DebugInfo: Track spilled variables in LiveDebugValues

When variables are spilled to the stack by the register allocator, keep track of their
debug locations in LiveDebugValues and insert DBG_VALUE instructions at the appropriate
place. Ensure that the locations are propagated down the dominator tree via the existing 
mechanisms.

Reviewer: aprantl

Differential Revision: https://reviews.llvm.org/D29500

llvm-svn: 294356

1 files changed, 137 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues.cpp b/llvm/lib/CodeGen/LiveDebugValues.cpp
index 131167630d6..2759b56f8c0 100644
--- a/llvm/lib/CodeGen/LiveDebugValues.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues.cpp
@@ -24,13 +24,16 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/UniqueVector.h"
 #include "llvm/CodeGen/LexicalScopes.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetFrameLowering.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetRegisterInfo.h"
@@ -61,6 +64,7 @@ class LiveDebugValues : public MachineFunctionPass {
 private:
   const TargetRegisterInfo *TRI;
   const TargetInstrInfo *TII;
+  const TargetFrameLowering *TFI;
   LexicalScopes LS;
 
   /// Keeps track of lexical scopes associated with a user value's source
@@ -169,6 +173,11 @@ private:
   typedef UniqueVector<VarLoc> VarLocMap;
   typedef SparseBitVector<> VarLocSet;
   typedef SmallDenseMap<const MachineBasicBlock *, VarLocSet> VarLocInMBB;
+  struct SpillDebugPair {
+    MachineInstr *SpillInst;
+    MachineInstr *DebugInst;
+  };
+  typedef SmallVector<SpillDebugPair, 4> SpillMap;
 
   /// This holds the working set of currently open ranges. For fast
   /// access, this is done both as a set of VarLocIDs, and a map of
@@ -218,14 +227,21 @@ private:
     }
   };
 
+  bool isSpillInstruction(const MachineInstr &MI, MachineFunction *MF,
+                          unsigned &Reg);
+  int extractSpillBaseRegAndOffset(const MachineInstr &MI, unsigned &Reg);
+
   void transferDebugValue(const MachineInstr &MI, OpenRangesSet &OpenRanges,
                           VarLocMap &VarLocIDs);
+  void transferSpillInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
+                         VarLocMap &VarLocIDs, SpillMap &Spills);
   void transferRegisterDef(MachineInstr &MI, OpenRangesSet &OpenRanges,
                            const VarLocMap &VarLocIDs);
   bool transferTerminatorInst(MachineInstr &MI, OpenRangesSet &OpenRanges,
                               VarLocInMBB &OutLocs, const VarLocMap &VarLocIDs);
   bool transfer(MachineInstr &MI, OpenRangesSet &OpenRanges,
-                VarLocInMBB &OutLocs, VarLocMap &VarLocIDs);
+                VarLocInMBB &OutLocs, VarLocMap &VarLocIDs, SpillMap &Spills,
+                bool transferSpills);
 
   bool join(MachineBasicBlock &MBB, VarLocInMBB &OutLocs, VarLocInMBB &InLocs,
             const VarLocMap &VarLocIDs,
@@ -305,6 +321,21 @@ void LiveDebugValues::printVarLocInMBB(const MachineFunction &MF,
 }
 #endif
 
+/// Given a spill instruction, extract the register and offset used to
+/// address the spill location in a target independent way.
+int LiveDebugValues::extractSpillBaseRegAndOffset(const MachineInstr &MI,
+                                                  unsigned &Reg) {
+  assert(MI.hasOneMemOperand() &&
+         "Spill instruction does not have exactly one memory operand?");
+  auto MMOI = MI.memoperands_begin();
+  const PseudoSourceValue *PVal = (*MMOI)->getPseudoValue();
+  assert(PVal->kind() == PseudoSourceValue::FixedStack &&
+         "Inconsistent memory operand in spill instruction");
+  int FI = cast<FixedStackPseudoSourceValue>(PVal)->getFrameIndex();
+  const MachineBasicBlock *MBB = MI.getParent();
+  return TFI->getFrameIndexReference(*MBB->getParent(), FI, Reg);
+}
+
 /// End all previous ranges related to @MI and start a new range from @MI
 /// if it is a DBG_VALUE instr.
 void LiveDebugValues::transferDebugValue(const MachineInstr &MI,
@@ -362,6 +393,85 @@ void LiveDebugValues::transferRegisterDef(MachineInstr &MI,
   OpenRanges.erase(KillSet, VarLocIDs);
 }
 
+/// Decide if @MI is a spill instruction and return true if it is. We use 2
+/// criteria to make this decision:
+/// - Is this instruction a store to a spill slot?
+/// - Is there a register operand that is both used and killed?
+bool LiveDebugValues::isSpillInstruction(const MachineInstr &MI,
+                                         MachineFunction *MF, unsigned &Reg) {
+  const MachineFrameInfo &FrameInfo = MF->getFrameInfo();
+  int FI;
+  const MachineMemOperand *MMO;
+
+  // To identify a spill instruction, use the same criteria as in AsmPrinter.
+  if (!((TII->isStoreToStackSlotPostFE(MI, FI) ||
+         TII->hasStoreToStackSlot(MI, MMO, FI)) &&
+        FrameInfo.isSpillSlotObjectIndex(FI)))
+    return false;
+
+  // In a spill instruction generated by the InlineSpiller the spilled register
+  // has its kill flag set. Return false if we don't find such a register.
+  Reg = 0;
+  for (const MachineOperand &MO : MI.operands()) {
+    if (MO.isReg() && MO.isUse() && MO.isKill()) {
+      Reg = MO.getReg();
+      break;
+    }
+  }
+  return Reg != 0;
+}
+
+/// A spilled register may indicate that we have to end the current range of
+/// a variable and create a new one for the spill location.
+/// We don't want to insert any instructions in transfer(), so we just create
+/// the DBG_VALUE witout inserting it and keep track of it in @Spills.
+/// It will be inserted into the BB when we're done iterating over the
+/// instructions.
+void LiveDebugValues::transferSpillInst(MachineInstr &MI,
+                                        OpenRangesSet &OpenRanges,
+                                        VarLocMap &VarLocIDs,
+                                        SpillMap &Spills) {
+  unsigned Reg;
+  MachineFunction *MF = MI.getParent()->getParent();
+  if (!isSpillInstruction(MI, MF, Reg))
+    return;
+
+  // Check if the register is the location of a debug value.
+  for (unsigned ID : OpenRanges.getVarLocs()) {
+    if (VarLocIDs[ID].isDescribedByReg() == Reg) {
+      DEBUG(dbgs() << "Spilling Register " << PrintReg(Reg, TRI) << '('
+                   << VarLocIDs[ID].Var.getVar()->getName() << ")\n");
+
+      // Create a DBG_VALUE instruction to describe the Var in its spilled
+      // location, but don't insert it yet to avoid invalidating the
+      // iterator in our caller.
+      unsigned SpillBase;
+      int SpillOffset = extractSpillBaseRegAndOffset(MI, SpillBase);
+      const MachineInstr *DMI = &VarLocIDs[ID].MI;
+      MachineInstr *SpDMI =
+          BuildMI(*MF, DMI->getDebugLoc(), DMI->getDesc(), true, SpillBase, 0,
+                  DMI->getDebugVariable(), DMI->getDebugExpression());
+      SpDMI->getOperand(1).setImm(SpillOffset);
+      DEBUG(dbgs() << "Creating DBG_VALUE inst for spill: ";
+            SpDMI->print(dbgs(), false, TII));
+
+      // The newly created DBG_VALUE instruction SpDMI must be inserted after
+      // MI. Keep track of the pairing.
+      SpillDebugPair MIP = {&MI, SpDMI};
+      Spills.push_back(MIP);
+
+      // End all previous ranges of Var.
+      OpenRanges.erase(VarLocIDs[ID].Var);
+
+      // Add the VarLoc to OpenRanges.
+      VarLoc VL(*SpDMI, LS);
+      unsigned SpillLocID = VarLocIDs.insert(VL);
+      OpenRanges.insert(SpillLocID, VL.Var);
+      return;
+    }
+  }
+}
+
 /// Terminate all open ranges at the end of the current basic block.
 bool LiveDebugValues::transferTerminatorInst(MachineInstr &MI,
                                              OpenRangesSet &OpenRanges,
@@ -387,10 +497,13 @@ bool LiveDebugValues::transferTerminatorInst(MachineInstr &MI,
 
 /// This routine creates OpenRanges and OutLocs.
 bool LiveDebugValues::transfer(MachineInstr &MI, OpenRangesSet &OpenRanges,
-                               VarLocInMBB &OutLocs, VarLocMap &VarLocIDs) {
+                               VarLocInMBB &OutLocs, VarLocMap &VarLocIDs,
+                               SpillMap &Spills, bool transferSpills) {
   bool Changed = false;
   transferDebugValue(MI, OpenRanges, VarLocIDs);
   transferRegisterDef(MI, OpenRanges, VarLocIDs);
+  if (transferSpills)
+    transferSpillInst(MI, OpenRanges, VarLocIDs, Spills);
   Changed = transferTerminatorInst(MI, OpenRanges, OutLocs, VarLocIDs);
   return Changed;
 }
@@ -479,10 +592,11 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
   bool OLChanged = false;
   bool MBBJoined = false;
 
-  VarLocMap VarLocIDs;   // Map VarLoc<>unique ID for use in bitvectors.
+  VarLocMap VarLocIDs;      // Map VarLoc<>unique ID for use in bitvectors.
   OpenRangesSet OpenRanges; // Ranges that are open until end of bb.
-  VarLocInMBB OutLocs;   // Ranges that exist beyond bb.
-  VarLocInMBB InLocs;    // Ranges that are incoming after joining.
+  VarLocInMBB OutLocs;      // Ranges that exist beyond bb.
+  VarLocInMBB InLocs;       // Ranges that are incoming after joining.
+  SpillMap Spills;          // DBG_VALUEs associated with spills.
 
   DenseMap<unsigned int, MachineBasicBlock *> OrderToBB;
   DenseMap<MachineBasicBlock *, unsigned int> BBToOrder;
@@ -494,9 +608,14 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
       Pending;
 
   // Initialize every mbb with OutLocs.
+  // We are not looking at any spill instructions during the initial pass
+  // over the BBs. The LiveDebugVariables pass has already created DBG_VALUE
+  // instructions for spills of registers that are known to be user variables
+  // within the BB in which the spill occurs.
   for (auto &MBB : MF)
     for (auto &MI : MBB)
-      transfer(MI, OpenRanges, OutLocs, VarLocIDs);
+      transfer(MI, OpenRanges, OutLocs, VarLocIDs, Spills,
+               /*transferSpills=*/false);
 
   DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs, "OutLocs after initialization",
                          dbgs()));
@@ -528,8 +647,18 @@ bool LiveDebugValues::ExtendRanges(MachineFunction &MF) {
       if (MBBJoined) {
         MBBJoined = false;
         Changed = true;
+        // Now that we have started to extend ranges across BBs we need to
+        // examine spill instructions to see whether they spill registers that
+        // correspond to user variables.
         for (auto &MI : *MBB)
-          OLChanged |= transfer(MI, OpenRanges, OutLocs, VarLocIDs);
+          OLChanged |= transfer(MI, OpenRanges, OutLocs, VarLocIDs, Spills,
+                                /*transferSpills=*/true);
+
+        // Add any DBG_VALUE instructions necessitated by spills.
+        for (auto &SP : Spills)
+          MBB->insertAfter(MachineBasicBlock::iterator(*SP.SpillInst),
+                           SP.DebugInst);
+        Spills.clear();
 
         DEBUG(printVarLocInMBB(MF, OutLocs, VarLocIDs,
                                "OutLocs after propagating", dbgs()));
@@ -563,6 +692,7 @@ bool LiveDebugValues::runOnMachineFunction(MachineFunction &MF) {
 
   TRI = MF.getSubtarget().getRegisterInfo();
   TII = MF.getSubtarget().getInstrInfo();
+  TFI = MF.getSubtarget().getFrameLowering();
   LS.initialize(MF);
 
   bool Changed = ExtendRanges(MF);