Fix bug in PEI's virtual-register scavenging

This change fixes a bug that I introduced in r178058. After a register is
scavenged using one of the available spills slots the instruction defining the
virtual register needs to be moved to after the spill code. The scavenger has
already processed the defining instruction so that registers killed by that
instruction are available for definition in that same instruction. Unfortunately,
after this, the scavenger needs to iterate through the spill code and then
visit, again, the instruction that defines the now-scavenged register. In order
to avoid confusion, the register scavenger needs the ability to 'back up'
through the spill code so that it can again process the instructions in the
appropriate order. Prior to this fix, once the scavenger reached the
just-moved instruction, it would assert if it killed any registers because,
having already processed the instruction, it believed they were undefined.

Unfortunately, I don't yet have a small test case. Thanks to Pranav Bhandarkar
for diagnosing the problem and testing this fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178845 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/RegisterScavenging.h

@@ -93,6 +93,15 @@ public:
     while (MBBI != I) forward();
   }
 
+  /// Invert the behavior of forward() on the current instruction (undo the
+  /// changes to the available registers made by forward()).
+  void unprocess();
+
+  /// Unprocess instructions until you reach the provided iterator.
+  void unprocess(MachineBasicBlock::iterator I) {
+    while (MBBI != I) unprocess();
+  }
+
   /// skipTo - Move the internal MBB iterator but do not update register states.
   void skipTo(MachineBasicBlock::iterator I) {
     if (I == MachineBasicBlock::iterator(NULL))
@@ -100,6 +109,10 @@ public:
     MBBI = I;
   }
 
+  MachineBasicBlock::iterator getCurrentPosition() const {
+    return MBBI;
+  }
+
   /// getRegsUsed - return all registers currently in use in used.
   void getRegsUsed(BitVector &used, bool includeReserved);
 
@@ -171,6 +184,10 @@ private:
     RegsAvailable |= Regs;
   }
 
+  /// Processes the current instruction and fill the KillRegs and DefRegs bit
+  /// vectors.
+  void determineKillsAndDefs();
+
   /// Add Reg and all its sub-registers to BV.
   void addRegWithSubRegs(BitVector &BV, unsigned Reg);
 

lib/CodeGen/PrologEpilogInserter.cpp

@@ -826,6 +826,8 @@ void PEI::scavengeFrameVirtualRegs(MachineFunction &Fn) {
     for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ) {
       MachineInstr *MI = I;
       MachineBasicBlock::iterator J = llvm::next(I);
+      MachineBasicBlock::iterator P = I == BB->begin() ?
+        MachineBasicBlock::iterator(NULL) : llvm::prior(I);
 
       // RS should process this instruction before we might scavenge at this
       // location. This is because we might be replacing a virtual register
@@ -869,8 +871,20 @@ void PEI::scavengeFrameVirtualRegs(MachineFunction &Fn) {
       // problem because we need the spill code before I: Move I to just
       // prior to J.
       if (I != llvm::prior(J)) {
-        BB->splice(J, BB, I++);
-        RS->skipTo(I == BB->begin() ? NULL : llvm::prior(I));
+        BB->splice(J, BB, I);
+
+        // Before we move I, we need to prepare the RS to visit I again.
+        // Specifically, RS will assert if it sees uses of registers that
+        // it believes are undefined. Because we have already processed
+        // register kills in I, when it visits I again, it will believe that
+        // those registers are undefined. To avoid this situation, unprocess
+        // the instruction I.
+        assert(RS->getCurrentPosition() == I &&
+          "The register scavenger has an unexpected position");
+        I = P;
+        RS->unprocess(P);
+
+        // RS->skipTo(I == BB->begin() ? NULL : llvm::prior(I));
       } else
         ++I;
     }

lib/CodeGen/RegisterScavenging.cpp

@@ -110,30 +110,11 @@ void RegScavenger::addRegWithSubRegs(BitVector &BV, unsigned Reg) {
     BV.set(*SubRegs);
 }
 
-void RegScavenger::forward() {
-  // Move ptr forward.
-  if (!Tracking) {
-    MBBI = MBB->begin();
-    Tracking = true;
-  } else {
-    assert(MBBI != MBB->end() && "Already past the end of the basic block!");
-    MBBI = llvm::next(MBBI);
-  }
-  assert(MBBI != MBB->end() && "Already at the end of the basic block!");
+void RegScavenger::determineKillsAndDefs() {
+  assert(Tracking && "Must be tracking to determine kills and defs");
 
   MachineInstr *MI = MBBI;
-
-  for (SmallVector<ScavengedInfo, 2>::iterator I = Scavenged.begin(),
-       IE = Scavenged.end(); I != IE; ++I) {
-    if (I->Restore != MI)
-      continue;
-
-    I->Reg = 0;
-    I->Restore = NULL;
-  }
-
-  if (MI->isDebugValue())
-    return;
+  assert(!MI->isDebugValue() && "Debug values have no kills or defs");
 
   // Find out which registers are early clobbered, killed, defined, and marked
   // def-dead in this instruction.
@@ -167,6 +148,54 @@ void RegScavenger::forward() {
         addRegWithSubRegs(DefRegs, Reg);
     }
   }
+}
+
+void RegScavenger::unprocess() {
+  assert(Tracking && "Cannot unprocess because we're not tracking");
+
+  MachineInstr *MI = MBBI;
+  if (MI->isDebugValue())
+    return;
+
+  determineKillsAndDefs();
+
+  // Commit the changes.
+  setUsed(KillRegs);
+  setUnused(DefRegs);
+
+  if (MBBI == MBB->begin()) {
+    MBBI = MachineBasicBlock::iterator(NULL);
+    Tracking = false;
+  } else
+    --MBBI;
+}
+
+void RegScavenger::forward() {
+  // Move ptr forward.
+  if (!Tracking) {
+    MBBI = MBB->begin();
+    Tracking = true;
+  } else {
+    assert(MBBI != MBB->end() && "Already past the end of the basic block!");
+    MBBI = llvm::next(MBBI);
+  }
+  assert(MBBI != MBB->end() && "Already at the end of the basic block!");
+
+  MachineInstr *MI = MBBI;
+
+  for (SmallVector<ScavengedInfo, 2>::iterator I = Scavenged.begin(),
+       IE = Scavenged.end(); I != IE; ++I) {
+    if (I->Restore != MI)
+      continue;
+
+    I->Reg = 0;
+    I->Restore = NULL;
+  }
+
+  if (MI->isDebugValue())
+    return;
+
+  determineKillsAndDefs();
 
   // Verify uses and defs.
 #ifndef NDEBUG