image/draw: optimize drawFillOver as drawFillSrc for opaque fills.

Benchmarks are much better for opaque fills and slightly worse on non
opaque fills. I think that on balance, this is still a win.

When the source is uniform(color.RGBA{0x11, 0x22, 0x33, 0xff}):
name        old time/op  new time/op  delta
FillOver-8   966µs ± 1%    32µs ± 1%  -96.67%  (p=0.000 n=10+10)
FillSrc-8   32.4µs ± 1%  32.2µs ± 1%     ~      (p=0.053 n=9+10)

When the source is uniform(color.RGBA{0x11, 0x22, 0x33, 0x44}):
name        old time/op  new time/op  delta
FillOver-8   962µs ± 0%  1018µs ± 0%  +5.85%   (p=0.000 n=9+10)
FillSrc-8   32.2µs ± 1%  32.1µs ± 0%    ~     (p=0.148 n=10+10)

Change-Id: I52ec6d5fcd0fbc6710cef0e973a21ee7827c0dd9
Reviewed-on: https://go-review.googlesource.com/28790Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/image/draw/bench_test.go

@@ -74,7 +74,7 @@ func bench(b *testing.B, dcm, scm, mcm color.Model, op Op) {
 	var src image.Image
 	switch scm {
 	case nil:
-		src = &image.Uniform{C: color.RGBA{0x11, 0x22, 0x33, 0xff}}
+		src = &image.Uniform{C: color.RGBA{0x11, 0x22, 0x33, 0x44}}
 	case color.CMYKModel:
 		src1 := image.NewCMYK(image.Rect(0, 0, srcw, srch))
 		for y := 0; y < srch; y++ {

src/image/draw/draw.go

@@ -116,7 +116,12 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 			if mask == nil {
 				switch src0 := src.(type) {
 				case *image.Uniform:
-					drawFillOver(dst0, r, src0)
+					sr, sg, sb, sa := src0.RGBA()
+					if sa == 0xffff {
+						drawFillSrc(dst0, r, sr, sg, sb, sa)
+					} else {
+						drawFillOver(dst0, r, sr, sg, sb, sa)
+					}
 					return
 				case *image.RGBA:
 					drawCopyOver(dst0, r, src0, sp)
@@ -150,7 +155,8 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 			if mask == nil {
 				switch src0 := src.(type) {
 				case *image.Uniform:
-					drawFillSrc(dst0, r, src0)
+					sr, sg, sb, sa := src0.RGBA()
+					drawFillSrc(dst0, r, sr, sg, sb, sa)
 					return
 				case *image.RGBA:
 					drawCopySrc(dst0, r, src0, sp)
@@ -232,8 +238,7 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 	}
 }
 
-func drawFillOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
-	sr, sg, sb, sa := src.RGBA()
+func drawFillOver(dst *image.RGBA, r image.Rectangle, sr, sg, sb, sa uint32) {
 	// The 0x101 is here for the same reason as in drawRGBA.
 	a := (m - sa) * 0x101
 	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
@@ -255,8 +260,7 @@ func drawFillOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
 	}
 }
 
-func drawFillSrc(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
-	sr, sg, sb, sa := src.RGBA()
+func drawFillSrc(dst *image.RGBA, r image.Rectangle, sr, sg, sb, sa uint32) {
 	sr8 := uint8(sr >> 8)
 	sg8 := uint8(sg >> 8)
 	sb8 := uint8(sb >> 8)