Add a really quick hack at a machine code sinking pass, enabled with --enable-sinking.

It is missing validity checks, so it is known broken.  However, it is powerful enough
to compile this contrived code:

void test1(int C, double A, double B, double *P) {
  double Tmp = A*A+B*B;
  *P = C ? Tmp : A;
}

into:

_test1:
	movsd	8(%esp), %xmm0
	cmpl	$0, 4(%esp)
	je	LBB1_2	# entry
LBB1_1:	# entry
	movsd	16(%esp), %xmm1
	mulsd	%xmm1, %xmm1
	mulsd	%xmm0, %xmm0
	addsd	%xmm1, %xmm0
LBB1_2:	# entry
	movl	24(%esp), %eax
	movsd	%xmm0, (%eax)
	ret

instead of:

_test1:
	movsd	16(%esp), %xmm0
	mulsd	%xmm0, %xmm0
	movsd	8(%esp), %xmm1
	movapd	%xmm1, %xmm2
	mulsd	%xmm2, %xmm2
	addsd	%xmm0, %xmm2
	cmpl	$0, 4(%esp)
	je	LBB1_2	# entry
LBB1_1:	# entry
	movapd	%xmm2, %xmm1
LBB1_2:	# entry
	movl	24(%esp), %eax
	movsd	%xmm1, (%eax)
	ret

woo.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45570 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -157,6 +157,10 @@ namespace llvm {
   /// 
   FunctionPass *createMachineLICMPass();
 
+  /// createMachineSinkingPass - This pass performs sinking on machine
+  /// instructions.
+  FunctionPass *createMachineSinkingPass();
+  
 } // End llvm namespace
 
 #endif

lib/CodeGen/LLVMTargetMachine.cpp

@@ -29,6 +29,12 @@ static cl::opt<bool> PrintISelInput("print-isel-input", cl::Hidden,
 static cl::opt<bool> PrintEmittedAsm("print-emitted-asm", cl::Hidden,
     cl::desc("Dump emitter generated instructions as assembly"));
 
+// Hidden options to help debugging
+static cl::opt<bool>
+EnableSinking("enable-sinking", cl::init(false), cl::Hidden,
+              cl::desc("Perform sinking on machine code"));
+
+
 FileModel::Model
 LLVMTargetMachine::addPassesToEmitFile(FunctionPassManager &PM,
                                        std::ostream &Out,
@@ -68,6 +74,9 @@ LLVMTargetMachine::addPassesToEmitFile(FunctionPassManager &PM,
     PM.add(createMachineFunctionPrinterPass(cerr));
 
   PM.add(createMachineLICMPass());
+  
+  if (EnableSinking)
+    PM.add(createMachineSinkingPass());
 
   // Perform register allocation to convert to a concrete x86 representation
   PM.add(createRegisterAllocator());

lib/CodeGen/MachineSink.cpp

@@ -0,0 +1,206 @@
+//===-- MachineSink.cpp - Sinking for machine instructions ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass 
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "machine-sink"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/Target/MRegisterInfo.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+using namespace llvm;
+
+STATISTIC(NumSunk, "Number of machine instructions sunk");
+
+namespace {
+  class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass {
+    const TargetMachine   *TM;
+    const TargetInstrInfo *TII;
+    MachineFunction       *CurMF; // Current MachineFunction
+    MachineRegisterInfo  *RegInfo; // Machine register information
+    MachineDominatorTree *DT;   // Machine dominator tree for the current Loop
+
+  public:
+    static char ID; // Pass identification
+    MachineSinking() : MachineFunctionPass((intptr_t)&ID) {}
+    
+    virtual bool runOnMachineFunction(MachineFunction &MF);
+    
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      MachineFunctionPass::getAnalysisUsage(AU);
+      AU.addRequired<MachineDominatorTree>();
+      AU.addPreserved<MachineDominatorTree>();
+    }
+  private:
+    bool ProcessBlock(MachineBasicBlock &MBB);
+    bool SinkInstruction(MachineInstr *MI);
+    bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
+  };
+  
+  char MachineSinking::ID = 0;
+  RegisterPass<MachineSinking> X("machine-sink", "Machine code sinking");
+} // end anonymous namespace
+
+FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
+
+/// AllUsesDominatedByBlock - Return true if all uses of the specified register
+/// occur in blocks dominated by the specified block.
+bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg, 
+                                             MachineBasicBlock *MBB) const {
+  assert(MRegisterInfo::isVirtualRegister(Reg) && "Only makes sense for vregs");
+  for (MachineRegisterInfo::reg_iterator I = RegInfo->reg_begin(Reg),
+       E = RegInfo->reg_end(); I != E; ++I) {
+    if (I.getOperand().isDef()) continue;  // ignore def.
+    
+    // Determine the block of the use.
+    MachineInstr *UseInst = &*I;
+    MachineBasicBlock *UseBlock = UseInst->getParent();
+    if (UseInst->getOpcode() == TargetInstrInfo::PHI) {
+      // PHI nodes use the operand in the predecessor block, not the block with
+      // the PHI.
+      UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
+    }
+    // Check that it dominates.
+    if (!DT->dominates(MBB, UseBlock))
+      return false;
+  }
+  return true;
+}
+
+
+
+bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
+  DOUT << "******** Machine Sinking ********\n";
+  
+  CurMF = &MF;
+  TM = &CurMF->getTarget();
+  TII = TM->getInstrInfo();
+  RegInfo = &CurMF->getRegInfo();
+  DT = &getAnalysis<MachineDominatorTree>();
+
+  bool EverMadeChange = false;
+  
+  while (1) {
+    bool MadeChange = false;
+
+    // Process all basic blocks.
+    for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end(); 
+         I != E; ++I)
+      MadeChange |= ProcessBlock(*I);
+    
+    // If this iteration over the code changed anything, keep iterating.
+    if (!MadeChange) break;
+    EverMadeChange = true;
+  } 
+  return EverMadeChange;
+}
+
+bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
+  bool MadeChange = false;
+  
+  // Can't sink anything out of a block that has less than two successors.
+  if (MBB.succ_size() <= 1) return false;
+  
+  // Walk the basic block bottom-up
+  for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
+    MachineBasicBlock::iterator LastIt = I;
+    if (SinkInstruction(--I)) {
+      I = LastIt;
+      ++NumSunk;
+    }
+  }
+  
+  return MadeChange;
+}
+
+/// SinkInstruction - Determine whether it is safe to sink the specified machine
+/// instruction out of its current block into a successor.
+bool MachineSinking::SinkInstruction(MachineInstr *MI) {
+  // Loop over all the operands of the specified instruction.  If there is
+  // anything we can't handle, bail out.
+  MachineBasicBlock *ParentBlock = MI->getParent();
+  
+  // SuccToSinkTo - This is the successor to sink this instruction to, once we
+  // decide.
+  MachineBasicBlock *SuccToSinkTo = 0;
+  
+  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+    const MachineOperand &MO = MI->getOperand(i);
+    if (!MO.isReg()) continue;  // Ignore non-register operands.
+    
+    unsigned Reg = MO.getReg();
+    if (Reg == 0) continue;
+    
+    if (MRegisterInfo::isPhysicalRegister(Reg)) {
+      // If this is a physical register use, we can't move it.  If it is a def,
+      // we can move it, but only if the def is dead.
+      if (MO.isUse() || !MO.isDead())
+        return false;
+    } else {
+      // Virtual register uses are always safe to sink.
+      if (MO.isUse()) continue;
+      
+      // Virtual register defs can only be sunk if all their uses are in blocks
+      // dominated by one of the successors.
+      if (SuccToSinkTo) {
+        // If a previous operand picked a block to sink to, then this operand
+        // must be sinkable to the same block.
+        if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo)) 
+          return false;
+        continue;
+      }
+      
+      // Otherwise, we should look at all the successors and decide which one
+      // we should sink to.
+      for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
+           E = ParentBlock->succ_end(); SI != E; ++SI) {
+        if (AllUsesDominatedByBlock(Reg, *SI)) {
+          SuccToSinkTo = *SI;
+          break;
+        }
+      }
+      
+      // If we couldn't find a block to sink to, ignore this instruction.
+      if (SuccToSinkTo == 0)
+        return false;
+    }
+  }
+  
+  // FIXME: Check that the instr doesn't have side effects etc.
+  
+  DEBUG(cerr << "Sink instr " << *MI);
+  DEBUG(cerr << "to block " << *SuccToSinkTo);
+  
+  // If the block has multiple predecessors, this would introduce computation on
+  // a path that it doesn't already exist.  We could split the critical edge,
+  // but for now we just punt.
+  if (SuccToSinkTo->pred_size() > 1) {
+    DEBUG(cerr << " *** PUNTING: Critical edge found\n");
+    return false;
+  }
+  
+  // Determine where to insert into.  Skip phi nodes.
+  MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
+  while (InsertPos != SuccToSinkTo->end() && 
+         InsertPos->getOpcode() == TargetInstrInfo::PHI)
+    ++InsertPos;
+  
+  // Move the instruction.
+  SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
+                       ++MachineBasicBlock::iterator(MI));
+  return true;
+}