Emit 'no line' information for interesting 'orphan' instructions.

DWARF specifies that "line 0" really means "no appropriate source
location" in the line table.  Use this for branch targets and some
other cases that have no specified source location, to prevent
inheriting unfortunate line numbers from physically preceding
instructions (which might be from completely unrelated source).

Differential Revision: http://reviews.llvm.org/D24180

llvm-svn: 288212

3 files changed, 54 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
index d30f106a939..5607242a7f1 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
@@ -201,8 +201,10 @@ void DebugHandlerBase::endInstruction() {
   assert(CurMI != nullptr);
   // Don't create a new label after DBG_VALUE instructions.
   // They don't generate code.
-  if (!CurMI->isDebugValue())
+  if (!CurMI->isDebugValue()) {
     PrevLabel = nullptr;
+    PrevInstBB = CurMI->getParent();
+  }
 
   DenseMap<const MachineInstr *, MCSymbol *>::iterator I =
       LabelsAfterInsn.find(CurMI);
diff --git a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h
index b8bbcec133f..cd02c4446a7 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.h
@@ -38,10 +38,12 @@ protected:
   MachineModuleInfo *MMI;
 
   /// Previous instruction's location information. This is used to
-  /// determine label location to indicate scope boundries in dwarf
-  /// debug info.
+  /// determine label location to indicate scope boundaries in debug info.
+  /// We track the previous instruction's source location (if not line 0),
+  /// whether it was a label, and its parent BB.
   DebugLoc PrevInstLoc;
   MCSymbol *PrevLabel = nullptr;
+  const MachineBasicBlock *PrevInstBB = nullptr;
 
   /// This location indicates end of function prologue and beginning of
   /// function body.
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 884e03d3f0a..949516b17a0 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1010,31 +1010,70 @@ void DwarfDebug::beginInstruction(const MachineInstr *MI) {
   if (MI->isDebugValue())
     return;
   const DebugLoc &DL = MI->getDebugLoc();
-  if (DL == PrevInstLoc)
+  // When we emit a line-0 record, we don't update PrevInstLoc; so look at
+  // the last line number actually emitted, to see if it was line 0.
+  unsigned LastAsmLine =
+    Asm->OutStreamer->getContext().getCurrentDwarfLoc().getLine();
+
+  if (DL == PrevInstLoc) {
+    // If we have an ongoing unspecified location, nothing to do here.
+    if (!DL)
+      return;
+    // We have an explicit location, same as the previous location.
+    // But we might be coming back to it after a line 0 record.
+    if (LastAsmLine == 0 && DL.getLine() != 0) {
+      // Reinstate the source location but not marked as a statement.
+      const MDNode *Scope = DL.getScope();
+      recordSourceLine(DL.getLine(), DL.getCol(), Scope, /*Flags=*/0);
+    }
     return;
+  }
 
   if (!DL) {
     // We have an unspecified location, which might want to be line 0.
-    if (UnknownLocations) {
-      PrevInstLoc = DL;
-      recordSourceLine(0, 0, nullptr, 0);
+    // If we have already emitted a line-0 record, don't repeat it.
+    if (LastAsmLine == 0)
+      return;
+    // See if we have a reason to emit a line-0 record now.
+    // Reasons to emit a line-0 record include:
+    // - User asked for it (UnknownLocations).
+    // - Instruction has a label, so it's referenced from somewhere else,
+    //   possibly debug information; we want it to have a source location.
+    // - Instruction is at the top of a block; we don't want to inherit the
+    //   location from the physically previous (maybe unrelated) block.
+    if (UnknownLocations || PrevLabel ||
+        (PrevInstBB && PrevInstBB != MI->getParent())) {
+      // Preserve the file number, if we can, to save space in the line table.
+      // Do not update PrevInstLoc, it remembers the last non-0 line.
+      // FIXME: Also preserve the column number, to save more space?
+      const MDNode *Scope = PrevInstLoc ? PrevInstLoc.getScope() : nullptr;
+      recordSourceLine(0, 0, Scope, 0);
     }
     return;
   }
 
-  // We have a new, explicit location.
+  // We have an explicit location, different from the previous location.
+  // Don't repeat a line-0 record, but otherwise emit the new location.
+  // (The new location might be an explicit line 0, which we do emit.)
+  if (DL.getLine() == 0 && LastAsmLine == 0)
+    return;
   unsigned Flags = 0;
-  PrevInstLoc = DL;
   if (DL == PrologEndLoc) {
     Flags |= DWARF2_FLAG_PROLOGUE_END | DWARF2_FLAG_IS_STMT;
     PrologEndLoc = DebugLoc();
   }
-  if (DL.getLine() !=
-      Asm->OutStreamer->getContext().getCurrentDwarfLoc().getLine())
+  // If the line changed, we call that a new statement; unless we went to
+  // line 0 and came back, in which case it is not a new statement.
+  unsigned OldLine = PrevInstLoc ? PrevInstLoc.getLine() : LastAsmLine;
+  if (DL.getLine() && DL.getLine() != OldLine)
     Flags |= DWARF2_FLAG_IS_STMT;
 
   const MDNode *Scope = DL.getScope();
   recordSourceLine(DL.getLine(), DL.getCol(), Scope, Flags);
+
+  // If we're not at line 0, remember this location.
+  if (DL.getLine())
+    PrevInstLoc = DL;
 }
 
 static DebugLoc findPrologueEndLoc(const MachineFunction *MF) {