[DebugInfo] Emit DWARF reference for DIVariable 'count' in DISubrange

Summary:
This patch implements the codegen of DWARF debug info for non-constant
'count' fields for DISubrange.

This is patch [2/3] in a series to extend LLVM's DISubrange Metadata
node to support debugging of C99 variable length arrays and vectors with
runtime length like the Scalable Vector Extension for AArch64. It is
also a first step towards representing more complex cases like arrays
in Fortran.

Reviewers: echristo, pcc, aprantl, dexonsmith, clayborg, kristof.beyls, dblaikie

Reviewed By: aprantl

Subscribers: fhahn, aemerson, rengolin, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D41696

llvm-svn: 323323

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 907ede0a633..71f452b6d8d 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1335,7 +1335,13 @@ void DwarfUnit::constructSubrangeDIE(DIE &Buffer, const DISubrange *SR,
   if (DefaultLowerBound == -1 || LowerBound != DefaultLowerBound)
     addUInt(DW_Subrange, dwarf::DW_AT_lower_bound, None, LowerBound);
 
-  if (Count != -1)
+  if (auto *CV = SR->getCount().dyn_cast<DIVariable*>()) {
+    // 'finishVariableDefinition' that creates the types for a variable is
+    // always called _after_ the DIEs for variables are created.
+    auto *CountVarDIE = getDIE(CV);
+    assert(CountVarDIE && "DIE for count is not yet instantiated");
+    addDIEEntry(DW_Subrange, dwarf::DW_AT_count, *CountVarDIE);
+  } else if (Count != -1)
     // FIXME: An unbounded array should reference the expression that defines
     // the array.
     addUInt(DW_Subrange, dwarf::DW_AT_count, None, Count);