SimplifyCFG: If convert single conditional stores

This transformation will transform a conditional store with a preceeding
uncondtional store to the same location:

 a[i] =
 may-alias with a[i] load
 if (cond)
   a[i] = Y

into an unconditional store.

 a[i] = X
 may-alias with a[i] load
 tmp = cond ? Y : X;
 a[i] = tmp

We assume that on average the cost of a mispredicted branch is going to be
higher than the cost of a second store to the same location, and that the
secondary benefits of creating a bigger basic block for other optimizations to
work on outway the potential case were the branch would be correctly predicted
and the cost of the executing the second store would be noticably reflected in
performance.

hmmer's execution time improves by 30% on an imac12,2 on ref data sets. With
this change we are on par with gcc's performance (gcc also performs this
transformation). There was a 1.2 % performance improvement on a ARM swift chip.
Other tests in the test-suite+external seem to be mostly uninfluenced in my
experiments:
This optimization was triggered on 41 tests such that the executable was
different before/after the patch. Only 1 out of the 40 tests (dealII) was
reproducable below 100% (by about .4%). Given that hmmer benefits so much I
believe this to be a fair trade off.

I am going to watch performance numbers across the builtbots and will revert
this if anything unexpected comes up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179957 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 88 insertions, 4 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 6de602e..88c44a7 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -59,6 +59,10 @@ static cl::opt<bool>
 SinkCommon("simplifycfg-sink-common", cl::Hidden, cl::init(true),
        cl::desc("Sink common instructions down to the end block"));
 
+static cl::opt<bool>
+HoistCondStores("simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true),
+       cl::desc("Hoist conditional stores if an unconditional store preceeds"));
+
 STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
 STATISTIC(NumLookupTables, "Number of switch instructions turned into lookup tables");
 STATISTIC(NumSinkCommons, "Number of common instructions sunk down to the end block");
@@ -1332,6 +1336,64 @@ static bool SinkThenElseCodeToEnd(BranchInst *BI1) {
   return Changed;
 }
 
+/// \brief Determine if we can hoist sink a sole store instruction out of a
+/// conditional block.
+///
+/// We are looking for code like the following:
+///   BrBB:
+///     store i32 %add, i32* %arrayidx2
+///     ... // No other stores or function calls (we could be calling a memory
+///     ... // function).
+///     %cmp = icmp ult %x, %y
+///     br i1 %cmp, label %EndBB, label %ThenBB
+///   ThenBB:
+///     store i32 %add5, i32* %arrayidx2
+///     br label EndBB
+///   EndBB:
+///     ...
+///   We are going to transform this into:
+///   BrBB:
+///     store i32 %add, i32* %arrayidx2
+///     ... //
+///     %cmp = icmp ult %x, %y
+///     %add.add5 = select i1 %cmp, i32 %add, %add5
+///     store i32 %add.add5, i32* %arrayidx2
+///     ...
+///
+/// \return The pointer to the value of the previous store if the store can be
+///         hoisted into the predecessor block. 0 otherwise.
+Value *isSafeToSpeculateStore(Instruction *I, BasicBlock *BrBB,
+                              BasicBlock *StoreBB, BasicBlock *EndBB) {
+  StoreInst *StoreToHoist = dyn_cast<StoreInst>(I);
+  if (!StoreToHoist)
+    return 0;
+
+  Value *StorePtr = StoreToHoist->getPointerOperand();
+  StoreInst *PreviousStore = 0;
+
+  // Look for a store to the same pointer in BrBB.
+  unsigned MaxNumInstToLookAt = 10;
+  for (BasicBlock::reverse_iterator RI = BrBB->rbegin(),
+       RE = BrBB->rend(); RI != RE && (--MaxNumInstToLookAt); ++RI) {
+    Instruction *CurI = &*RI;
+
+    // Could be calling an instruction that effects memory like free().
+    if (CurI->mayHaveSideEffects() && !isa<StoreInst>(CurI))
+      return 0;
+
+    // Found the previous store make sure it stores to the same location.
+    if ((PreviousStore = dyn_cast<StoreInst>(CurI))) {
+      if (PreviousStore->getPointerOperand() == StorePtr)
+        // Found the previous store, return its value operand.
+        return PreviousStore->getValueOperand();
+      else
+        return 0; // Unknown store.
+    }
+  }
+
+  return 0;
+}
+
 /// \brief Speculate a conditional basic block flattening the CFG.
 ///
 /// Note that this is a very risky transform currently. Speculating
@@ -1395,6 +1457,8 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
   SmallDenseMap<Instruction *, unsigned, 4> SinkCandidateUseCounts;
 
   unsigned SpeculationCost = 0;
+  Value *SpeculatedStoreValue = 0;
+  StoreInst *SpeculatedStore = 0;
   for (BasicBlock::iterator BBI = ThenBB->begin(),
                             BBE = llvm::prior(ThenBB->end());
        BBI != BBE; ++BBI) {
@@ -1410,13 +1474,21 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
       return false;
 
     // Don't hoist the instruction if it's unsafe or expensive.
-    if (!isSafeToSpeculativelyExecute(I))
+    if (!isSafeToSpeculativelyExecute(I) &&
+        !(HoistCondStores &&
+          (SpeculatedStoreValue = isSafeToSpeculateStore(I, BB, ThenBB,
+                                                         EndBB))))
       return false;
-    if (ComputeSpeculationCost(I) > PHINodeFoldingThreshold)
+    if (!SpeculatedStoreValue &&
+        ComputeSpeculationCost(I) > PHINodeFoldingThreshold)
       return false;
 
+    // Store the store speculation candidate.
+    if (SpeculatedStoreValue)
+      SpeculatedStore = cast<StoreInst>(I);
+
     // Do not hoist the instruction if any of its operands are defined but not
-    // used in this BB. The transformation will prevent the operand from
+    // used in BB. The transformation will prevent the operand from
     // being sunk into the use block.
     for (User::op_iterator i = I->op_begin(), e = I->op_end();
          i != e; ++i) {
@@ -1473,12 +1545,24 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
 
   // If there are no PHIs to process, bail early. This helps ensure idempotence
   // as well.
-  if (!HaveRewritablePHIs)
+  if (!HaveRewritablePHIs && !(HoistCondStores && SpeculatedStoreValue))
     return false;
 
   // If we get here, we can hoist the instruction and if-convert.
   DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";);
 
+  // Insert a select of the value of the speculated store.
+  if (SpeculatedStoreValue) {
+    IRBuilder<true, NoFolder> Builder(BI);
+    Value *TrueV = SpeculatedStore->getValueOperand();
+    Value *FalseV = SpeculatedStoreValue;
+    if (Invert)
+      std::swap(TrueV, FalseV);
+    Value *S = Builder.CreateSelect(BrCond, TrueV, FalseV, TrueV->getName() +
+                                    "." + FalseV->getName());
+    SpeculatedStore->setOperand(0, S);
+  }
+
   // Hoist the instructions.
   BB->getInstList().splice(BI, ThenBB->getInstList(), ThenBB->begin(),
                            llvm::prior(ThenBB->end()));