image/png: hoist repetitive pixels per byte out of loop in Encode

The existing implementation has calculated pixels per byte for each pixel.
reduce the calculation of pixels per byte.

name                        old time/op    new time/op     delta
EncodeGray-4                  2.16ms ± 1%     2.16ms ± 1%    -0.28%  (p=0.000 n=86+84)
EncodeGrayWithBufferPool-4    1.99ms ± 0%     1.97ms ± 0%    -0.72%  (p=0.000 n=97+92)
EncodeNRGBOpaque-4            6.51ms ± 1%     6.48ms ± 1%    -0.45%  (p=0.000 n=90+85)
EncodeNRGBA-4                 7.33ms ± 1%     7.28ms ± 0%    -0.69%  (p=0.000 n=89+87)
EncodePaletted-4              5.10ms ± 1%     2.29ms ± 0%   -55.11%  (p=0.000 n=90+85)
EncodeRGBOpaque-4             6.51ms ± 1%     6.51ms ± 0%      ~     (p=0.311 n=94+88)
EncodeRGBA-4                  24.3ms ± 2%     24.1ms ± 1%    -0.87%  (p=0.000 n=91+91)

name                        old speed      new speed       delta
EncodeGray-4                 142MB/s ± 1%    143MB/s ± 1%    +0.26%  (p=0.000 n=86+85)
EncodeGrayWithBufferPool-4   154MB/s ± 0%    156MB/s ± 0%    +0.73%  (p=0.000 n=97+92)
EncodeNRGBOpaque-4           189MB/s ± 1%    190MB/s ± 1%    +0.44%  (p=0.000 n=90+86)
EncodeNRGBA-4                168MB/s ± 1%    169MB/s ± 0%    +0.69%  (p=0.000 n=89+87)
EncodePaletted-4            60.3MB/s ± 1%  134.2MB/s ± 0%  +122.74%  (p=0.000 n=90+85)
EncodeRGBOpaque-4            189MB/s ± 1%    189MB/s ± 0%      ~     (p=0.326 n=94+88)
EncodeRGBA-4                50.6MB/s ± 2%   51.1MB/s ± 1%    +0.87%  (p=0.000 n=91+91)

name                        old alloc/op   new alloc/op    delta
EncodeGray-4                   852kB ± 0%      852kB ± 0%    +0.00%  (p=0.000 n=100+100)
EncodeGrayWithBufferPool-4    1.49kB ± 2%     1.47kB ± 1%    -0.88%  (p=0.000 n=95+90)
EncodeNRGBOpaque-4             860kB ± 0%      860kB ± 0%    +0.00%  (p=0.003 n=98+58)
EncodeNRGBA-4                  864kB ± 0%      864kB ± 0%    +0.00%  (p=0.021 n=100+99)
EncodePaletted-4               849kB ± 0%      849kB ± 0%    +0.00%  (p=0.040 n=100+100)
EncodeRGBOpaque-4              860kB ± 0%      860kB ± 0%      ~     (p=0.062 n=66+98)
EncodeRGBA-4                  3.32MB ± 0%     3.32MB ± 0%    -0.00%  (p=0.044 n=99+99)

name                        old allocs/op  new allocs/op   delta
EncodeGray-4                    32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodeGrayWithBufferPool-4      3.00 ± 0%       3.00 ± 0%      ~     (all equal)
EncodeNRGBOpaque-4              32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodeNRGBA-4                   32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodePaletted-4                36.0 ± 0%       36.0 ± 0%      ~     (all equal)
EncodeRGBOpaque-4               32.0 ± 0%       32.0 ± 0%      ~     (all equal)
EncodeRGBA-4                    614k ± 0%       614k ± 0%      ~     (all equal)

Change-Id: I716bde2dc80d8111d75d3d765fc09223f770d5a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/187417
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/image/png/writer.go b/src/image/png/writer.go
index 2bd4fde692c3cabc5dd5de3d1157597072b74f5a..53adc1633c0dba6a7e82b19fc9b898d1c222d1ba 100644 (file)
--- a/src/image/png/writer.go
+++ b/src/image/png/writer.go
@@ -426,10 +426,11 @@ func (e *encoder) writeImage(w io.Writer, m image.Image, cb int, level int) erro
 
                        var a uint8
                        var c int
+                       pixelsPerByte := 8 / bitsPerPixel
                        for x := b.Min.X; x < b.Max.X; x++ {
                                a = a<<uint(bitsPerPixel) | pi.ColorIndexAt(x, y)
                                c++
-                               if c == 8/bitsPerPixel {
+                               if c == pixelsPerByte {
                                        cr[0][i] = a
                                        i += 1
                                        a = 0
@@ -437,7 +438,7 @@ func (e *encoder) writeImage(w io.Writer, m image.Image, cb int, level int) erro
                                }
                        }
                        if c != 0 {
-                               for c != 8/bitsPerPixel {
+                               for c != pixelsPerByte {
                                        a = a << uint(bitsPerPixel)
                                        c++
                                }