Debug info: Infrastructure to support debug locations for fragmented

variables (for example, by-value struct arguments passed in registers, or
large integer values split across several smaller registers).
On the IR level, this adds a new type of complex address operation OpPiece
to DIVariable that describes size and offset of a variable fragment.
On the DWARF emitter level, all pieces describing the same variable are
collected, sorted and emitted as DWARF expressions using the DW_OP_piece
and DW_OP_bit_piece operators.

http://reviews.llvm.org/D3373
rdar://problem/15928306

What this patch doesn't do / Future work:
- This patch only adds the backend machinery to make this work, patches
  that change SROA and SelectionDAG's type legalizer to actually create
  such debug info will follow. (http://reviews.llvm.org/D2680)
- Making the DIVariable complex expressions into an argument of dbg.value
  will reduce the memory footprint of the debug metadata.
- The sorting/uniquing of pieces should be moved into DebugLocEntry,
  to facilitate the merging of multi-piece entries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214576 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/DIBuilder.cpp

@@ -1100,6 +1100,28 @@ DIVariable DIBuilder::createComplexVariable(unsigned Tag, DIDescriptor Scope,
   return DIVariable(MDNode::get(VMContext, Elts));
 }
 
+/// createVariablePiece - Create a descriptor to describe one part
+/// of aggregate variable that is fragmented across multiple Values.
+DIVariable DIBuilder::createVariablePiece(DIVariable Variable,
+                                          unsigned OffsetInBytes,
+                                          unsigned SizeInBytes) {
+  assert(SizeInBytes > 0 && "zero-size piece");
+  Value *Addr[] = {
+    ConstantInt::get(Type::getInt32Ty(VMContext), OpPiece),
+    ConstantInt::get(Type::getInt32Ty(VMContext), OffsetInBytes),
+    ConstantInt::get(Type::getInt32Ty(VMContext), SizeInBytes)
+  };
+
+  assert((Variable->getNumOperands() == 8 || Variable.isVariablePiece()) &&
+         "variable already has a complex address");
+  SmallVector<Value *, 9> Elts;
+  for (unsigned i = 0; i < 8; ++i)
+    Elts.push_back(Variable->getOperand(i));
+
+  Elts.push_back(MDNode::get(VMContext, Addr));
+  return DIVariable(MDNode::get(VMContext, Elts));
+}
+
 /// createFunction - Create a new descriptor for the specified function.
 /// FIXME: this is added for dragonegg. Once we update dragonegg
 /// to call resolve function, this will be removed.