[x86] generalize reassociation optimization in machine combiner to 2 instructions

[oota-llvm.git] / lib / CodeGen / MachineCombiner.cpp

diff --git a/lib/CodeGen/MachineCombiner.cpp b/lib/CodeGen/MachineCombiner.cpp
index 5019e8eef19b45cf9e33d44891854a83b913a2ef..66b9fce4e460eb06972bbe7f38a81ad8b74491c7 100644 (file)
--- a/lib/CodeGen/MachineCombiner.cpp
+++ b/lib/CodeGen/MachineCombiner.cpp
@@ -67,10 +67,11 @@ private:
   unsigned getLatency(MachineInstr *Root, MachineInstr *NewRoot,
                       MachineTraceMetrics::Trace BlockTrace);
   bool
-  preservesCriticalPathLen(MachineBasicBlock *MBB, MachineInstr *Root,
+  improvesCriticalPathLen(MachineBasicBlock *MBB, MachineInstr *Root,
                            MachineTraceMetrics::Trace BlockTrace,
                            SmallVectorImpl<MachineInstr *> &InsInstrs,
-                           DenseMap<unsigned, unsigned> &InstrIdxForVirtReg);
+                           DenseMap<unsigned, unsigned> &InstrIdxForVirtReg,
+                           bool NewCodeHasLessInsts);
   bool preservesResourceLen(MachineBasicBlock *MBB,
                             MachineTraceMetrics::Trace BlockTrace,
                             SmallVectorImpl<MachineInstr *> &InsInstrs,
@@ -208,19 +209,24 @@ unsigned MachineCombiner::getLatency(MachineInstr *Root, MachineInstr *NewRoot,
   return NewRootLatency;
 }
 
-/// True when the new instruction sequence does not
-/// lengthen the critical path. The DAGCombine code sequence ends in MI
-/// (Machine Instruction) Root. The new code sequence ends in MI NewRoot. A
-/// necessary condition for the new sequence to replace the old sequence is that
-/// it cannot lengthen the critical path. This is decided by the formula
+/// True when the new instruction sequence does not lengthen the critical path
+/// and the new sequence has less instructions or the new sequence improves the
+/// critical path.
+/// The DAGCombine code sequence ends in MI (Machine Instruction) Root.
+/// The new code sequence ends in MI NewRoot. A necessary condition for the new
+/// sequence to replace the old sequence is that it cannot lengthen the critical
+/// path. This is decided by the formula:
 /// (NewRootDepth + NewRootLatency) <= (RootDepth + RootLatency + RootSlack)).
-/// The slack is the number of cycles Root can be delayed before the critical
-/// patch becomes longer.
-bool MachineCombiner::preservesCriticalPathLen(
+/// If the new sequence has an equal length critical path but does not reduce
+/// the number of instructions (NewCodeHasLessInsts is false), then it is not
+/// considered an improvement. The slack is the number of cycles Root can be
+/// delayed before the critical patch becomes longer.
+bool MachineCombiner::improvesCriticalPathLen(
     MachineBasicBlock *MBB, MachineInstr *Root,
     MachineTraceMetrics::Trace BlockTrace,
     SmallVectorImpl<MachineInstr *> &InsInstrs,
-    DenseMap<unsigned, unsigned> &InstrIdxForVirtReg) {
+    DenseMap<unsigned, unsigned> &InstrIdxForVirtReg,
+    bool NewCodeHasLessInsts) {
 
   assert(TSchedModel.hasInstrSchedModel() && "Missing machine model\n");
   // NewRoot is the last instruction in the \p InsInstrs vector.
@@ -245,9 +251,13 @@ bool MachineCombiner::preservesCriticalPathLen(
         dbgs() << " RootDepth + RootLatency + RootSlack "
                << RootDepth + RootLatency + RootSlack << "\n";);
 
-  /// True when the new sequence does not lengthen the critical path.
-  return ((NewRootDepth + NewRootLatency) <=
-          (RootDepth + RootLatency + RootSlack));
+  unsigned NewCycleCount = NewRootDepth + NewRootLatency;
+  unsigned OldCycleCount = RootDepth + RootLatency + RootSlack;
+  
+  if (NewCodeHasLessInsts)
+    return NewCycleCount <= OldCycleCount;
+  else
+    return NewCycleCount < OldCycleCount;
 }
 
 /// helper routine to convert instructions into SC
@@ -359,18 +369,21 @@ bool MachineCombiner::combineInstructions(MachineBasicBlock *MBB) {
         Traces->verifyAnalysis();
         TII->genAlternativeCodeSequence(MI, P, InsInstrs, DelInstrs,
                                         InstrIdxForVirtReg);
+        unsigned NewInstCount = InsInstrs.size();
+        unsigned OldInstCount = DelInstrs.size();
         // Found pattern, but did not generate alternative sequence.
         // This can happen e.g. when an immediate could not be materialized
         // in a single instruction.
-        if (!InsInstrs.size())
+        if (!NewInstCount)
           continue;
         // Substitute when we optimize for codesize and the new sequence has
         // fewer instructions OR
         // the new sequence neither lengthens the critical path nor increases
         // resource pressure.
-        if (doSubstitute(InsInstrs.size(), DelInstrs.size()) ||
-            (preservesCriticalPathLen(MBB, &MI, BlockTrace, InsInstrs,
-                                      InstrIdxForVirtReg) &&
+        if (doSubstitute(NewInstCount, OldInstCount) ||
+            (improvesCriticalPathLen(MBB, &MI, BlockTrace, InsInstrs,
+                                      InstrIdxForVirtReg,
+                                      NewInstCount < OldInstCount) &&
              preservesResourceLen(MBB, BlockTrace, InsInstrs, DelInstrs))) {
           for (auto *InstrPtr : InsInstrs)
             MBB->insert((MachineBasicBlock::iterator) &MI, InstrPtr);