[Pkg-javascript-commits] [pdf.js] 26/207: Optimize JPG.js receiveAndExtend for 1 bit case.

[David Prévot]

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in repository pdf.js.

commit 0a42edf82f1d36a1681e5d770ffd151ee1569958
Author: Fabian Lange <lange.fabian at gmail.com>
Date:   Thu Jun 5 12:07:04 2014 +0200

    Optimize JPG.js receiveAndExtend for 1 bit case.
    
    Profiling showed that receiveAndExtend is frequently called with the length of
    one bit. This happens for example in decodeBaseline.
    For a single bit, the loop and shift in receive, as well as the shifts in
    receiveAndExtend are overhead.
    This shortcut manually calculates the shifts by either returning 1 or -1 from
    receiveAndExtend by reading the bit and deciding on the return value.
    While it comes with an overhead for each non-one length, the speedup is at about
    10% in the hot parse/decode path.
---
 src/core/jpg.js | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/core/jpg.js b/src/core/jpg.js
index 350d695..26de996 100644
--- a/src/core/jpg.js
+++ b/src/core/jpg.js
@@ -159,6 +159,9 @@ var JpegImage = (function jpegImage() {
     }
 
     function receiveAndExtend(length) {
+      if (length === 1) {
+        return readBit() === 1 ? 1 : -1;
+      }
       var n = receive(length);
       if (n >= 1 << (length - 1)) {
         return n;

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