[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, 23 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index c87d74657d7..bcd5338ac05 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -3168,6 +3168,7 @@ llvm::DISubprogram *CGDebugInfo::getFunctionFwdDeclOrStub(GlobalDecl GD,
   QualType FnType = CGM.getContext().getFunctionType(
       FD->getReturnType(), ArgTypes, FunctionProtoType::ExtProtoInfo(CC));
   if (Stub) {
+    Flags |= getCallSiteRelatedAttrs();
     return DBuilder.createFunction(
         DContext, Name, LinkageName, Unit, Line,
         getOrCreateFunctionType(GD.getDecl(), FnType, Unit),
@@ -3407,6 +3408,8 @@ void CGDebugInfo::EmitFunctionStart(GlobalDecl GD, SourceLocation Loc,
   if (CurFuncIsThunk)
     Flags |= llvm::DINode::FlagThunk;
 
+  llvm::DINode::DIFlags FlagsForDef = Flags | getCallSiteRelatedAttrs();
+
   unsigned LineNo = getLineNumber(Loc);
   unsigned ScopeLine = getLineNumber(ScopeLoc);
 
@@ -3418,7 +3421,7 @@ void CGDebugInfo::EmitFunctionStart(GlobalDecl GD, SourceLocation Loc,
   llvm::DISubprogram *SP = DBuilder.createFunction(
       FDContext, Name, LinkageName, Unit, LineNo,
       getOrCreateFunctionType(D, FnType, Unit), Fn->hasLocalLinkage(),
-      true /*definition*/, ScopeLine, Flags, CGM.getLangOpts().Optimize,
+      true /*definition*/, ScopeLine, FlagsForDef, CGM.getLangOpts().Optimize,
       TParamsArray.get(), getFunctionDeclaration(D));
   Fn->setSubprogram(SP);
   // We might get here with a VarDecl in the case we're generating
@@ -4422,3 +4425,22 @@ llvm::DebugLoc CGDebugInfo::SourceLocToDebugLoc(SourceLocation Loc) {
   llvm::MDNode *Scope = LexicalBlockStack.back();
   return llvm::DebugLoc::get(getLineNumber(Loc), getColumnNumber(Loc), Scope);
 }
+
+llvm::DINode::DIFlags CGDebugInfo::getCallSiteRelatedAttrs() const {
+  // Call site-related attributes are only useful in optimized programs, and
+  // when there's a possibility of debugging backtraces.
+  if (!CGM.getLangOpts().Optimize || DebugKind == codegenoptions::NoDebugInfo ||
+      DebugKind == codegenoptions::LocTrackingOnly)
+    return llvm::DINode::FlagZero;
+
+  // Call site-related attributes are available in DWARF v5. Some debuggers,
+  // while not fully DWARF v5-compliant, may accept these attributes as if they
+  // were part of DWARF v4.
+  bool SupportsDWARFv4Ext =
+      CGM.getCodeGenOpts().DwarfVersion == 4 &&
+      CGM.getCodeGenOpts().getDebuggerTuning() == llvm::DebuggerKind::LLDB;
+  if (!SupportsDWARFv4Ext && CGM.getCodeGenOpts().DwarfVersion < 5)
+    return llvm::DINode::FlagZero;
+
+  return llvm::DINode::FlagAllCallsDescribed;
+}