Extend EarlyCSE to handle basic cases from JumpThreading and CVP

This patch extends EarlyCSE to take advantage of the information that a controlling branch gives us about the value of a Value within this and dominated basic blocks. If the current block has a single predecessor with a controlling branch, we can infer what the branch condition must have been to execute this block. The actual change to support this is downright simple because EarlyCSE's existing scoped hash table logic deals with most of the complexity around merging.

The patch actually implements two optimizations.
1) The first is analogous to JumpThreading in that it enables EarlyCSE's CSE handling to fold branches which are exactly redundant due to a previous branch to branches on constants. (It doesn't actually replace the branch or change the CFG.) This is pretty clearly a win since it enables substantial CFG simplification before we start trying to inline.
2) The second is analogous to CVP in that it exploits the knowledge gained to replace dominated *uses* of the original value. EarlyCSE does not otherwise reason about specific uses, so this is the more arguable one. It does enable further simplication and constant folding within the rest of the visit by EarlyCSE.

In both cases, the added code only handles the easy dominance based case of each optimization. The general case is deferred to the existing passes.

Differential Revision: http://reviews.llvm.org/D9763

llvm-svn: 238071

3 files changed, 46 insertions, 21 deletions

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index d5b9e03677f..d536a937dce 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -461,6 +461,30 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
   if (!BB->getSinglePredecessor())
     ++CurrentGeneration;
 
+  // If this node has a single predecessor which ends in a conditional branch,
+  // we can infer the value of the branch condition given that we took this
+  // path.  We need the single predeccesor to ensure there's not another path
+  // which reaches this block where the condition might hold a different
+  // value.  Since we're adding this to the scoped hash table (like any other
+  // def), it will have been popped if we encounter a future merge block.
+  if (BasicBlock *Pred = BB->getSinglePredecessor())
+    if (auto *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
+      if (BI->isConditional())
+        if (auto *CondInst = dyn_cast<Instruction>(BI->getCondition()))
+          if (SimpleValue::canHandle(CondInst)) {
+            assert(BI->getSuccessor(0) == BB || BI->getSuccessor(1) == BB);
+            auto *ConditionalConstant = (BI->getSuccessor(0) == BB) ?
+              ConstantInt::getTrue(BB->getContext()) :
+              ConstantInt::getFalse(BB->getContext());
+            AvailableValues.insert(CondInst, ConditionalConstant);
+            DEBUG(dbgs() << "EarlyCSE CVP: Add conditional value for '"
+                  << CondInst->getName() << "' as " << *ConditionalConstant
+                  << " in " << BB->getName() << "\n");
+            // Replace all dominated uses with the known value
+            replaceDominatedUsesWith(CondInst, ConditionalConstant, DT,
+                                     BasicBlockEdge(Pred, BB));
+          }
+
   /// LastStore - Keep track of the last non-volatile store that we saw... for
   /// as long as there in no instruction that reads memory.  If we see a store
   /// to the same location, we delete the dead store.  This zaps trivial dead
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 97d5b6d59fa..7770ddcb9d7 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -716,8 +716,6 @@ namespace {
     void verifyRemoved(const Instruction *I) const;
     bool splitCriticalEdges();
     BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);
-    unsigned replaceAllDominatedUsesWith(Value *From, Value *To,
-                                         const BasicBlockEdge &Root);
     bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root);
     bool processFoldableCondBr(BranchInst *BI);
     void addDeadBlock(BasicBlock *BB);
@@ -2033,23 +2031,6 @@ Value *GVN::findLeader(const BasicBlock *BB, uint32_t num) {
   return Val;
 }
 
-/// Replace all uses of 'From' with 'To' if the use is dominated by the given
-/// basic block.  Returns the number of uses that were replaced.
-unsigned GVN::replaceAllDominatedUsesWith(Value *From, Value *To,
-                                          const BasicBlockEdge &Root) {
-  unsigned Count = 0;
-  for (Value::use_iterator UI = From->use_begin(), UE = From->use_end();
-       UI != UE; ) {
-    Use &U = *UI++;
-
-    if (DT->dominates(Root, U)) {
-      U.set(To);
-      ++Count;
-    }
-  }
-  return Count;
-}
-
 /// There is an edge from 'Src' to 'Dst'.  Return
 /// true if every path from the entry block to 'Dst' passes via this edge.  In
 /// particular 'Dst' must not be reachable via another edge from 'Src'.
@@ -2126,7 +2107,7 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS,
     // LHS always has at least one use that is not dominated by Root, this will
     // never do anything if LHS has only one use.
     if (!LHS->hasOneUse()) {
-      unsigned NumReplacements = replaceAllDominatedUsesWith(LHS, RHS, Root);
+      unsigned NumReplacements = replaceDominatedUsesWith(LHS, RHS, *DT, Root);
       Changed |= NumReplacements > 0;
       NumGVNEqProp += NumReplacements;
     }
@@ -2198,7 +2179,7 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS,
         Value *NotCmp = findLeader(Root.getEnd(), Num);
         if (NotCmp && isa<Instruction>(NotCmp)) {
           unsigned NumReplacements =
-            replaceAllDominatedUsesWith(NotCmp, NotVal, Root);
+            replaceDominatedUsesWith(NotCmp, NotVal, *DT, Root);
           Changed |= NumReplacements > 0;
           NumGVNEqProp += NumReplacements;
         }
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 4d40ba842a0..70c77b06d62 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1343,3 +1343,23 @@ void llvm::combineMetadata(Instruction *K, const Instruction *J, ArrayRef<unsign
     }
   }
 }
+
+unsigned llvm::replaceDominatedUsesWith(Value *From, Value *To,
+                                        DominatorTree &DT,
+                                        const BasicBlockEdge &Root) {
+  assert(From->getType() == To->getType());
+  
+  unsigned Count = 0;
+  for (Value::use_iterator UI = From->use_begin(), UE = From->use_end();
+       UI != UE; ) {
+    Use &U = *UI++;
+    if (DT.dominates(Root, U)) {
+      U.set(To);
+      DEBUG(dbgs() << "Replace dominated use of '"
+            << From->getName() << "' as "
+            << *To << " in " << *U << "\n");
+      ++Count;
+    }
+  }
+  return Count;
+}