[DebugInfo] Add support for DWARF5 call site-related attributes

DWARF v5 introduces DW_AT_call_all_calls, a subprogram attribute which
indicates that all calls (both regular and tail) within the subprogram
have call site entries. The information within these call site entries
can be used by a debugger to populate backtraces with synthetic tail
call frames.

Tail calling frames go missing in backtraces because the frame of the
caller is reused by the callee. Call site entries allow a debugger to
reconstruct a sequence of (tail) calls which led from one function to
another. This improves backtrace quality. There are limitations: tail
recursion isn't handled, variables within synthetic frames may not
survive to be inspected, etc. This approach is not novel, see:

  https://gcc.gnu.org/wiki/summit2010?action=AttachFile&do=get&target=jelinek.pdf

This patch adds an IR-level flag (DIFlagAllCallsDescribed) which lowers
to DW_AT_call_all_calls. It adds the minimal amount of DWARF generation
support needed to emit standards-compliant call site entries. For easier
deployment, when the debugger tuning is LLDB, the DWARF requirement is
adjusted to v4.

Testing: Apart from check-{llvm, clang}, I built a stage2 RelWithDebInfo
clang binary. Its dSYM passed verification and grew by 1.4% compared to
the baseline. 151,879 call site entries were added.

rdar://42001377

Differential Revision: https://reviews.llvm.org/D49887

llvm-svn: 343883

1 files changed, 30 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
index d8d6f76d71e..443c8879f13 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
@@ -730,7 +730,8 @@ DIE *DwarfCompileUnit::createScopeChildrenDIE(LexicalScope *Scope,
   return ObjectPointer;
 }
 
-void DwarfCompileUnit::constructSubprogramScopeDIE(const DISubprogram *Sub, LexicalScope *Scope) {
+DIE &DwarfCompileUnit::constructSubprogramScopeDIE(const DISubprogram *Sub,
+                                                   LexicalScope *Scope) {
   DIE &ScopeDIE = updateSubprogramScopeDIE(Sub);
 
   if (Scope) {
@@ -753,6 +754,8 @@ void DwarfCompileUnit::constructSubprogramScopeDIE(const DISubprogram *Sub, Lexi
       !includeMinimalInlineScopes())
     ScopeDIE.addChild(
         DIE::get(DIEValueAllocator, dwarf::DW_TAG_unspecified_parameters));
+
+  return ScopeDIE;
 }
 
 DIE *DwarfCompileUnit::createAndAddScopeChildren(LexicalScope *Scope,
@@ -807,6 +810,32 @@ void DwarfCompileUnit::constructAbstractSubprogramScopeDIE(
     ContextCU->addDIEEntry(*AbsDef, dwarf::DW_AT_object_pointer, *ObjectPointer);
 }
 
+DIE &DwarfCompileUnit::constructCallSiteEntryDIE(DIE &ScopeDIE,
+                                                 const DISubprogram &CalleeSP,
+                                                 bool IsTail,
+                                                 const MCSymbol *ReturnPC) {
+  // Insert a call site entry DIE within ScopeDIE.
+  DIE &CallSiteDIE =
+      createAndAddDIE(dwarf::DW_TAG_call_site, ScopeDIE, nullptr);
+
+  // For the purposes of showing tail call frames in backtraces, a key piece of
+  // information is DW_AT_call_origin, a pointer to the callee DIE.
+  DIE *CalleeDIE = getOrCreateSubprogramDIE(&CalleeSP);
+  assert(CalleeDIE && "Could not create DIE for call site entry origin");
+  addDIEEntry(CallSiteDIE, dwarf::DW_AT_call_origin, *CalleeDIE);
+
+  if (IsTail) {
+    // Attach DW_AT_call_tail_call to tail calls for standards compliance.
+    addFlag(CallSiteDIE, dwarf::DW_AT_call_tail_call);
+  } 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);
+  }
+  return CallSiteDIE;
+}
+
 DIE *DwarfCompileUnit::constructImportedEntityDIE(
     const DIImportedEntity *Module) {
   DIE *IMDie = DIE::get(DIEValueAllocator, (dwarf::Tag)Module->getTag());