[DebugInfo] Add DW_OP_LLVM_user extension point

The extension codespace for DWARF expressions (DW_OP_LLVM_{lo,hi}_user)
has shrunk over time, as no extension is ever "retired" in practice. To
facilitate future extensions, this patch reserves one open opcode as an extension
point (0xfe), which is followed by a ULEB128-encoded SubOperation, and
then by the subop's operands.

There is some prior-art, namely DW_OP_AARCH64_operation
(see https://github.com/ARM-software/abi-aa/blob/edd7460d87493fff124b8b5713acf71ffc06ee91/aadwarf64/aadwarf64.rst#45dwarf-expression-operations).

This version makes some different tradeoffs, opting to use a ULEB128 for
the subop encoding for future-proofing.

Reviewed By: #debug-info, dblaikie

Differential Revision: https://reviews.llvm.org/D147271

1 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/BinaryFormat/Dwarf.cpp b/llvm/lib/BinaryFormat/Dwarf.cpp
index 12e9c73..e4e5b5d 100644
--- a/llvm/lib/BinaryFormat/Dwarf.cpp
+++ b/llvm/lib/BinaryFormat/Dwarf.cpp
@@ -172,6 +172,40 @@ unsigned llvm::dwarf::getOperationEncoding(StringRef OperationEncodingString) {
       .Default(0);
 }
 
+static StringRef LlvmUserOperationEncodingString(unsigned Encoding) {
+  switch (Encoding) {
+  default:
+    llvm_unreachable("unhandled DWARF operation with LLVM user op");
+#define HANDLE_DW_OP_LLVM_USEROP(ID, NAME)                                     \
+  case DW_OP_LLVM_##NAME:                                                      \
+    return "DW_OP_LLVM_" #NAME;
+#include "llvm/BinaryFormat/Dwarf.def"
+  }
+}
+
+static unsigned
+getLlvmUserOperationEncoding(StringRef LlvmUserOperationEncodingString) {
+  unsigned E = StringSwitch<unsigned>(LlvmUserOperationEncodingString)
+#define HANDLE_DW_OP_LLVM_USEROP(ID, NAME) .Case(#NAME, DW_OP_LLVM_##NAME)
+#include "llvm/BinaryFormat/Dwarf.def"
+                   .Default(0);
+  assert(E && "unhandled DWARF operation string with LLVM user op");
+  return E;
+}
+
+StringRef llvm::dwarf::SubOperationEncodingString(unsigned OpEncoding,
+                                                  unsigned SubOpEncoding) {
+  assert(OpEncoding == DW_OP_LLVM_user);
+  return LlvmUserOperationEncodingString(SubOpEncoding);
+}
+
+unsigned
+llvm::dwarf::getSubOperationEncoding(unsigned OpEncoding,
+                                     StringRef SubOperationEncodingString) {
+  assert(OpEncoding == DW_OP_LLVM_user);
+  return getLlvmUserOperationEncoding(SubOperationEncodingString);
+}
+
 unsigned llvm::dwarf::OperationVersion(dwarf::LocationAtom Op) {
   switch (Op) {
   default: