[TurboProp] Optimize BitVector::Iterator::Advance.

Optimizes BitVector::Iterator::Advance by using base::bits::CountTrailingZeros to
skip through bitvector. Also inlines Advance in the header. This reduces the
LiveRangeAnalysis phase of TurboFan/Prop by about 2-5% on Octane.

BUG=v8:9684

Change-Id: I3954d50d8ae9bd062a153e1fa2cb0abfd43d73eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1910948
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64920}

src/utils/bit-vector.cc

@@ -25,21 +25,6 @@ void BitVector::Print() {
 }
 #endif
 
-void BitVector::Iterator::Advance() {
-  current_++;
-  uintptr_t val = current_value_;
-  while (val == 0) {
-    current_index_++;
-    if (Done()) return;
-    DCHECK(!target_->is_inline());
-    val = target_->data_.ptr_[current_index_];
-    current_ = current_index_ << kDataBitShift;
-  }
-  val = SkipZeroBytes(val);
-  val = SkipZeroBits(val);
-  current_value_ = val >> 1;
-}
-
 int BitVector::Count() const {
   if (is_inline()) return base::bits::CountPopulation(data_.inline_);
 

src/utils/bit-vector.h

@@ -5,6 +5,7 @@
 #ifndef V8_UTILS_BIT_VECTOR_H_
 #define V8_UTILS_BIT_VECTOR_H_
 
+#include "src/base/bits.h"
 #include "src/utils/allocation.h"
 #include "src/zone/zone.h"
 
@@ -34,7 +35,27 @@ class V8_EXPORT_PRIVATE BitVector : public ZoneObject {
     ~Iterator() = default;
 
     bool Done() const { return current_index_ >= target_->data_length_; }
-    V8_EXPORT_PRIVATE void Advance();
+
+    V8_EXPORT_PRIVATE inline void Advance() {
+      current_++;
+
+      // Skip zeroed words.
+      while (current_value_ == 0) {
+        current_index_++;
+        if (Done()) return;
+        DCHECK(!target_->is_inline());
+        current_value_ = target_->data_.ptr_[current_index_];
+        current_ = current_index_ << kDataBitShift;
+      }
+
+      // Skip zeroed bits.
+      uintptr_t trailing_zeros = base::bits::CountTrailingZeros(current_value_);
+      current_ += trailing_zeros;
+      current_value_ >>= trailing_zeros;
+
+      // Get current_value ready for next advance.
+      current_value_ >>= 1;
+    }
 
     int Current() const {
       DCHECK(!Done());
@@ -42,19 +63,10 @@ class V8_EXPORT_PRIVATE BitVector : public ZoneObject {
     }
 
    private:
-    uintptr_t SkipZeroBytes(uintptr_t val) {
-      while ((val & 0xFF) == 0) {
-        val >>= 8;
-        current_ += 8;
-      }
-      return val;
-    }
     uintptr_t SkipZeroBits(uintptr_t val) {
-      while ((val & 0x1) == 0) {
-        val >>= 1;
-        current_++;
-      }
-      return val;
+      uintptr_t trailing_zeros = base::bits::CountTrailingZeros(val);
+      current_ += trailing_zeros;
+      return val >> trailing_zeros;
     }
 
     BitVector* target_;