AsmPrinter: Don't emit empty .debug_loc entries

If we don't know how to represent a .debug_loc entry, skip the entry
entirely rather than emitting an empty one.  Similarly, if a .debug_loc
list has no entries, don't create the list.

We still want to create the variables, just in an optimized-out form
that doesn't have a DW_AT_location.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240244 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/CMakeLists.txt

@@ -5,6 +5,7 @@ add_llvm_library(LLVMAsmPrinter
   AsmPrinterDwarf.cpp
   AsmPrinterInlineAsm.cpp
   DbgValueHistoryCalculator.cpp
+  DebugLocStream.cpp
   DIE.cpp
   DIEHash.cpp
   DwarfAccelTable.cpp

lib/CodeGen/AsmPrinter/DebugLocEntry.h

@@ -142,7 +142,7 @@ public:
   }
 
   /// \brief Lower this entry into a DWARF expression.
-  void finalize(const AsmPrinter &AP, DebugLocStream &Locs,
+  void finalize(const AsmPrinter &AP, DebugLocStream::ListBuilder &List,
                 const DIBasicType *BT);
 };
 

lib/CodeGen/AsmPrinter/DebugLocStream.cpp

@@ -0,0 +1,46 @@
+//===- DebugLocStream.cpp - DWARF debug_loc stream --------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "DebugLocStream.h"
+#include "DwarfDebug.h"
+#include "llvm/CodeGen/AsmPrinter.h"
+
+using namespace llvm;
+
+bool DebugLocStream::finalizeList(AsmPrinter &Asm) {
+  if (Lists.back().EntryOffset == Entries.size()) {
+    // Empty list.  Delete it.
+    Lists.pop_back();
+    return false;
+  }
+
+  // Real list.  Generate a label for it.
+  Lists.back().Label = Asm.createTempSymbol("debug_loc");
+  return true;
+}
+
+void DebugLocStream::finalizeEntry() {
+  if (Entries.back().ByteOffset != DWARFBytes.size())
+    return;
+
+  // The last entry was empty.  Delete it.
+  Comments.erase(Comments.begin() + Entries.back().CommentOffset,
+                 Comments.end());
+  Entries.pop_back();
+
+  assert(Lists.back().EntryOffset <= Entries.size() &&
+         "Popped off more entries than are in the list");
+}
+
+DebugLocStream::ListBuilder::~ListBuilder() {
+  if (!Locs.finalizeList(Asm))
+    return;
+  V.initializeDbgValue(&MI);
+  V.setDebugLocListIndex(ListIndex);
+}

lib/CodeGen/AsmPrinter/DebugLocStream.h

@@ -15,7 +15,11 @@
 #include "ByteStreamer.h"
 
 namespace llvm {
+
+class AsmPrinter;
+class DbgVariable;
 class DwarfCompileUnit;
+class MachineInstr;
 class MCSymbol;
 
 /// \brief Byte stream of .debug_loc entries.
@@ -29,10 +33,10 @@ class DebugLocStream {
 public:
   struct List {
     DwarfCompileUnit *CU;
-    MCSymbol *Label;
+    MCSymbol *Label = nullptr;
     size_t EntryOffset;
-    List(DwarfCompileUnit *CU, MCSymbol *Label, size_t EntryOffset)
-        : CU(CU), Label(Label), EntryOffset(EntryOffset) {}
+    List(DwarfCompileUnit *CU, size_t EntryOffset)
+        : CU(CU), EntryOffset(EntryOffset) {}
   };
   struct Entry {
     const MCSymbol *BeginSym;
@@ -61,18 +65,30 @@ public:
   const List &getList(size_t LI) const { return Lists[LI]; }
   ArrayRef<List> getLists() const { return Lists; }
 
+  class ListBuilder;
+  class EntryBuilder;
+
+private:
   /// \brief Start a new .debug_loc entry list.
   ///
   /// Start a new .debug_loc entry list.  Return the new list's index so it can
   /// be retrieved later via \a getList().
   ///
   /// Until the next call, \a startEntry() will add entries to this list.
-  size_t startList(DwarfCompileUnit *CU, MCSymbol *Label) {
+  size_t startList(DwarfCompileUnit *CU) {
     size_t LI = Lists.size();
-    Lists.emplace_back(CU, Label, Entries.size());
+    Lists.emplace_back(CU, Entries.size());
     return LI;
   }
 
+  /// Finalize a .debug_loc entry list.
+  ///
+  /// If there are no entries in this list, delete it outright.  Otherwise,
+  /// create a label with \a Asm.
+  ///
+  /// \return false iff the list is deleted.
+  bool finalizeList(AsmPrinter &Asm);
+
   /// \brief Start a new .debug_loc entry.
   ///
   /// Until the next call, bytes added to the stream will be added to this
@@ -81,6 +97,10 @@ public:
     Entries.emplace_back(BeginSym, EndSym, DWARFBytes.size(), Comments.size());
   }
 
+  /// Finalize a .debug_loc entry, deleting if it's empty.
+  void finalizeEntry();
+
+public:
   BufferByteStreamer getStreamer() {
     return BufferByteStreamer(DWARFBytes, Comments, GenerateComments);
   }
@@ -129,5 +149,45 @@ private:
     return Entries[EI + 1].CommentOffset - Entries[EI].CommentOffset;
   }
 };
+
+/// Builder for DebugLocStream lists.
+class DebugLocStream::ListBuilder {
+  DebugLocStream &Locs;
+  AsmPrinter &Asm;
+  DbgVariable &V;
+  const MachineInstr &MI;
+  size_t ListIndex;
+
+public:
+  ListBuilder(DebugLocStream &Locs, DwarfCompileUnit &CU, AsmPrinter &Asm,
+              DbgVariable &V, const MachineInstr &MI)
+      : Locs(Locs), Asm(Asm), V(V), MI(MI), ListIndex(Locs.startList(&CU)) {}
+
+  /// Finalize the list.
+  ///
+  /// If the list is empty, delete it.  Otherwise, finalize it by creating a
+  /// temp symbol in \a Asm and setting up the \a DbgVariable.
+  ~ListBuilder();
+
+  DebugLocStream &getLocs() { return Locs; }
+};
+
+/// Builder for DebugLocStream entries.
+class DebugLocStream::EntryBuilder {
+  DebugLocStream &Locs;
+
+public:
+  EntryBuilder(ListBuilder &List, const MCSymbol *Begin, const MCSymbol *End)
+      : Locs(List.getLocs()) {
+    Locs.startEntry(Begin, End);
+  }
+
+  /// Finalize the entry, deleting it if it's empty.
+  ~EntryBuilder() { Locs.finalizeEntry(); }
+
+  BufferByteStreamer getStreamer() { return Locs.getStreamer(); }
+};
+
 } // namespace llvm
+
 #endif

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -908,15 +908,15 @@ void DwarfDebug::collectVariableInfo(DwarfCompileUnit &TheCU,
 
     const MachineInstr *MInsn = Ranges.front().first;
     assert(MInsn->isDebugValue() && "History must begin with debug value");
-    RegVar->initializeDbgValue(MInsn);
 
     // Check if the first DBG_VALUE is valid for the rest of the function.
-    if (Ranges.size() == 1 && Ranges.front().second == nullptr)
+    if (Ranges.size() == 1 && Ranges.front().second == nullptr) {
+      RegVar->initializeDbgValue(MInsn);
       continue;
+    }
 
     // Handle multiple DBG_VALUE instructions describing one variable.
-    RegVar->setDebugLocListIndex(
-        DebugLocs.startList(&TheCU, Asm->createTempSymbol("debug_loc")));
+    DebugLocStream::ListBuilder List(DebugLocs, TheCU, *Asm, *RegVar, *MInsn);
 
     // Build the location list for this variable.
     SmallVector<DebugLocEntry, 8> Entries;
@@ -930,7 +930,7 @@ void DwarfDebug::collectVariableInfo(DwarfCompileUnit &TheCU,
 
     // Finalize the entry by lowering it into a DWARF bytestream.
     for (auto &Entry : Entries)
-      Entry.finalize(*Asm, DebugLocs, BT);
+      Entry.finalize(*Asm, List, BT);
   }
 
   // Collect info for variables that were optimized out.
@@ -1504,10 +1504,11 @@ static void emitDebugLocValue(const AsmPrinter &AP, const DIBasicType *BT,
   // FIXME: ^
 }
 
-void DebugLocEntry::finalize(const AsmPrinter &AP, DebugLocStream &Locs,
+void DebugLocEntry::finalize(const AsmPrinter &AP,
+                             DebugLocStream::ListBuilder &List,
                              const DIBasicType *BT) {
-  Locs.startEntry(Begin, End);
-  BufferByteStreamer Streamer = Locs.getStreamer();
+  DebugLocStream::EntryBuilder Entry(List, Begin, End);
+  BufferByteStreamer Streamer = Entry.getStreamer();
   const DebugLocEntry::Value &Value = Values[0];
   if (Value.isBitPiece()) {
     // Emit all pieces that belong to the same variable and range.

test/DebugInfo/X86/debug-loc-empty-entries.ll

@@ -0,0 +1,66 @@
+; RUN: llc -mtriple=x86_64-apple-macosx <%s | FileCheck %s
+; Test that we don't generate empty .debug_loc entries.  Originally, there were
+; two empty .debug_loc entries for 'a' in an otherwise empty .debug_loc list.
+;
+; CHECK:      .section __DWARF,__debug_loc,regular,debug
+; CHECK-NEXT: Lsection_debug_loc:
+; CHECK-NEXT: .section __DWARF,__debug_abbrev,regular,debug
+;
+; Test that the variable stuck around.
+; CHECK:      .section __DWARF,__debug_info,regular,debug
+; CHECK:      DW_TAG_variable
+; CHECK-NOT:  DW_AT_location
+
+; Generated using clang -cc1 with the following args:
+;
+;   -triple x86_64-apple-macosx -emit-llvm -gdwarf-4 -O1
+;
+; From this testcase:
+;
+;;   void fn1() {
+;;     float a = 1;
+;;     for (;;)
+;;       a = 0;
+;;   }
+
+; Function Attrs: noreturn nounwind readnone
+define void @_Z3fn1v() #0 {
+entry:
+  tail call void @llvm.dbg.value(metadata float 1.000000e+00, i64 0, metadata !9, metadata !14), !dbg !15
+  br label %for.cond, !dbg !16
+
+for.cond:                                         ; preds = %for.cond, %entry
+  tail call void @llvm.dbg.value(metadata float 0.000000e+00, i64 0, metadata !9, metadata !14), !dbg !15
+  br label %for.cond, !dbg !17
+}
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata) #1
+
+attributes #0 = { noreturn nounwind readnone "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-realign-stack" "stack-protector-buffer-size"="8" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind readnone }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!11, !12}
+!llvm.ident = !{!13}
+
+!0 = !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 3.7.0 (trunk 238517) (llvm/trunk 238524)", isOptimized: true, runtimeVersion: 0, emissionKind: 1, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2)
+!1 = !DIFile(filename: "<stdin>", directory: "/Users/dexonsmith/data/llvm/bootstrap/play/delta2/testcase")
+!2 = !{}
+!3 = !{!4}
+!4 = !DISubprogram(name: "fn1", linkageName: "_Z3fn1v", scope: !5, file: !5, line: 1, type: !6, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, function: void ()* @_Z3fn1v, variables: !8)
+!5 = !DIFile(filename: "t.cpp", directory: "/Users/dexonsmith/data/llvm/bootstrap/play/delta2/testcase")
+!6 = !DISubroutineType(types: !7)
+!7 = !{null}
+!8 = !{!9}
+!9 = !DILocalVariable(tag: DW_TAG_auto_variable, name: "a", scope: !4, file: !5, line: 2, type: !10)
+!10 = !DIBasicType(name: "float", size: 32, align: 32, encoding: DW_ATE_float)
+!11 = !{i32 2, !"Dwarf Version", i32 4}
+!12 = !{i32 2, !"Debug Info Version", i32 3}
+!13 = !{!"clang version 3.7.0 (trunk 238517) (llvm/trunk 238524)"}
+!14 = !DIExpression()
+!15 = !DILocation(line: 2, scope: !4)
+!16 = !DILocation(line: 3, scope: !4)
+!17 = !DILocation(line: 3, scope: !18)
+!18 = distinct !DILexicalBlock(scope: !19, file: !5, line: 3)
+!19 = distinct !DILexicalBlock(scope: !4, file: !5, line: 3)