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

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/unittests/IR/MetadataTest.cpp b/llvm/unittests/IR/MetadataTest.cpp
index 3ab0ad4..76c1903 100644
--- a/llvm/unittests/IR/MetadataTest.cpp
+++ b/llvm/unittests/IR/MetadataTest.cpp
@@ -1314,6 +1314,11 @@ TEST_F(DICompositeTypeTest, replaceOperands) {
   EXPECT_EQ(nullptr, N->getVTableHolder());
   N->replaceVTableHolder(VTableHolder);
   EXPECT_EQ(VTableHolder, N->getVTableHolder());
+  // As an extension, the containing type can be anything.  This is
+  // used by Rust to associate vtables with their concrete type.
+  DIType *BasicType = getBasicType("basic");
+  N->replaceVTableHolder(BasicType);
+  EXPECT_EQ(BasicType, N->getVTableHolder());
   N->replaceVTableHolder(nullptr);
   EXPECT_EQ(nullptr, N->getVTableHolder());