From 10be4f7ffd20dca88cfbdc29f36152b73960bd90 Mon Sep 17 00:00:00 2001
From: "whesse@chromium.org"
 <whesse@chromium.org@ce2b1a6d-e550-0410-aec6-3dcde31c8c00>
Date: Wed, 18 Mar 2009 10:31:31 +0000
Subject: [PATCH] Speed up the inner loop of free register allocation. Review
 URL: http://codereview.chromium.org/42296

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1528 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
---
 src/register-allocator.cc |  9 ++++-----
 src/register-allocator.h  | 16 +++++++++++++---
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/src/register-allocator.cc b/src/register-allocator.cc
index 7a96882..b702af4 100644
--- a/src/register-allocator.cc
+++ b/src/register-allocator.cc
@@ -83,11 +83,10 @@ void RegisterFile::CopyTo(RegisterFile* other) {
 
 Result RegisterAllocator::AllocateWithoutSpilling() {
   // Return the first free register, if any.
-  for (int i = 0; i < kNumRegisters; i++) {
-    if (!is_used(i)) {
-      Register free_reg = { i };
-      return Result(free_reg, cgen_);
-    }
+  int free_reg = registers_.ScanForFreeRegister();
+  if (free_reg < kNumRegisters) {
+    Register free_result = { free_reg };
+    return Result(free_result, cgen_);
   }
   return Result(cgen_);
 }
diff --git a/src/register-allocator.h b/src/register-allocator.h
index 759ab14..42eb6e0 100644
--- a/src/register-allocator.h
+++ b/src/register-allocator.h
@@ -149,10 +149,9 @@ class RegisterFile BASE_EMBEDDED {
   // Record that a register will no longer be used by decrementing its
   // reference count.
   void Unuse(Register reg) {
+    ASSERT(!reg.is(no_reg));
     ASSERT(is_used(reg.code()));
-    if (is_used(reg.code())) {
-      ref_counts_[reg.code()]--;
-    }
+    ref_counts_[reg.code()]--;
   }
 
   // Copy the reference counts from this register file to the other.
@@ -161,6 +160,17 @@ class RegisterFile BASE_EMBEDDED {
  private:
   int ref_counts_[kNumRegisters];
 
+  // Very fast inlined loop to find a free register.
+  // Used in RegisterAllocator::AllocateWithoutSpilling.
+  // Returns kNumRegisters if no free register found.
+  inline int ScanForFreeRegister() {
+    int i = 0;
+    for (; i < kNumRegisters ; ++i) {
+      if (ref_counts_[i] == 0) break;
+    }
+    return i;
+  }
+
   friend class RegisterAllocator;
 };
 
-- 
2.7.4