Add a little heuristic to Value::isUsedInBasicBlock to speed it up for small basic blocks.

- Calling getUser in a loop is much more expensive than iterating over a few instructions.
- Use it instead of the open-coded loop in AddrModeMatcher.
- 5% speedup on ARMDisassembler.cpp Release builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145810 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 14 insertions, 8 deletions

diff --git a/lib/Transforms/Utils/AddrModeMatcher.cpp b/lib/Transforms/Utils/AddrModeMatcher.cpp
index 8e5a1eb..d831452 100644
--- a/lib/Transforms/Utils/AddrModeMatcher.cpp
+++ b/lib/Transforms/Utils/AddrModeMatcher.cpp
@@ -473,14 +473,7 @@ bool AddressingModeMatcher::ValueAlreadyLiveAtInst(Value *Val,Value *KnownLive1,
   // Check to see if this value is already used in the memory instruction's
   // block.  If so, it's already live into the block at the very least, so we
   // can reasonably fold it.
-  BasicBlock *MemBB = MemoryInst->getParent();
-  for (Value::use_iterator UI = Val->use_begin(), E = Val->use_end();
-       UI != E; ++UI)
-    // We know that uses of arguments and instructions have to be instructions.
-    if (cast<Instruction>(*UI)->getParent() == MemBB)
-      return true;
-  
-  return false;
+  return Val->isUsedInBasicBlock(MemoryInst->getParent());
 }
 
 
diff --git a/lib/VMCore/Value.cpp b/lib/VMCore/Value.cpp
index 291df91..a5f1918 100644
--- a/lib/VMCore/Value.cpp
+++ b/lib/VMCore/Value.cpp
@@ -108,6 +108,19 @@ bool Value::hasNUsesOrMore(unsigned N) const {
 /// isUsedInBasicBlock - Return true if this value is used in the specified
 /// basic block.
 bool Value::isUsedInBasicBlock(const BasicBlock *BB) const {
+  // Start by scanning over the instructions looking for a use before we start
+  // the expensive use iteration.
+  unsigned MaxBlockSize = 3;
+  for (BasicBlock::const_iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+    if (std::find(I->op_begin(), I->op_end(), this) != I->op_end())
+      return true;
+    if (MaxBlockSize-- == 0) // If the block is larger fall back to use_iterator
+      break;
+  }
+
+  if (MaxBlockSize != 0) // We scanned the entire block and found no use.
+    return false;
+
   for (const_use_iterator I = use_begin(), E = use_end(); I != E; ++I) {
     const Instruction *User = dyn_cast<Instruction>(*I);
     if (User && User->getParent() == BB)