[DebugInfo][InstrRef][3/4] Produce DBG_INSTR_REFs for all variable locations

This patch emits DBG_INSTR_REFs for two remaining flavours of variable
locations that weren't supported: copies, and inter-block VRegs. There are
still some locations that must be represented by DBG_VALUE such as
constants, but they're mostly independent of optimisations.

For variable locations that refer to values defined in different blocks,
vregs are allocated before isel begins, but the defining instruction
might not exist until late in isel. To get around this, emit
DBG_INSTR_REFs in a "half done" state, where the first operand refers to a
VReg. Then at the end of isel, patch these back up to refer to
instructions, using the finalizeDebugInstrRefs method.

Copies are something that I complained about the original RFC, and I
really don't want to have to put instruction numbers on copies. They don't
define a value: they move them. To address this isel, salvageCopySSA
interprets:
 * COPYs,
 * SUBREG_TO_REG,
 * Anything that isCopyInstr thinks is a copy.
And follows chains of copies back to the defining instruction that they
read from. This relies on any physical registers that COPYs read being
defined in the same block, or being entry-block arguments. For the former
we can put an instruction number on the defining instruction; for the
latter we can drop a DBG_PHI that reads the incoming value.

Differential Revision: https://reviews.llvm.org/D88896

Author: jmorse

llvm/include/llvm/CodeGen/MachineFunction.h

@@ -504,6 +504,25 @@ class MachineFunction {
   void substituteDebugValuesForInst(const MachineInstr &Old, MachineInstr &New,
                                     unsigned MaxOperand = UINT_MAX);
 
+  /// Find the underlying  defining instruction / operand for a COPY instruction
+  /// while in SSA form. Copies do not actually define values -- they move them
+  /// between registers. Labelling a COPY-like instruction with an instruction
+  /// number is to be avoided as it makes value numbers non-unique later in
+  /// compilation. This method follows the definition chain for any sequence of
+  /// COPY-like instructions to find whatever non-COPY-like instruction defines
+  /// the copied value; or for parameters, creates a DBG_PHI on entry.
+  /// May insert instructions into the entry block!
+  /// \p MI The copy-like instruction to salvage.
+  /// \returns An instruction/operand pair identifying the defining value.
+  DebugInstrOperandPair salvageCopySSA(MachineInstr &MI);
+
+  /// Finalise any partially emitted debug instructions. These are DBG_INSTR_REF
+  /// instructions where we only knew the vreg of the value they use, not the
+  /// instruction that defines that vreg. Once isel finishes, we should have
+  /// enough information for every DBG_INSTR_REF to point at an instruction
+  /// (or DBG_PHI).
+  void finalizeDebugInstrRefs();
+
   MachineFunction(Function &F, const LLVMTargetMachine &Target,
                   const TargetSubtargetInfo &STI, unsigned FunctionNum,
                   MachineModuleInfo &MMI);

llvm/lib/CodeGen/MachineFunction.cpp

@@ -1007,6 +1007,205 @@ void MachineFunction::substituteDebugValuesForInst(const MachineInstr &Old,
   }
 }
 
+auto MachineFunction::salvageCopySSA(MachineInstr &MI)
+    -> DebugInstrOperandPair {
+  MachineRegisterInfo &MRI = getRegInfo();
+  const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
+  const TargetInstrInfo &TII = *getSubtarget().getInstrInfo();
+
+  // Chase the value read by a copy-like instruction back to the instruction
+  // that ultimately _defines_ that value. This may pass:
+  //  * Through multiple intermediate copies, including subregister moves /
+  //    copies,
+  //  * Copies from physical registers that must then be traced back to the
+  //    defining instruction,
+  //  * Or, physical registers may be live-in to (only) the entry block, which
+  //    requires a DBG_PHI to be created.
+  // We can pursue this problem in that order: trace back through copies,
+  // optionally through a physical register, to a defining instruction. We
+  // should never move from physreg to vreg. As we're still in SSA form, no need
+  // to worry about partial definitions of registers.
+
+  // Helper lambda to interpret a copy-like instruction. Takes instruction,
+  // returns the register read and any subregister identifying which part is
+  // read.
+  auto GetRegAndSubreg =
+      [&](const MachineInstr &Cpy) -> std::pair<Register, unsigned> {
+    Register NewReg, OldReg;
+    unsigned SubReg;
+    if (Cpy.isCopy()) {
+      OldReg = Cpy.getOperand(0).getReg();
+      NewReg = Cpy.getOperand(1).getReg();
+      SubReg = Cpy.getOperand(1).getSubReg();
+    } else if (Cpy.isSubregToReg()) {
+      OldReg = Cpy.getOperand(0).getReg();
+      NewReg = Cpy.getOperand(2).getReg();
+      SubReg = Cpy.getOperand(3).getImm();
+    } else {
+      auto CopyDetails = *TII.isCopyInstr(Cpy);
+      const MachineOperand &Src = *CopyDetails.Source;
+      const MachineOperand &Dest = *CopyDetails.Destination;
+      OldReg = Dest.getReg();
+      NewReg = Src.getReg();
+      SubReg = Src.getSubReg();
+    }
+
+    return {NewReg, SubReg};
+  };
+
+  // First seek either the defining instruction, or a copy from a physreg.
+  // During search, the current state is the current copy instruction, and which
+  // register we've read. Accumulate qualifying subregisters into SubregsSeen;
+  // deal with those later.
+  auto State = GetRegAndSubreg(MI);
+  auto CurInst = MI.getIterator();
+  SmallVector<unsigned, 4> SubregsSeen;
+  while (true) {
+    // If we've found a copy from a physreg, first portion of search is over.
+    if (!State.first.isVirtual())
+      break;
+
+    // Record any subregister qualifier.
+    if (State.second)
+      SubregsSeen.push_back(State.second);
+
+    assert(MRI.hasOneDef(State.first));
+    MachineInstr &Inst = *MRI.def_begin(State.first)->getParent();
+    CurInst = Inst.getIterator();
+
+    // Any non-copy instruction is the defining instruction we're seeking.
+    if (!Inst.isCopyLike() && !TII.isCopyInstr(Inst))
+      break;
+    State = GetRegAndSubreg(Inst);
+  };
+
+  // Helper lambda to apply additional subregister substitutions to a known
+  // instruction/operand pair. Adds new (fake) substitutions so that we can
+  // record the subregister. FIXME: this isn't very space efficient if multiple
+  // values are tracked back through the same copies; cache something later.
+  auto ApplySubregisters =
+      [&](DebugInstrOperandPair P) -> DebugInstrOperandPair {
+    for (unsigned Subreg : reverse(SubregsSeen)) {
+      // Fetch a new instruction number, not attached to an actual instruction.
+      unsigned NewInstrNumber = getNewDebugInstrNum();
+      // Add a substitution from the "new" number to the known one, with a
+      // qualifying subreg.
+      makeDebugValueSubstitution({NewInstrNumber, 0}, P, Subreg);
+      // Return the new number; to find the underlying value, consumers need to
+      // deal with the qualifying subreg.
+      P = {NewInstrNumber, 0};
+    }
+    return P;
+  };
+
+  // If we managed to find the defining instruction after COPYs, return an
+  // instruction / operand pair after adding subregister qualifiers.
+  if (State.first.isVirtual()) {
+    // Virtual register def -- we can just look up where this happens.
+    MachineInstr *Inst = MRI.def_begin(State.first)->getParent();
+    for (auto &MO : Inst->operands()) {
+      if (!MO.isReg() || !MO.isDef() || MO.getReg() != State.first)
+        continue;
+      return ApplySubregisters(
+          {Inst->getDebugInstrNum(), Inst->getOperandNo(&MO)});
+    }
+
+    llvm_unreachable("Vreg def with no corresponding operand?");
+  }
+
+  // Our search ended in a copy from a physreg: walk back up the function
+  // looking for whatever defines the physreg.
+  assert(CurInst->isCopyLike() || TII.isCopyInstr(*CurInst));
+  State = GetRegAndSubreg(*CurInst);
+  Register RegToSeek = State.first;
+
+  auto RMII = CurInst->getReverseIterator();
+  auto PrevInstrs = make_range(RMII, CurInst->getParent()->instr_rend());
+  for (auto &ToExamine : PrevInstrs) {
+    for (auto &MO : ToExamine.operands()) {
+      // Test for operand that defines something aliasing RegToSeek.
+      if (!MO.isReg() || !MO.isDef() ||
+          !TRI.regsOverlap(RegToSeek, MO.getReg()))
+        continue;
+
+      return ApplySubregisters(
+          {ToExamine.getDebugInstrNum(), ToExamine.getOperandNo(&MO)});
+    }
+  }
+
+  // We reached the start of the block before finding a defining instruction.
+  // It must be an argument: assert that this is the entry block, and produce
+  // a DBG_PHI.
+  assert(!State.first.isVirtual());
+  MachineBasicBlock &TargetBB = *CurInst->getParent();
+  assert(&*TargetBB.getParent()->begin() == &TargetBB);
+
+  // Create DBG_PHI for specified physreg.
+  auto Builder = BuildMI(TargetBB, TargetBB.getFirstNonPHI(), DebugLoc(),
+                         TII.get(TargetOpcode::DBG_PHI));
+  Builder.addReg(State.first, RegState::Debug);
+  unsigned NewNum = getNewDebugInstrNum();
+  Builder.addImm(NewNum);
+  return ApplySubregisters({NewNum, 0u});
+}
+
+void MachineFunction::finalizeDebugInstrRefs() {
+  auto *TII = getSubtarget().getInstrInfo();
+
+  auto MakeDbgValue = [&](MachineInstr &MI) {
+    const MCInstrDesc &RefII = TII->get(TargetOpcode::DBG_VALUE);
+    MI.setDesc(RefII);
+    MI.getOperand(1).ChangeToRegister(0, false);
+    MI.getOperand(0).setIsDebug();
+  };
+
+  if (!getTarget().Options.ValueTrackingVariableLocations)
+    return;
+
+  for (auto &MBB : *this) {
+    for (auto &MI : MBB) {
+      if (!MI.isDebugRef() || !MI.getOperand(0).isReg())
+        continue;
+
+      Register Reg = MI.getOperand(0).getReg();
+
+      // Some vregs can be deleted as redundant in the meantime. Mark those
+      // as DBG_VALUE $noreg.
+      if (Reg == 0) {
+        MakeDbgValue(MI);
+        continue;
+      }
+
+      assert(Reg.isVirtual());
+      MachineInstr &DefMI = *RegInfo->def_instr_begin(Reg);
+      assert(RegInfo->hasOneDef(Reg));
+
+      // If we've found a copy-like instruction, follow it back to the
+      // instruction that defines the source value, see salvageCopySSA docs
+      // for why this is important.
+      if (DefMI.isCopyLike() || TII->isCopyInstr(DefMI)) {
+        auto Result = salvageCopySSA(DefMI);
+        MI.getOperand(0).ChangeToImmediate(Result.first);
+        MI.getOperand(1).setImm(Result.second);
+      } else {
+        // Otherwise, identify the operand number that the VReg refers to.
+        unsigned OperandIdx = 0;
+        for (const auto &MO : DefMI.operands()) {
+          if (MO.isReg() && MO.isDef() && MO.getReg() == Reg)
+            break;
+          ++OperandIdx;
+        }
+        assert(OperandIdx < DefMI.getNumOperands());
+
+        // Morph this instr ref to point at the given instruction and operand.
+        unsigned ID = DefMI.getDebugInstrNum();
+        MI.getOperand(0).ChangeToImmediate(ID);
+        MI.getOperand(1).setImm(OperandIdx);
+      }
+    }
+  }
+}
+
 /// \}
 
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