[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, 9 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
index 81eb0c2..1d9c1d3 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
@@ -821,7 +821,7 @@ void DwarfCompileUnit::constructAbstractSubprogramScopeDIE(
 DIE &DwarfCompileUnit::constructCallSiteEntryDIE(DIE &ScopeDIE,
                                                  const DISubprogram &CalleeSP,
                                                  bool IsTail,
-                                                 const MCSymbol *ReturnPC) {
+                                                 const MCExpr *PCOffset) {
   // Insert a call site entry DIE within ScopeDIE.
   DIE &CallSiteDIE =
       createAndAddDIE(dwarf::DW_TAG_call_site, ScopeDIE, nullptr);
@@ -838,8 +838,8 @@ DIE &DwarfCompileUnit::constructCallSiteEntryDIE(DIE &ScopeDIE,
   } else {
     // Attach the return PC to allow the debugger to disambiguate call paths
     // from one function to another.
-    assert(ReturnPC && "Missing return PC information for a call");
-    addLabelAddress(CallSiteDIE, dwarf::DW_AT_call_return_pc, ReturnPC);
+    assert(PCOffset && "Missing return PC information for a call");
+    addAddressExpr(CallSiteDIE, dwarf::DW_AT_call_return_pc, PCOffset);
   }
   return CallSiteDIE;
 }
@@ -1103,6 +1103,12 @@ void DwarfCompileUnit::addExpr(DIELoc &Die, dwarf::Form Form,
   Die.addValue(DIEValueAllocator, (dwarf::Attribute)0, Form, DIEExpr(Expr));
 }
 
+void DwarfCompileUnit::addAddressExpr(DIE &Die, dwarf::Attribute Attribute,
+                                      const MCExpr *Expr) {
+  Die.addValue(DIEValueAllocator, Attribute, dwarf::DW_FORM_addr,
+               DIEExpr(Expr));
+}
+
 void DwarfCompileUnit::applySubprogramAttributesToDefinition(
     const DISubprogram *SP, DIE &SPDie) {
   auto *SPDecl = SP->getDeclaration();