[DebugInstrRef] Create DBG_INSTR_REFs in SelectionDAG

When given the -experimental-debug-variable-locations option (via -Xclang
or to llc), have SelectionDAG generate DBG_INSTR_REF instructions instead
of DBG_VALUE. For now, this only happens in a limited circumstance: when
the value referred to is not a PHI and is defined in the current block.
Other situations introduce interesting problems, addresed in later patches.

Practically, this patch hooks into InstrEmitter and if it can find a
defining instruction for a value, gives it an instruction number, and
points the DBG_INSTR_REF at that <instr, operand> pair.

Differential Revision: https://reviews.llvm.org/D85747

llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp

@@ -695,6 +695,11 @@ InstrEmitter::EmitDbgValue(SDDbgValue *SD,
     return &*MIB;
   }
 
+  // Attempt to produce a DBG_INSTR_REF if we've been asked to.
+  if (EmitDebugInstrRefs)
+    if (auto *InstrRef = EmitDbgInstrRef(SD, VRBaseMap))
+      return InstrRef;
+
   if (SD->getKind() == SDDbgValue::FRAMEIX) {
     // Stack address; this needs to be lowered in target-dependent fashion.
     // EmitTargetCodeForFrameDebugValue is responsible for allocation.
@@ -761,6 +766,63 @@ InstrEmitter::EmitDbgValue(SDDbgValue *SD,
   return &*MIB;
 }
 
+MachineInstr *
+InstrEmitter::EmitDbgInstrRef(SDDbgValue *SD,
+                              DenseMap<SDValue, Register> &VRBaseMap) {
+  // Instruction referencing is still in a prototype state: for now we're only
+  // going to support SDNodes within a block. Copies are not supported, they
+  // don't actually define a value.
+  if (SD->getKind() != SDDbgValue::SDNODE)
+    return nullptr;
+
+  SDNode *Node = SD->getSDNode();
+  SDValue Op = SDValue(Node, SD->getResNo());
+  DenseMap<SDValue, Register>::iterator I = VRBaseMap.find(Op);
+  if (I==VRBaseMap.end())
+    return nullptr; // undef value: let EmitDbgValue produce a DBG_VALUE $noreg.
+
+  MDNode *Var = SD->getVariable();
+  MDNode *Expr = SD->getExpression();
+  DebugLoc DL = SD->getDebugLoc();
+
+  // Try to pick out a defining instruction at this point.
+  unsigned VReg = getVR(Op, VRBaseMap);
+  MachineInstr *ResultInstr = nullptr;
+
+  // No definition corresponds to scenarios where a vreg is live-in to a block,
+  // and doesn't have a defining instruction (yet). This can be patched up
+  // later; at this early stage of implementation, fall back to using DBG_VALUE.
+  if (!MRI->hasOneDef(VReg))
+    return nullptr;
+
+  MachineInstr &DefMI = *MRI->def_instr_begin(VReg);
+  // Some target specific opcodes can become copies. As stated above, we're
+  // ignoring those for now.
+  if (DefMI.isCopy() || DefMI.getOpcode() == TargetOpcode::SUBREG_TO_REG)
+    return nullptr;
+
+  const MCInstrDesc &RefII = TII->get(TargetOpcode::DBG_INSTR_REF);
+  auto MIB = BuildMI(*MF, DL, RefII);
+
+  // Find the operand which defines the specified VReg.
+  unsigned OperandIdx = 0;
+  for (const auto &MO : DefMI.operands()) {
+    if (MO.isReg() && MO.isDef() && MO.getReg() == VReg)
+      break;
+    ++OperandIdx;
+  }
+  assert(OperandIdx < DefMI.getNumOperands());
+
+  // Make the DBG_INSTR_REF refer to that instruction, and that operand.
+  unsigned InstrNum = DefMI.getDebugInstrNum();
+  MIB.addImm(InstrNum);
+  MIB.addImm(OperandIdx);
+  MIB.addMetadata(Var);
+  MIB.addMetadata(Expr);
+  ResultInstr = &*MIB;
+  return ResultInstr;
+}
+
 MachineInstr *
 InstrEmitter::EmitDbgLabel(SDDbgLabel *SD) {
   MDNode *Label = SD->getLabel();
@@ -1177,10 +1239,12 @@ EmitSpecialNode(SDNode *Node, bool IsClone, bool IsCloned,
 
 /// InstrEmitter - Construct an InstrEmitter and set it to start inserting
 /// at the given position in the given block.
-InstrEmitter::InstrEmitter(MachineBasicBlock *mbb,
+InstrEmitter::InstrEmitter(const TargetMachine &TM, MachineBasicBlock *mbb,
                            MachineBasicBlock::iterator insertpos)
     : MF(mbb->getParent()), MRI(&MF->getRegInfo()),
       TII(MF->getSubtarget().getInstrInfo()),
       TRI(MF->getSubtarget().getRegisterInfo()),
       TLI(MF->getSubtarget().getTargetLowering()), MBB(mbb),
-      InsertPos(insertpos) {}
+      InsertPos(insertpos) {
+  EmitDebugInstrRefs = TM.Options.ValueTrackingVariableLocations;
+}

llvm/lib/CodeGen/SelectionDAG/InstrEmitter.h

@@ -37,6 +37,9 @@ class LLVM_LIBRARY_VISIBILITY InstrEmitter {
   MachineBasicBlock *MBB;
   MachineBasicBlock::iterator InsertPos;
 
+  /// Should we try to produce DBG_INSTR_REF instructions?
+  bool EmitDebugInstrRefs;
+
   /// EmitCopyFromReg - Generate machine code for an CopyFromReg node or an
   /// implicit physical register output.
   void EmitCopyFromReg(SDNode *Node, unsigned ResNo,
@@ -109,6 +112,11 @@ class LLVM_LIBRARY_VISIBILITY InstrEmitter {
   MachineInstr *EmitDbgValue(SDDbgValue *SD,
                              DenseMap<SDValue, Register> &VRBaseMap);
 
+  /// Attempt to emit a dbg_value as a DBG_INSTR_REF. May fail and return
+  /// nullptr, in which case we fall back to plain EmitDbgValue.
+  MachineInstr *EmitDbgInstrRef(SDDbgValue *SD,
+                                DenseMap<SDValue, Register> &VRBaseMap);
+
   /// Generate machine instruction for a dbg_label node.
   MachineInstr *EmitDbgLabel(SDDbgLabel *SD);
 
@@ -130,7 +138,8 @@ class LLVM_LIBRARY_VISIBILITY InstrEmitter {
 
   /// InstrEmitter - Construct an InstrEmitter and set it to start inserting
   /// at the given position in the given block.
-  InstrEmitter(MachineBasicBlock *mbb, MachineBasicBlock::iterator insertpos);
+  InstrEmitter(const TargetMachine &TM, MachineBasicBlock *mbb,
+               MachineBasicBlock::iterator insertpos);
 
 private:
   void EmitMachineNode(SDNode *Node, bool IsClone, bool IsCloned,

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGFast.cpp

@@ -760,7 +760,7 @@ void ScheduleDAGLinearize::Schedule() {
 
 MachineBasicBlock*
 ScheduleDAGLinearize::EmitSchedule(MachineBasicBlock::iterator &InsertPos) {
-  InstrEmitter Emitter(BB, InsertPos);
+  InstrEmitter Emitter(DAG->getTarget(), BB, InsertPos);
   DenseMap<SDValue, Register> VRBaseMap;
 
   LLVM_DEBUG({ dbgs() << "\n*** Final schedule ***\n"; });

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp

@@ -829,7 +829,7 @@ EmitPhysRegCopy(SUnit *SU, DenseMap<SUnit*, Register> &VRBaseMap,
 /// not necessarily refer to returned BB. The emitter may split blocks.
 MachineBasicBlock *ScheduleDAGSDNodes::
 EmitSchedule(MachineBasicBlock::iterator &InsertPos) {
-  InstrEmitter Emitter(BB, InsertPos);
+  InstrEmitter Emitter(DAG->getTarget(), BB, InsertPos);
   DenseMap<SDValue, Register> VRBaseMap;
   DenseMap<SUnit*, Register> CopyVRBaseMap;
   SmallVector<std::pair<unsigned, MachineInstr*>, 32> Orders;

llvm/test/DebugInfo/X86/instr-ref-selectiondag.ll

@@ -0,0 +1,54 @@
+; RUN: llc %s -mtriple=x86_64-unknown-unknown -o - -stop-before=finalize-isel \
+; RUN:   | FileCheck %s --check-prefix=NORMAL \
+; RUN:     --implicit-check-not=debug-instr-number \
+; RUN:     --implicit-check-not=DBG_INSTR_REF
+; RUN: llc %s -mtriple=x86_64-unknown-unknown -o - -stop-before=finalize-isel \
+; RUN:     -experimental-debug-variable-locations -verify-machineinstrs \
+; RUN:   | FileCheck %s --check-prefix=INSTRREF \
+; RUN:     --implicit-check-not=DBG_VALUE
+
+; Test that SelectionDAG produces DBG_VALUEs normally, but DBG_INSTR_REFs when
+; asked.
+
+; NORMAL:      %[[REG0:[0-9]+]]:gr32 = ADD32rr
+; NORMAL-NEXT: DBG_VALUE %[[REG0]]
+; NORMAL-NEXT: %[[REG1:[0-9]+]]:gr32 = ADD32rr
+; NORMAL-NEXT: DBG_VALUE %[[REG1]]
+
+; Note that I'm baking in an assumption of one-based ordering here. We could
+; capture and check for the instruction numbers, we'd rely on machine verifier
+; ensuring there were no duplicates.
+
+; INSTRREF:      ADD32rr
+; INSTRREF-SAME: debug-instr-number 1
+; INSTRREF-NEXT: DBG_INSTR_REF 1, 0
+; INSTRREF-NEXT: ADD32rr
+; INSTRREF-SAME: debug-instr-number 2
+; INSTRREF-NEXT: DBG_INSTR_REF 2, 0
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+define i32 @foo(i32 %bar, i32 %baz, i32 %qux) !dbg !7 {
+entry:
+  %0 = add i32 %bar, %baz, !dbg !14
+  call void @llvm.dbg.value(metadata i32 %0, metadata !13, metadata !DIExpression()), !dbg !14
+  %1 = add i32 %0, %qux
+  call void @llvm.dbg.value(metadata i32 %1, metadata !13, metadata !DIExpression()), !dbg !14
+  ret i32 %1, !dbg !14
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "exprconflict.c", directory: "/home/jmorse")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 5, type: !8, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!13}
+!13 = !DILocalVariable(name: "baz", scope: !7, file: !1, line: 6, type: !10)
+!14 = !DILocation(line: 1, scope: !7)