[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

5 files changed, 38 insertions, 9 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();
diff --git a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h
index 4b0ce0e3f03..cdf8dc72b07 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h
@@ -125,6 +125,10 @@ public:
   /// Return Label immediately following the instruction.
   MCSymbol *getLabelAfterInsn(const MachineInstr *MI);
 
+  /// Return the function-local offset of an instruction. A label for the
+  /// instruction \p MI should exist (\ref getLabelAfterInsn).
+  const MCExpr *getFunctionLocalOffsetAfterInsn(const MachineInstr *MI);
+
   /// If this type is derived from a base type then return base type size.
   static uint64_t getBaseTypeSize(const DITypeRef TyRef);
 };
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
index 81eb0c2aa9e..1d9c1d38a24 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();
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
index 97a944e9b95..13679c37fe5 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
@@ -210,10 +210,10 @@ public:
 
   /// Construct a call site entry DIE describing a call within \p Scope to a
   /// callee described by \p CalleeSP. \p IsTail specifies whether the call is
-  /// a tail call. \p ReturnPC must be non-null for non-tail calls and point
-  /// to the PC value after the call returns.
+  /// a tail call. \p PCOffset must be non-zero for non-tail calls or be the
+  /// function-local offset to PC value after the call instruction.
   DIE &constructCallSiteEntryDIE(DIE &ScopeDIE, const DISubprogram &CalleeSP,
-                                 bool IsTail, const MCSymbol *ReturnPC);
+                                 bool IsTail, const MCExpr *PCOffset);
 
   /// Construct import_module DIE.
   DIE *constructImportedEntityDIE(const DIImportedEntity *Module);
@@ -292,6 +292,9 @@ public:
   /// Add a Dwarf expression attribute data and value.
   void addExpr(DIELoc &Die, dwarf::Form Form, const MCExpr *Expr);
 
+  /// Add an attribute containing an address expression to \p Die.
+  void addAddressExpr(DIE &Die, dwarf::Attribute Attribute, const MCExpr *Expr);
+
   void applySubprogramAttributesToDefinition(const DISubprogram *SP,
                                              DIE &SPDie);
 
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 2f14f5464fd..5f91674d9f0 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -548,14 +548,15 @@ void DwarfDebug::constructCallSiteEntryDIEs(const DISubprogram &SP,
       // For tail calls, no return PC information is needed. For regular calls,
       // the return PC is needed to disambiguate paths in the call graph which
       // could lead to some target function.
-      const MCSymbol *ReturnPC = IsTail ? nullptr : getLabelAfterInsn(&MI);
+      const MCExpr *PCOffset =
+          IsTail ? nullptr : getFunctionLocalOffsetAfterInsn(&MI);
 
-      assert((IsTail || ReturnPC) && "Call without return PC information");
+      assert((IsTail || PCOffset) && "Call without return PC information");
       LLVM_DEBUG(dbgs() << "CallSiteEntry: " << MF.getName() << " -> "
                         << CalleeDecl->getName() << (IsTail ? " [tail]" : "")
                         << "\n");
       CU.constructCallSiteEntryDIE(ScopeDIE, *CalleeDecl->getSubprogram(),
-                                   IsTail, ReturnPC);
+                                   IsTail, PCOffset);
     }
   }
 }