Let replaceVTableHolder accept any type.

In Rust, a trait can be implemented for any type, and if a trait
object pointer is used for the type, then a virtual table will be
emitted for that trait/type combination.

We would like debuggers to be able to inspect trait objects, which
requires finding the concrete type associated with a given vtable.

This patch changes LLVM so that any type can be passed to
replaceVTableHolder. This allows the Rust compiler to emit the needed
debug info -- associating a vtable with the concrete type for which it
was emitted.

This is a DWARF extension: DWARF only specifies the meaning of
DW_AT_containing_type in one specific situation. This style of DWARF
extension is routine, though, and LLVM already has one such case for
DW_AT_containing_type.

Patch by Tom Tromey!

Differential Revision: https://reviews.llvm.org/D39503

llvm-svn: 317730

2 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 784a1d512c1..ce7465910fc 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -960,8 +960,9 @@ void DwarfUnit::constructTypeDIE(DIE &Buffer, const DICompositeType *CTy) {
 
     // This is outside the DWARF spec, but GDB expects a DW_AT_containing_type
     // inside C++ composite types to point to the base class with the vtable.
-    if (auto *ContainingType =
-            dyn_cast_or_null<DICompositeType>(resolve(CTy->getVTableHolder())))
+    // Rust uses DW_AT_containing_type to link a vtable to the type
+    // for which it was created.
+    if (auto *ContainingType = resolve(CTy->getVTableHolder()))
       addDIEEntry(Buffer, dwarf::DW_AT_containing_type,
                   *getOrCreateTypeDIE(ContainingType));
 
diff --git a/llvm/lib/IR/DIBuilder.cpp b/llvm/lib/IR/DIBuilder.cpp
index 18979a8d5cf..837b1ec5857 100644
--- a/llvm/lib/IR/DIBuilder.cpp
+++ b/llvm/lib/IR/DIBuilder.cpp
@@ -874,7 +874,7 @@ Instruction *DIBuilder::insertDbgValueIntrinsic(
 }
 
 void DIBuilder::replaceVTableHolder(DICompositeType *&T,
-                                    DICompositeType *VTableHolder) {
+                                    DIType *VTableHolder) {
   {
     TypedTrackingMDRef<DICompositeType> N(T);
     N->replaceVTableHolder(VTableHolder);