Fix the ExecutionDepsFix pass to handle AVX instructions.

This pass is needed to break false dependencies. Without it, unlucky
register assignment can result in wild (5x) swings in
performance. This pass was trying to handle AVX but not getting it
right. AVX doesn't have partial register defs, it has unused register
reads in which the high bits of a source operand are copied into the
unused bits of the dest.

Fixing this requires conservative liveness analysis. This is awkard
because the pass already has its own pseudo-liveness. However, proper
liveness is expensive, and we would like to use a generic utility to
compute it. The fix only invokes liveness on-demand. It is rare to
detect a case that needs undef-read dependence breaking, but when it
happens, it can be needed many times within a very large block.

I think the existing heuristic which uses a register window of 16 is
too conservative for loop-carried false dependencies. If the loop is a
reduction. The out-of-order engine may be able to execute several loop
iterations in parallel. However, I'll leave this tuning exercise for
next time.

llvm-svn: 192635

1 files changed, 93 insertions, 22 deletions

diff --git a/llvm/lib/CodeGen/ExecutionDepsFix.cpp b/llvm/lib/CodeGen/ExecutionDepsFix.cpp
index e277f5c664a..0d26f9d4cba 100644
--- a/llvm/lib/CodeGen/ExecutionDepsFix.cpp
+++ b/llvm/lib/CodeGen/ExecutionDepsFix.cpp
@@ -23,6 +23,7 @@
 #define DEBUG_TYPE "execution-fix"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Support/Allocator.h"
@@ -136,6 +137,12 @@ class ExeDepsFix : public MachineFunctionPass {
   typedef DenseMap<MachineBasicBlock*, LiveReg*> LiveOutMap;
   LiveOutMap LiveOuts;
 
+  /// List of undefined register reads in this block in forward order.
+  std::vector<std::pair<MachineInstr*, unsigned> > UndefReads;
+
+  /// Storage for register unit liveness.
+  LiveRegUnits LiveUnits;
+
   /// Current instruction number.
   /// The first instruction in each basic block is 0.
   int CurInstr;
@@ -185,6 +192,8 @@ private:
   void processDefs(MachineInstr*, bool Kill);
   void visitSoftInstr(MachineInstr*, unsigned mask);
   void visitHardInstr(MachineInstr*, unsigned domain);
+  bool shouldBreakDependence(MachineInstr*, unsigned OpIdx, unsigned Pref);
+  void processUndefReads(MachineBasicBlock*);
 };
 }
 
@@ -341,6 +350,10 @@ void ExeDepsFix::enterBasicBlock(MachineBasicBlock *MBB) {
   // Reset instruction counter in each basic block.
   CurInstr = 0;
 
+  // Set up UndefReads to track undefined register reads.
+  UndefReads.clear();
+  LiveUnits.clear();
+
   // Set up LiveRegs to represent registers entering MBB.
   if (!LiveRegs)
     LiveRegs = new LiveReg[NumRegs];
@@ -448,10 +461,46 @@ void ExeDepsFix::visitInstr(MachineInstr *MI) {
   processDefs(MI, !DomP.first);
 }
 
+/// \brief Return true to if it makes sense to break dependence on a partial def
+/// or undef use.
+bool ExeDepsFix::shouldBreakDependence(MachineInstr *MI, unsigned OpIdx,
+                                       unsigned Pref) {
+  int rx = regIndex(MI->getOperand(OpIdx).getReg());
+  if (rx < 0)
+    return false;
+
+  unsigned Clearance = CurInstr - LiveRegs[rx].Def;
+  DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
+
+  if (Pref > Clearance) {
+    DEBUG(dbgs() << ": Break dependency.\n");
+    return true;
+  }
+  // The current clearance seems OK, but we may be ignoring a def from a
+  // back-edge.
+  if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
+    DEBUG(dbgs() << ": OK .\n");
+    return false;
+  }
+  // A def from an unprocessed back-edge may make us break this dependency.
+  DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
+  return false;
+}
+
 // Update def-ages for registers defined by MI.
 // If Kill is set, also kill off DomainValues clobbered by the defs.
+//
+// Also break dependencies on partial defs and undef uses.
 void ExeDepsFix::processDefs(MachineInstr *MI, bool Kill) {
   assert(!MI->isDebugValue() && "Won't process debug values");
+
+  // Break dependence on undef uses. Do this before updating LiveRegs below.
+  unsigned OpNum;
+  unsigned Pref = TII->getUndefRegClearance(MI, OpNum, TRI);
+  if (Pref) {
+    if (shouldBreakDependence(MI, OpNum, Pref))
+      UndefReads.push_back(std::make_pair(MI, OpNum));
+  }
   const MCInstrDesc &MCID = MI->getDesc();
   for (unsigned i = 0,
          e = MI->isVariadic() ? MI->getNumOperands() : MCID.getNumDefs();
@@ -471,37 +520,56 @@ void ExeDepsFix::processDefs(MachineInstr *MI, bool Kill) {
     DEBUG(dbgs() << TRI->getName(RC->getRegister(rx)) << ":\t" << CurInstr
                  << '\t' << *MI);
 
+    // Check clearance before partial register updates.
+    // Call breakDependence before setting LiveRegs[rx].Def.
+    unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
+    if (Pref && shouldBreakDependence(MI, i, Pref))
+      TII->breakPartialRegDependency(MI, i, TRI);
+
     // How many instructions since rx was last written?
-    unsigned Clearance = CurInstr - LiveRegs[rx].Def;
     LiveRegs[rx].Def = CurInstr;
 
     // Kill off domains redefined by generic instructions.
     if (Kill)
       kill(rx);
+  }
+  ++CurInstr;
+}
 
-    // Verify clearance before partial register updates.
-    unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
-    if (!Pref)
-      continue;
-    DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
-    if (Pref > Clearance) {
-      DEBUG(dbgs() << ": Break dependency.\n");
-      TII->breakPartialRegDependency(MI, i, TRI);
-      continue;
-    }
-
-    // The current clearance seems OK, but we may be ignoring a def from a
-    // back-edge.
-    if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
-      DEBUG(dbgs() << ": OK.\n");
-      continue;
-    }
+/// \break Break false dependencies on undefined register reads.
+///
+/// Walk the block backward computing precise liveness. This is expensive, so we
+/// only do it on demand. Note that the occurrence of undefined register reads
+/// that should be broken is very rare, but when they occur we may have many in
+/// a single block.
+void ExeDepsFix::processUndefReads(MachineBasicBlock *MBB) {
+  if (UndefReads.empty())
+    return;
 
-    // A def from an unprocessed back-edge may make us break this dependency.
-    DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
+  // Collect this block's live out register units.
+  LiveUnits.init(TRI);
+  for (MachineBasicBlock::const_succ_iterator SI = MBB->succ_begin(),
+         SE = MBB->succ_end(); SI != SE; ++SI) {
+    LiveUnits.addLiveIns(*SI, *TRI);
   }
+  MachineInstr *UndefMI = UndefReads.back().first;
+  unsigned OpIdx = UndefReads.back().second;
 
-  ++CurInstr;
+  for (MachineBasicBlock::reverse_iterator I = MBB->rbegin(), E = MBB->rend();
+       I != E; ++I) {
+    if (UndefMI == &*I) {
+      if (!LiveUnits.contains(UndefMI->getOperand(OpIdx).getReg(), *TRI))
+        TII->breakPartialRegDependency(UndefMI, OpIdx, TRI);
+
+      UndefReads.pop_back();
+      if (UndefReads.empty())
+        return;
+
+      UndefMI = UndefReads.back().first;
+      OpIdx = UndefReads.back().second;
+    }
+    LiveUnits.stepBackward(*I, *TRI);
+  }
 }
 
 // A hard instruction only works in one domain. All input registers will be
@@ -549,7 +617,7 @@ void ExeDepsFix::visitSoftInstr(MachineInstr *mi, unsigned mask) {
         // Is it possible to use this collapsed register for free?
         if (dv->isCollapsed()) {
           // Restrict available domains to the ones in common with the operand.
-          // If there are no common domains, we must pay the cross-domain 
+          // If there are no common domains, we must pay the cross-domain
           // penalty for this operand.
           if (common) available = common;
         } else if (common)
@@ -686,6 +754,7 @@ bool ExeDepsFix::runOnMachineFunction(MachineFunction &mf) {
     for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
         ++I)
       visitInstr(I);
+    processUndefReads(MBB);
     leaveBasicBlock(MBB);
   }
 
@@ -698,6 +767,7 @@ bool ExeDepsFix::runOnMachineFunction(MachineFunction &mf) {
         ++I)
       if (!I->isDebugValue())
         processDefs(I, false);
+    processUndefReads(MBB);
     leaveBasicBlock(MBB);
   }
 
@@ -713,6 +783,7 @@ bool ExeDepsFix::runOnMachineFunction(MachineFunction &mf) {
     delete[] FI->second;
   }
   LiveOuts.clear();
+  UndefReads.clear();
   Avail.clear();
   Allocator.DestroyAll();