Revert r359906, "RegAllocFast: Add heuristic to detect values not live-out of a block"

Makes clang/test/Misc/backend-stack-frame-diagnostics-fallback.cpp fail.

llvm-svn: 359912

1 files changed, 4 insertions, 41 deletions

diff --git a/llvm/lib/CodeGen/RegAllocFast.cpp b/llvm/lib/CodeGen/RegAllocFast.cpp
index c11ae9cce1a..a711db1c0eb 100644
--- a/llvm/lib/CodeGen/RegAllocFast.cpp
+++ b/llvm/lib/CodeGen/RegAllocFast.cpp
@@ -101,10 +101,6 @@ namespace {
 
     DenseMap<unsigned, SmallVector<MachineInstr *, 2>> LiveDbgValueMap;
 
-    /// Has a bit set for every virtual register for which it was determined
-    /// that it is alive across blocks.
-    BitVector MayLiveAcrossBlocks;
-
     /// State of a physical register.
     enum RegState {
       /// A disabled register is not available for allocation, but an alias may
@@ -212,7 +208,7 @@ namespace {
                             unsigned Hint);
     LiveReg &reloadVirtReg(MachineInstr &MI, unsigned OpNum, unsigned VirtReg,
                            unsigned Hint);
-    void spillAll(MachineBasicBlock::iterator MI, bool OnlyLiveOut);
+    void spillAll(MachineBasicBlock::iterator MI);
     bool setPhysReg(MachineInstr &MI, MachineOperand &MO, MCPhysReg PhysReg);
 
     int getStackSpaceFor(unsigned VirtReg);
@@ -221,8 +217,6 @@ namespace {
     void reload(MachineBasicBlock::iterator Before, unsigned VirtReg,
                 MCPhysReg PhysReg);
 
-    bool mayLiveOut(unsigned VirtReg);
-
     void dumpState();
   };
 
@@ -257,33 +251,6 @@ int RegAllocFast::getStackSpaceFor(unsigned VirtReg) {
   return FrameIdx;
 }
 
-/// Returns false if \p VirtReg is known to not live out of the current block.
-bool RegAllocFast::mayLiveOut(unsigned VirtReg) {
-  if (MayLiveAcrossBlocks.test(TargetRegisterInfo::virtReg2Index(VirtReg))) {
-    // Cannot be live-out if there are no successors.
-    return !MBB->succ_empty();
-  }
-
-  // If this block loops back to itself, it would be necessary to check whether
-  // the use comes after the def.
-  if (MBB->isSuccessor(MBB))
-    return true;
-
-  // See if the first \p Limit uses of the register are all in the current
-  // block.
-  static const unsigned Limit = 8;
-  unsigned C = 0;
-  for (const MachineInstr &UseInst : MRI->reg_nodbg_instructions(VirtReg)) {
-    if (UseInst.getParent() != MBB || ++C >= Limit) {
-      MayLiveAcrossBlocks.set(TargetRegisterInfo::virtReg2Index(VirtReg));
-      // Cannot be live-out if there are no successors.
-      return !MBB->succ_empty();
-    }
-  }
-
-  return false;
-}
-
 /// Insert spill instruction for \p AssignedReg before \p Before. Update
 /// DBG_VALUEs with \p VirtReg operands with the stack slot.
 void RegAllocFast::spill(MachineBasicBlock::iterator Before, unsigned VirtReg,
@@ -407,7 +374,7 @@ void RegAllocFast::spillVirtReg(MachineBasicBlock::iterator MI, LiveReg &LR) {
 }
 
 /// Spill all dirty virtregs without killing them.
-void RegAllocFast::spillAll(MachineBasicBlock::iterator MI, bool OnlyLiveOut) {
+void RegAllocFast::spillAll(MachineBasicBlock::iterator MI) {
   if (LiveVirtRegs.empty())
     return;
   // The LiveRegMap is keyed by an unsigned (the virtreg number), so the order
@@ -415,8 +382,6 @@ void RegAllocFast::spillAll(MachineBasicBlock::iterator MI, bool OnlyLiveOut) {
   for (LiveReg &LR : LiveVirtRegs) {
     if (!LR.PhysReg)
       continue;
-    if (OnlyLiveOut && !mayLiveOut(LR.VirtReg))
-      continue;
     spillVirtReg(MI, LR);
   }
   LiveVirtRegs.clear();
@@ -1054,7 +1019,7 @@ void RegAllocFast::allocateInstruction(MachineInstr &MI) {
     // definitions may be used later on and we do not want to reuse
     // those for virtual registers in between.
     LLVM_DEBUG(dbgs() << "  Spilling remaining registers before call.\n");
-    spillAll(MI, /*OnlyLiveOut*/ false);
+    spillAll(MI);
   }
 
   // Third scan.
@@ -1164,7 +1129,7 @@ void RegAllocFast::allocateBasicBlock(MachineBasicBlock &MBB) {
 
   // Spill all physical registers holding virtual registers now.
   LLVM_DEBUG(dbgs() << "Spilling live registers at end of block.\n");
-  spillAll(MBB.getFirstTerminator(), /*OnlyLiveOut*/ true);
+  spillAll(MBB.getFirstTerminator());
 
   // Erase all the coalesced copies. We are delaying it until now because
   // LiveVirtRegs might refer to the instrs.
@@ -1193,8 +1158,6 @@ bool RegAllocFast::runOnMachineFunction(MachineFunction &MF) {
   unsigned NumVirtRegs = MRI->getNumVirtRegs();
   StackSlotForVirtReg.resize(NumVirtRegs);
   LiveVirtRegs.setUniverse(NumVirtRegs);
-  MayLiveAcrossBlocks.clear();
-  MayLiveAcrossBlocks.resize(NumVirtRegs);
 
   // Loop over all of the basic blocks, eliminating virtual register references
   for (MachineBasicBlock &MBB : MF)