<InitListExpr>(Init)) {
+    if (const RecordType *RType = ILE->getType()->getAs<RecordType>()) {
+      if (RType->getDecl()->isStruct()) {
+        unsigned NumFields = 0;
+        for (auto *Field : RType->getDecl()->fields())
+          if (!Field->isUnnamedBitfield())
+            ++NumFields;
+        if (ILE->getNumInits() == NumFields)
+          for (unsigned i = 0, e = ILE->getNumInits(); i != e; ++i)
+            if (!isa<ImplicitValueInitExpr>(ILE->getInit(i)))
+              --NumFields;
+        if (ILE->getNumInits() == NumFields && TryMemsetInitialization())
+          return;
+      }
+    }
+  }
+
   // Create the loop blocks.
   llvm::BasicBlock *EntryBB = Builder.GetInsertBlock();
   llvm::BasicBlock *LoopBB = createBasicBlock("new.loop");