[LiveDebugValues] Extend var ranges through artificial blocks

ASan often introduces basic blocks consisting exclusively of
instructions without debug locations, or with line 0 debug locations.

LiveDebugValues needs to extend variable ranges through these artificial
blocks. Otherwise, a lot of variables disappear -- even at -O0.

Typically, LiveDebugValues does not extend a variable's range into a
block unless the block is essentially "part of" the variable's scope
(for a precise definition, see LexicalScopes::dominates). This patch
relaxes the lexical dominance check for artificial blocks.

This makes the following Swift program debuggable at -O0:
```
  1| var x = 100
  2| print("x = \(x)")
```

rdar://39127144

Differential Revision: https://reviews.llvm.org/D52921

llvm-svn: 343890

llvm/lib/CodeGen/LiveDebugValues.cpp

@@ -258,7 +258,8 @@ private:
 
   bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
             const VarLocMap &VarLocIDs,
-            SmallPtrSet<const MachineBasicBlock *, 16> &Visited);
+            SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
+            SmallPtrSetImpl<const MachineBasicBlock *> &ArtificialBlocks);
 
   bool ExtendRanges(MachineFunction &MF);
 
@@ -633,9 +634,11 @@ bool LiveDebugValues::process(MachineInstr &MI, OpenRangesSet &OpenRanges,
 /// This routine joins the analysis results of all incoming edges in @MBB by
 /// inserting a new DBG_VALUE instruction at the start of the @MBB - if the same
 /// source variable in all the predecessors of @MBB reside in the same location.
-bool LiveDebugValues::join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs,
+bool LiveDebugValues::join(
-                           VarLocInMBB &InLocs, const VarLocMap &VarLocIDs,
+    MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
-                           SmallPtrSet<const MachineBasicBlock *, 16> &Visited) {
+    const VarLocMap &VarLocIDs,
+    SmallPtrSet<const MachineBasicBlock *, 16> &Visited,
+    SmallPtrSetImpl<const MachineBasicBlock *> &ArtificialBlocks) {
   LLVM_DEBUG(dbgs() << "join MBB: " << MBB.getNumber() << "\n");
   bool Changed = false;
 
@@ -678,6 +681,8 @@ bool LiveDebugValues::join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs,
 
   // Filter out DBG_VALUES that are out of scope.
   VarLocSet KillSet;
+  bool IsArtificial = ArtificialBlocks.count(&MBB);
+  if (!IsArtificial) {
     for (auto ID : InLocsT) {
       if (!VarLocIDs[ID].dominates(MBB)) {
         KillSet.set(ID);
@@ -687,6 +692,7 @@ bool LiveDebugValues::join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs,
         });
       }
     }
+  }
   InLocsT.intersectWithComplement(KillSet);
 
   // As we are processing blocks in reverse post-order we
@@ -737,6 +743,10 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
   VarLocInMBB InLocs;       // Ranges that are incoming after joining.
   TransferMap Transfers;    // DBG_VALUEs associated with spills.
 
+  // Blocks which are artificial, i.e. blocks which exclusively contain
+  // instructions without locations, or with line 0 locations.
+  SmallPtrSet<const MachineBasicBlock *, 16> ArtificialBlocks;
+
   DenseMap<unsigned int, MachineBasicBlock *> OrderToBB;
   DenseMap<MachineBasicBlock *, unsigned int> BBToOrder;
   std::priority_queue<unsigned int, std::vector<unsigned int>,
@@ -758,6 +768,15 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
       process(MI, OpenRanges, OutLocs, VarLocIDs, Transfers,
               dontTransferChanges);
 
+  auto hasNonArtificialLocation = [](const MachineInstr &MI) -> bool {
+    if (const DebugLoc &DL = MI.getDebugLoc())
+      return DL.getLine() != 0;
+    return false;
+  };
+  for (auto &MBB : MF)
+    if (none_of(MBB.instrs(), hasNonArtificialLocation))
+      ArtificialBlocks.insert(&MBB);
+
   LLVM_DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs,
                               "OutLocs after initialization", dbgs()));
 
@@ -783,7 +802,8 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
     while (!Worklist.empty()) {
       MachineBasicBlock *MBB = OrderToBB[Worklist.top()];
       Worklist.pop();
-      MBBJoined = join(*MBB, OutLocs, InLocs, VarLocIDs, Visited);
+      MBBJoined =
+          join(*MBB, OutLocs, InLocs, VarLocIDs, Visited, ArtificialBlocks);
       Visited.insert(MBB);
       if (MBBJoined) {
         MBBJoined = false;

llvm/test/DebugInfo/AArch64/compiler-gen-bbs-livedebugvalues.ll

@@ -0,0 +1,66 @@
+; RUN: llc -O0 -regalloc=fast -stop-after=livedebugvalues -o - < %s | \
+; RUN:   FileCheck %s -implicit-check-not=DBG_VALUE
+
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+target triple = "arm64-apple-ios12.1.0"
+
+declare void @use(i32 %x)
+
+define void @f1(i32 %x) !dbg !6 {
+; CHECK-LABEL: name: f1
+entry:
+; CHECK-LABEL: bb.0.entry:
+  %var = add i32 %x, 1, !dbg !12
+  call void @llvm.dbg.value(metadata i32 %var, metadata !9, metadata !DIExpression()), !dbg !12
+; CHECK: DBG_VALUE debug-use renamable $w0, debug-use $noreg, !9, !DIExpression(), debug-location !12
+; CHECK-NEXT: STRWui killed $w0, $sp, 3 :: (store 4 into %stack.0)
+; CHECK-NEXT: DBG_VALUE debug-use $sp, 0, !9, !DIExpression(DW_OP_plus_uconst, 12)
+
+  br label %artificial-bb-1, !dbg !13
+
+artificial-bb-1:                                  ; preds = %entry
+; CHECK-LABEL: bb.1.artificial-bb-1:
+; CHECK: DBG_VALUE debug-use $sp, 0, !9, !DIExpression(DW_OP_plus_uconst, 12)
+
+  br label %artificial-bb-2
+
+artificial-bb-2:                                  ; preds = %artificial-bb-1
+; CHECK-LABEL: bb.2.artificial-bb-2:
+; CHECK: DBG_VALUE debug-use $sp, 0, !9, !DIExpression(DW_OP_plus_uconst, 12)
+
+  %invisible = add i32 %var, 1
+  br label %return, !dbg !14
+
+return:                                           ; preds = %artificial-bb-2
+; CHECK-LABEL: bb.3.return:
+; CHECK: DBG_VALUE debug-use $sp, 0, !9, !DIExpression(DW_OP_plus_uconst, 12)
+
+  call void @use(i32 %var)
+  ret void, !dbg !15
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+attributes #0 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "compiler-gen-bbs-livedebugvalues.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 6}
+!4 = !{i32 2}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "f1", linkageName: "f1", scope: null, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !0, retainedNodes: !8)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9, !11}
+!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+!10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+!11 = !DILocalVariable(name: "2", scope: !6, file: !1, line: 4, type: !10)
+!12 = !DILocation(line: 1, column: 1, scope: !6)
+!13 = !DILocation(line: 2, column: 1, scope: !6)
+!14 = !DILocation(line: 0, column: 1, scope: !6)
+!15 = !DILocation(line: 4, column: 1, scope: !6)