[DWARF] Use a function-local offset for AT_call_return_pc

Logs provided by @stella.stamenova indicate that on Linux, lldb adds a
spurious slide offset to the return PC it loads from AT_call_return_pc
attributes (see the list thread: "[PATCH] D50478: Add support for
artificial tail call frames").

This patch side-steps the issue by getting rid of the load address
calculation in lldb's CallEdge::GetReturnPCAddress.

The idea is to have the DWARF writer emit function-local offsets to the
instruction after a call. I.e. return-pc = label-after-call-insn -
function-entry. LLDB can simply add this offset to the base address of a
function to get the return PC.

Differential Revision: https://reviews.llvm.org/D53469

llvm-svn: 344960

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
index 580f682b9a6..a362dd40e3b 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
@@ -125,6 +125,21 @@ MCSymbol *DebugHandlerBase::getLabelAfterInsn(const MachineInstr *MI) {
   return LabelsAfterInsn.lookup(MI);
 }
 
+// Return the function-local offset of an instruction.
+const MCExpr *
+DebugHandlerBase::getFunctionLocalOffsetAfterInsn(const MachineInstr *MI) {
+  MCContext &MC = Asm->OutContext;
+
+  MCSymbol *Start = Asm->getFunctionBegin();
+  const auto *StartRef = MCSymbolRefExpr::create(Start, MC);
+
+  MCSymbol *AfterInsn = getLabelAfterInsn(MI);
+  assert(AfterInsn && "Expected label after instruction");
+  const auto *AfterRef = MCSymbolRefExpr::create(AfterInsn, MC);
+
+  return MCBinaryExpr::createSub(AfterRef, StartRef, MC);
+}
+
 /// If this type is derived from a base type then return base type size.
 uint64_t DebugHandlerBase::getBaseTypeSize(const DITypeRef TyRef) {
   DIType *Ty = TyRef.resolve();