Teach machine-sink to break critical edges when appropriate. Work in progress.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111530 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineSink.cpp

@@ -26,11 +26,21 @@
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
+static cl::opt<bool> 
+SplitEdges("machine-sink-split",
+           cl::desc("Split critical edges during machine sinking"),
+           cl::init(false), cl::Hidden);
+static cl::opt<unsigned>
+SplitLimit("split-limit",
+           cl::init(~0u), cl::Hidden);
+
 STATISTIC(NumSunk,  "Number of machine instructions sunk");
+STATISTIC(NumSplit, "Number of critical edges split");
 
 namespace {
   class MachineSinking : public MachineFunctionPass {
@@ -59,6 +69,8 @@ namespace {
     }
   private:
     bool ProcessBlock(MachineBasicBlock &MBB);
+    MachineBasicBlock *SplitCriticalEdge(MachineBasicBlock *From,
+                                         MachineBasicBlock *To);
     bool SinkInstruction(MachineInstr *MI, bool &SawStore);
     bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB,
                                MachineBasicBlock *DefMBB, bool &LocalUse) const;
@@ -175,6 +187,66 @@ bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
   return MadeChange;
 }
 
+MachineBasicBlock *MachineSinking::SplitCriticalEdge(MachineBasicBlock *FromBB,
+                                                     MachineBasicBlock *ToBB) {
+  // Avoid breaking back edge. From == To means backedge for single BB loop.
+  if (!SplitEdges || NumSplit == SplitLimit || FromBB == ToBB)
+    return 0;
+
+  // Check for more "complex" loops.
+  if (LI->getLoopFor(FromBB) != LI->getLoopFor(ToBB) ||
+      !LI->isLoopHeader(ToBB)) {
+    // It's not always legal to break critical edges and sink the computation
+    // to the edge.
+    //
+    // BB#1:
+    // v1024
+    // Beq BB#3
+    // <fallthrough>
+    // BB#2:
+    // ... no uses of v1024
+    // <fallthrough>
+    // BB#3:
+    // ...
+    //       = v1024
+    //
+    // If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted:
+    //
+    // BB#1:
+    // ...
+    // Bne BB#2
+    // BB#4:
+    // v1024 =
+    // B BB#3
+    // BB#2:
+    // ... no uses of v1024
+    // <fallthrough>
+    // BB#3:
+    // ...
+    //       = v1024
+    //
+    // This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3
+    // flow. We need to ensure the new basic block where the computation is
+    // sunk to dominates all the uses.
+    // It's only legal to break critical edge and sink the computation to the
+    // new block if all the predecessors of "To", except for "From", are
+    // not dominated by "From". Given SSA property, this means these
+    // predecessors are dominated by "To".
+    for (MachineBasicBlock::pred_iterator PI = ToBB->pred_begin(),
+           E = ToBB->pred_end(); PI != E; ++PI) {
+      if (*PI == FromBB)
+        continue;
+      if (!DT->dominates(ToBB, *PI))
+        return 0;
+    }
+
+    // FIXME: Determine if it's cost effective to break this edge.
+    return FromBB->SplitCriticalEdge(ToBB, this);
+  }
+
+  return 0;
+}
+
 /// 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, bool &SawStore) {
@@ -316,27 +388,46 @@ bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
   if (SuccToSinkTo->pred_size() > 1) {
     // We cannot sink a load across a critical edge - there may be stores in
     // other code paths.
+    bool TryBreak = false;
     bool store = true;
     if (!MI->isSafeToMove(TII, AA, store)) {
-      DEBUG(dbgs() << " *** PUNTING: Wont sink load along critical edge.\n");
+      DEBUG(dbgs() << " *** PUNTING: Won't sink load along critical edge.\n");
-      return false;
+      TryBreak = true;
     }
 
     // We don't want to sink across a critical edge if we don't dominate the
     // successor. We could be introducing calculations to new code paths.
-    if (!DT->dominates(ParentBlock, SuccToSinkTo)) {
+    if (!TryBreak && !DT->dominates(ParentBlock, SuccToSinkTo)) {
       DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
-      return false;
+      TryBreak = true;
     }
 
     // Don't sink instructions into a loop.
-    if (LI->isLoopHeader(SuccToSinkTo)) {
+    if (!TryBreak && LI->isLoopHeader(SuccToSinkTo)) {
       DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
-      return false;
+      TryBreak = true;
     }
 
     // Otherwise we are OK with sinking along a critical edge.
+    if (!TryBreak)
       DEBUG(dbgs() << "Sinking along critical edge.\n");
+    else {
+      MachineBasicBlock *NewSucc = SplitCriticalEdge(ParentBlock, SuccToSinkTo);
+      if (!NewSucc) {
+        DEBUG(dbgs() <<
+              " *** PUNTING: Not legal or profitable to break critical edge\n");
+        return false;
+      } else {
+        DEBUG(dbgs() << "*** Splitting critical edge:"
+              " BB#" << ParentBlock->getNumber()
+              << " -- BB#" << NewSucc->getNumber()
+              << " -- BB#" << SuccToSinkTo->getNumber() << '\n');
+        //assert(DT->dominates(NewSucc, SuccToSinkTo) &&
+        //"New BB doesn't dominate all uses!");
+        SuccToSinkTo = NewSucc;
+        ++NumSplit;
+      }
+    }
   }
 
   // Determine where to insert into. Skip phi nodes.