[arrow] branch master updated: ARROW-8537: [C++] Revert Optimizing BitmapReader

[apitrou]

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apitrou pushed a commit to branch master
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The following commit(s) were added to refs/heads/master by this push:
     new c4accca  ARROW-8537: [C++] Revert Optimizing BitmapReader
c4accca is described below

commit c4accca7a55559e1dc1a5340facf44a69008367a
Author: Yibo Cai <yibo....@arm.com>
AuthorDate: Wed Apr 22 14:04:51 2020 +0200

    ARROW-8537: [C++] Revert Optimizing BitmapReader
    
    Revert PR https://github.com/apache/arrow/pull/6986 as it introduces
    big performance regression to BitmapAnd unaligned benchmark.
    
    Closes #7007 from cyb70289/bitmap
    
    Authored-by: Yibo Cai <yibo....@arm.com>
    Signed-off-by: Antoine Pitrou <anto...@python.org>
---
 cpp/src/arrow/util/bit_util.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/cpp/src/arrow/util/bit_util.h b/cpp/src/arrow/util/bit_util.h
index 5b6cbf7..68bc0b4 100644
--- a/cpp/src/arrow/util/bit_util.h
+++ b/cpp/src/arrow/util/bit_util.h
@@ -507,21 +507,21 @@ class BitmapReader {
       : bitmap_(bitmap), position_(0), length_(length) {
     current_byte_ = 0;
     byte_offset_ = start_offset / 8;
-    bit_offset_mask_ = 1 << (start_offset % 8);
+    bit_offset_ = start_offset % 8;
     if (length > 0) {
       current_byte_ = bitmap[byte_offset_];
     }
   }
 
-  bool IsSet() const { return (current_byte_ & bit_offset_mask_) != 0; }
+  bool IsSet() const { return (current_byte_ & (1 << bit_offset_)) != 0; }
 
-  bool IsNotSet() const { return (current_byte_ & bit_offset_mask_) == 0; }
+  bool IsNotSet() const { return (current_byte_ & (1 << bit_offset_)) == 0; }
 
   void Next() {
-    bit_offset_mask_ <<= 1;
+    ++bit_offset_;
     ++position_;
-    if (ARROW_PREDICT_FALSE(bit_offset_mask_ == 0)) {
-      bit_offset_mask_ = 1;
+    if (ARROW_PREDICT_FALSE(bit_offset_ == 8)) {
+      bit_offset_ = 0;
       ++byte_offset_;
       if (ARROW_PREDICT_TRUE(position_ < length_)) {
         current_byte_ = bitmap_[byte_offset_];
@@ -537,8 +537,8 @@ class BitmapReader {
   int64_t length_;
 
   uint8_t current_byte_;
-  uint8_t bit_offset_mask_;
   int64_t byte_offset_;
+  int64_t bit_offset_;
 };
 
 class BitmapWriter {