[DebugInstrRef] Pass DBG_INSTR_REFs through register allocation

Both FastRegAlloc and LiveDebugVariables/greedy need to cope with
DBG_INSTR_REFs. None of them actually need to take any action, other than
passing DBG_INSTR_REFs through: variable location information doesn't refer
to any registers at this stage.

LiveDebugVariables stashes the instruction information in a tuple, then
re-creates it later. This is only necessary as the register allocator
doesn't expect to see any debug instructions while it's working. No
equivalence classes or interval splitting is required at all!

No changes are needed for the fast register allocator, as it just ignores
debug instructions. The test added checks that both of them preserve
DBG_INSTR_REFs.

This also expands ScheduleDAGInstrs.cpp to treat DBG_INSTR_REFs the same as
DBG_VALUEs when rescheduling instructions around. The current movement of
DBG_VALUEs around is less than ideal, but it's not a regression to make
DBG_INSTR_REFs subject to the same movement.

Differential Revision: https://reviews.llvm.org/D85757

Author: jmorse

llvm/lib/CodeGen/LiveDebugVariables.cpp

@@ -395,6 +395,11 @@ class LDVImpl {
   LiveIntervals *LIS;
   const TargetRegisterInfo *TRI;
 
+  using StashedInstrRef =
+      std::tuple<unsigned, unsigned, const DILocalVariable *,
+                 const DIExpression *, DebugLoc>;
+  std::map<SlotIndex, std::vector<StashedInstrRef>> StashedInstrReferences;
+
   /// Whether emitDebugValues is called.
   bool EmitDone = false;
 
@@ -431,6 +436,16 @@ class LDVImpl {
   /// \returns True if the DBG_VALUE instruction should be deleted.
   bool handleDebugValue(MachineInstr &MI, SlotIndex Idx);
 
+  /// Track a DBG_INSTR_REF. This needs to be removed from the MachineFunction
+  /// during regalloc -- but there's no need to maintain live ranges, as we
+  /// refer to a value rather than a location.
+  ///
+  /// \param MI DBG_INSTR_REF instruction
+  /// \param Idx Last valid SlotIndex before instruction
+  ///
+  /// \returns True if the DBG_VALUE instruction should be deleted.
+  bool handleDebugInstrRef(MachineInstr &MI, SlotIndex Idx);
+
   /// Add DBG_LABEL instruction to UserLabel.
   ///
   /// \param MI DBG_LABEL instruction
@@ -459,6 +474,7 @@ class LDVImpl {
   /// Release all memory.
   void clear() {
     MF = nullptr;
+    StashedInstrReferences.clear();
     userValues.clear();
     userLabels.clear();
     virtRegToEqClass.clear();
@@ -666,6 +682,19 @@ bool LDVImpl::handleDebugValue(MachineInstr &MI, SlotIndex Idx) {
   return true;
 }
 
+bool LDVImpl::handleDebugInstrRef(MachineInstr &MI, SlotIndex Idx) {
+  assert(MI.isDebugRef());
+  unsigned InstrNum = MI.getOperand(0).getImm();
+  unsigned OperandNum = MI.getOperand(1).getImm();
+  auto *Var = MI.getDebugVariable();
+  auto *Expr = MI.getDebugExpression();
+  auto &DL = MI.getDebugLoc();
+  StashedInstrRef Stashed =
+      std::make_tuple(InstrNum, OperandNum, Var, Expr, DL);
+  StashedInstrReferences[Idx].push_back(Stashed);
+  return true;
+}
+
 bool LDVImpl::handleDebugLabel(MachineInstr &MI, SlotIndex Idx) {
   // DBG_LABEL label
   if (MI.getNumOperands() != 1 || !MI.getOperand(0).isMetadata()) {
@@ -713,6 +742,7 @@ bool LDVImpl::collectDebugValues(MachineFunction &mf) {
         // Only handle DBG_VALUE in handleDebugValue(). Skip all other
         // kinds of debug instructions.
         if ((MBBI->isDebugValue() && handleDebugValue(*MBBI, Idx)) ||
+            (MBBI->isDebugRef() && handleDebugInstrRef(*MBBI, Idx)) ||
             (MBBI->isDebugLabel() && handleDebugLabel(*MBBI, Idx))) {
           MBBI = MBB->erase(MBBI);
           Changed = true;
@@ -1435,6 +1465,28 @@ void LDVImpl::emitDebugValues(VirtRegMap *VRM) {
     LLVM_DEBUG(userLabel->print(dbgs(), TRI));
     userLabel->emitDebugLabel(*LIS, *TII);
   }
+
+  LLVM_DEBUG(dbgs() << "********** EMITTING INSTR REFERENCES **********\n");
+
+  // Re-insert any DBG_INSTR_REFs back in the position they were. Ordering
+  // is preserved by vector.
+  auto Slots = LIS->getSlotIndexes();
+  const MCInstrDesc &RefII = TII->get(TargetOpcode::DBG_INSTR_REF);
+  for (auto &P : StashedInstrReferences) {
+    const SlotIndex &Idx = P.first;
+    auto *MBB = Slots->getMBBFromIndex(Idx);
+    MachineBasicBlock::iterator insertPos = findInsertLocation(MBB, Idx, *LIS);
+    for (auto &Stashed : P.second) {
+      auto MIB = BuildMI(*MF, std::get<4>(Stashed), RefII);
+      MIB.addImm(std::get<0>(Stashed));
+      MIB.addImm(std::get<1>(Stashed));
+      MIB.addMetadata(std::get<2>(Stashed));
+      MIB.addMetadata(std::get<3>(Stashed));
+      MachineInstr *New = MIB;
+      MBB->insert(insertPos, New);
+    }
+  }
+
   EmitDone = true;
 }
 

llvm/lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -807,7 +807,7 @@ void ScheduleDAGInstrs::buildSchedGraph(AAResults *AA,
       DbgMI = nullptr;
     }
 
-    if (MI.isDebugValue()) {
+    if (MI.isDebugValue() || MI.isDebugRef()) {
       DbgMI = &MI;
       continue;
     }

llvm/test/DebugInfo/MIR/InstrRef/survives-livedebugvars.mir

@@ -0,0 +1,142 @@
+# RUN: llc -start-after=phi-node-elimination -stop-after=virtregrewriter %s -mtriple=x86_64-unknown-unknown -o - -experimental-debug-variable-locations | FileCheck %s
+# RUN: llc -O0 -start-after=phi-node-elimination -stop-after=regallocfast %s -mtriple=x86_64-unknown-unknown -o - -experimental-debug-variable-locations | FileCheck %s --check-prefix=FASTREG
+#
+# Test that DBG_INSTR_REFs can pass through livedebugvariables to the end of
+# regalloc without problem. Program body copied from
+# livedebugvars-crossbb-interval.mir.
+#
+# CHECK-LABEL: bb.0:
+# CHECK:       DBG_INSTR_REF 1, 0
+# CHECK-NEXT:  JMP_1
+# CHECK-LABEL: bb.1:
+# CHECK:       DBG_INSTR_REF 2, 0
+# CHECK-NEXT:  JMP_1
+# CHECK-LABEL: bb.2:
+# CHECK:       DBG_INSTR_REF 3, 0
+# CHECK-NEXT:  CALL64pcrel32
+# CHECK-LABEL: bb.3:
+# CHECK:       DBG_INSTR_REF 4, 0
+# CHECK-NEXT:  JMP_1
+#
+#
+# The fast register allocator puts some spills in -- these are no-ops as far
+# as the slot indexes are concerned. It doesn't matter which side of spills
+# the DBG_INSTR_REF lands on.
+#
+# FASTREG-LABEL: bb.0:
+# FASTREG-DAG:   DBG_INSTR_REF 1, 0
+# FASTREG-DAG:   MOV64mr
+# FASTREG-DAG:   MOV32mr
+# FASTREG-NEXT:  JMP_1
+# FASTREG-LABEL: bb.1:
+# FASTREG:       DBG_INSTR_REF 2, 0
+# FASTREG-NEXT:  JMP_1
+# FASTREG-LABEL: bb.2:
+# FASTREG:       DBG_INSTR_REF 3, 0
+# FASTREG-NEXT:  CALL64pcrel32
+# FASTREG-LABEL: bb.3:
+# FASTREG-DAG:   MOV32rm
+# FASTREG-DAG:   DBG_INSTR_REF 4, 0
+# FASTREG-DAG:   MOV32mr
+# FASTREG-NEXT:  JMP_1
+# FASTREG-LABEL: bb.4:
+# FASTREG:       DBG_INSTR_REF 5, 0
+# FASTREG-NEXT:  RETQ
+
+--- |
+  ; ModuleID = 'tmp.ll'
+  source_filename = "tmp.ll"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+  %struct.a = type { i32 }
+
+  ; Function Attrs: nounwind ssp
+  define i32 @bar() !dbg !4 {
+    ret i32 0, !dbg !18
+  }
+
+  declare i32 @foo();
+
+  ; Function Attrs: nounwind readnone speculatable willreturn
+  declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3}
+  
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "asdf", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2)
+  !1 = !DIFile(filename: "bar.c", directory: "asdf")
+  !2 = !{}
+  !3 = !{i32 1, !"Debug Info Version", i32 3}
+  !4 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 5, type: !5, virtualIndex: 6, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+  !5 = !DISubroutineType(types: !6)
+  !6 = !{!7}
+  !7 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+  !8 = !{!9, !14}
+  !9 = !DILocalVariable(name: "b", arg: 1, scope: !4, file: !1, line: 5, type: !10)
+  !10 = !DIDerivedType(tag: DW_TAG_pointer_type, scope: !0, baseType: !11, size: 64, align: 64)
+  !11 = !DICompositeType(tag: DW_TAG_structure_type, name: "a", scope: !0, file: !1, line: 1, size: 32, align: 32, elements: !12)
+  !12 = !{!13}
+  !13 = !DIDerivedType(tag: DW_TAG_member, name: "c", scope: !1, file: !1, line: 2, baseType: !7, size: 32, align: 32)
+  !14 = !DILocalVariable(name: "x", scope: !15, file: !1, line: 6, type: !7)
+  !15 = distinct !DILexicalBlock(scope: !4, file: !1, line: 5, column: 22)
+  !16 = !DILocation(line: 5, column: 19, scope: !4)
+  !17 = !DILocation(line: 6, column: 14, scope: !15)
+  !18 = !DILocation(line: 8, column: 2, scope: !15)
+  !19 = !DILocation(line: 7, column: 2, scope: !15)
+
+...
+---
+name:            bar
+alignment:       16
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gr64 }
+  - { id: 1, class: gr32 }
+  - { id: 2, class: gr64 }
+  - { id: 3, class: gr64 }
+  - { id: 4, class: gr32 }
+  - { id: 5, class: gr32 }
+  - { id: 6, class: gr32 }
+  - { id: 7, class: gr32 }
+liveins:
+  - { reg: '$rdi', virtual-reg: '%2' }
+  - { reg: '$esi', virtual-reg: '%4' }
+frameInfo:
+  hasCalls:        true
+machineFunctionInfo: {}
+body:             |
+  bb.0:
+    liveins: $rdi, $esi
+
+    %4:gr32 = COPY $esi
+    %2:gr64 = COPY $rdi
+    %3:gr64 = COPY killed %2
+    %5:gr32 = COPY killed %4
+    DBG_INSTR_REF 1, 0, !9, !DIExpression(), debug-location !16
+    JMP_1 %bb.3
+
+  bb.1:
+    DBG_INSTR_REF 2, 0, !9, !DIExpression(), debug-location !16
+    JMP_1 %bb.4
+
+  bb.2:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp, debug-location !19
+    $edi = COPY %6, debug-location !19
+    $al = MOV8ri 0, debug-location !19
+    DBG_INSTR_REF 3, 0, !9, !DIExpression(), debug-location !16
+    CALL64pcrel32 @foo, csr_64, implicit $rsp, implicit $ssp, implicit $al, implicit $edi, implicit-def $eax, debug-location !19
+    ADJCALLSTACKUP64 0, 0, implicit-def $rsp, implicit-def $eflags, implicit-def $ssp, implicit $rsp, implicit $ssp, debug-location !19
+    %7:gr32 = COPY $eax, debug-location !19
+    JMP_1 %bb.1
+
+  bb.3:
+    %6:gr32 = MOV32rm %3, 1, $noreg, 0, $noreg, debug-location !17
+    DBG_INSTR_REF 4, 0, !9, !DIExpression(), debug-location !16
+    JMP_1 %bb.2
+
+  bb.4:
+    $eax = COPY %5, debug-location !18
+    DBG_INSTR_REF 5, 0, !9, !DIExpression(), debug-location !16
+    RETQ implicit $eax, debug-location !18
+
+...