image/jpeg: reduce bound checks from idct and fdct

Before -
$gotip build -gcflags="-d=ssa/check_bce/debug=1" fdct.go idct.go
./fdct.go:89:10: Found IsInBounds
./fdct.go:90:10: Found IsInBounds
./fdct.go:91:10: Found IsInBounds
./fdct.go:92:10: Found IsInBounds
./fdct.go:93:10: Found IsInBounds
./fdct.go:94:10: Found IsInBounds
./fdct.go:95:10: Found IsInBounds
./fdct.go:96:10: Found IsInBounds
./idct.go:77:9: Found IsInBounds
./idct.go:77:27: Found IsInBounds
./idct.go:77:45: Found IsInBounds
./idct.go:78:7: Found IsInBounds
./idct.go:78:25: Found IsInBounds
./idct.go:78:43: Found IsInBounds
./idct.go:78:61: Found IsInBounds
./idct.go:79:13: Found IsInBounds
./idct.go:92:13: Found IsInBounds
./idct.go:93:12: Found IsInBounds
./idct.go:94:12: Found IsInBounds
./idct.go:95:12: Found IsInBounds
./idct.go:97:12: Found IsInBounds
./idct.go:98:12: Found IsInBounds
./idct.go:99:12: Found IsInBounds

After -
$gotip build -gcflags="-d=ssa/check_bce/debug=1" fdct.go idct.go
./fdct.go:90:9: Found IsSliceInBounds
./idct.go:76:11: Found IsSliceInBounds
./idct.go:145:11: Found IsSliceInBounds

name                 old time/op    new time/op    delta
FDCT-4                 1.85µs ± 2%    1.74µs ± 1%  -5.95%  (p=0.000 n=10+10)
IDCT-4                 1.94µs ± 2%    1.89µs ± 1%  -2.67%  (p=0.000 n=10+9)
DecodeBaseline-4       1.45ms ± 2%    1.46ms ± 1%    ~     (p=0.156 n=9+10)
DecodeProgressive-4    2.21ms ± 1%    2.21ms ± 1%    ~     (p=0.796 n=10+10)
EncodeRGBA-4           24.9ms ± 1%    25.0ms ± 1%    ~     (p=0.075 n=10+10)
EncodeYCbCr-4          26.1ms ± 1%    26.2ms ± 1%    ~     (p=0.573 n=8+10)

name                 old speed      new speed      delta
DecodeBaseline-4     42.5MB/s ± 2%  42.4MB/s ± 1%    ~     (p=0.162 n=9+10)
DecodeProgressive-4  27.9MB/s ± 1%  27.9MB/s ± 1%    ~     (p=0.796 n=10+10)
EncodeRGBA-4         49.4MB/s ± 1%  49.1MB/s ± 1%    ~     (p=0.066 n=10+10)
EncodeYCbCr-4        35.3MB/s ± 1%  35.2MB/s ± 1%    ~     (p=0.586 n=8+10)

name                 old alloc/op   new alloc/op   delta
DecodeBaseline-4       63.0kB ± 0%    63.0kB ± 0%    ~     (all equal)
DecodeProgressive-4     260kB ± 0%     260kB ± 0%    ~     (all equal)
EncodeRGBA-4           4.40kB ± 0%    4.40kB ± 0%    ~     (all equal)
EncodeYCbCr-4          4.40kB ± 0%    4.40kB ± 0%    ~     (all equal)

name                 old allocs/op  new allocs/op  delta
DecodeBaseline-4         5.00 ± 0%      5.00 ± 0%    ~     (all equal)
DecodeProgressive-4      13.0 ± 0%      13.0 ± 0%    ~     (all equal)
EncodeRGBA-4             4.00 ± 0%      4.00 ± 0%    ~     (all equal)
EncodeYCbCr-4            4.00 ± 0%      4.00 ± 0%    ~     (all equal)

Updates #24499

Change-Id: I6828d077b851817503a7c1a08235763f81bdadf9
Reviewed-on: https://go-review.googlesource.com/c/go/+/167417
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

2 files changed, 63 insertions, 59 deletions

diff --git a/src/image/jpeg/fdct.go b/src/image/jpeg/fdct.go
index 201a5abd0b..c7a973ec3c 100644
--- a/src/image/jpeg/fdct.go
+++ b/src/image/jpeg/fdct.go
@@ -86,14 +86,16 @@ const (
 func fdct(b *block) {
 	// Pass 1: process rows.
 	for y := 0; y < 8; y++ {
-		x0 := b[y*8+0]
-		x1 := b[y*8+1]
-		x2 := b[y*8+2]
-		x3 := b[y*8+3]
-		x4 := b[y*8+4]
-		x5 := b[y*8+5]
-		x6 := b[y*8+6]
-		x7 := b[y*8+7]
+		y8 := y * 8
+		s := b[y8 : y8+8 : y8+8] // Small cap improves performance, see https://golang.org/issue/27857
+		x0 := s[0]
+		x1 := s[1]
+		x2 := s[2]
+		x3 := s[3]
+		x4 := s[4]
+		x5 := s[5]
+		x6 := s[6]
+		x7 := s[7]
 
 		tmp0 := x0 + x7
 		tmp1 := x1 + x6
@@ -110,12 +112,12 @@ func fdct(b *block) {
 		tmp2 = x2 - x5
 		tmp3 = x3 - x4
 
-		b[y*8+0] = (tmp10 + tmp11 - 8*centerJSample) << pass1Bits
-		b[y*8+4] = (tmp10 - tmp11) << pass1Bits
+		s[0] = (tmp10 + tmp11 - 8*centerJSample) << pass1Bits
+		s[4] = (tmp10 - tmp11) << pass1Bits
 		z1 := (tmp12 + tmp13) * fix_0_541196100
 		z1 += 1 << (constBits - pass1Bits - 1)
-		b[y*8+2] = (z1 + tmp12*fix_0_765366865) >> (constBits - pass1Bits)
-		b[y*8+6] = (z1 - tmp13*fix_1_847759065) >> (constBits - pass1Bits)
+		s[2] = (z1 + tmp12*fix_0_765366865) >> (constBits - pass1Bits)
+		s[6] = (z1 - tmp13*fix_1_847759065) >> (constBits - pass1Bits)
 
 		tmp10 = tmp0 + tmp3
 		tmp11 = tmp1 + tmp2
@@ -134,10 +136,10 @@ func fdct(b *block) {
 
 		tmp12 += z1
 		tmp13 += z1
-		b[y*8+1] = (tmp0 + tmp10 + tmp12) >> (constBits - pass1Bits)
-		b[y*8+3] = (tmp1 + tmp11 + tmp13) >> (constBits - pass1Bits)
-		b[y*8+5] = (tmp2 + tmp11 + tmp12) >> (constBits - pass1Bits)
-		b[y*8+7] = (tmp3 + tmp10 + tmp13) >> (constBits - pass1Bits)
+		s[1] = (tmp0 + tmp10 + tmp12) >> (constBits - pass1Bits)
+		s[3] = (tmp1 + tmp11 + tmp13) >> (constBits - pass1Bits)
+		s[5] = (tmp2 + tmp11 + tmp12) >> (constBits - pass1Bits)
+		s[7] = (tmp3 + tmp10 + tmp13) >> (constBits - pass1Bits)
 	}
 	// Pass 2: process columns.
 	// We remove pass1Bits scaling, but leave results scaled up by an overall factor of 8.
diff --git a/src/image/jpeg/idct.go b/src/image/jpeg/idct.go
index 46fcaecb79..a3957c8ada 100644
--- a/src/image/jpeg/idct.go
+++ b/src/image/jpeg/idct.go
@@ -73,30 +73,31 @@ func idct(src *block) {
 	// Horizontal 1-D IDCT.
 	for y := 0; y < 8; y++ {
 		y8 := y * 8
+		s := src[y8 : y8+8 : y8+8] // Small cap improves performance, see https://golang.org/issue/27857
 		// If all the AC components are zero, then the IDCT is trivial.
-		if src[y8+1] == 0 && src[y8+2] == 0 && src[y8+3] == 0 &&
-			src[y8+4] == 0 && src[y8+5] == 0 && src[y8+6] == 0 && src[y8+7] == 0 {
-			dc := src[y8+0] << 3
-			src[y8+0] = dc
-			src[y8+1] = dc
-			src[y8+2] = dc
-			src[y8+3] = dc
-			src[y8+4] = dc
-			src[y8+5] = dc
-			src[y8+6] = dc
-			src[y8+7] = dc
+		if s[1] == 0 && s[2] == 0 && s[3] == 0 &&
+			s[4] == 0 && s[5] == 0 && s[6] == 0 && s[7] == 0 {
+			dc := s[0] << 3
+			s[0] = dc
+			s[1] = dc
+			s[2] = dc
+			s[3] = dc
+			s[4] = dc
+			s[5] = dc
+			s[6] = dc
+			s[7] = dc
 			continue
 		}
 
 		// Prescale.
-		x0 := (src[y8+0] << 11) + 128
-		x1 := src[y8+4] << 11
-		x2 := src[y8+6]
-		x3 := src[y8+2]
-		x4 := src[y8+1]
-		x5 := src[y8+7]
-		x6 := src[y8+5]
-		x7 := src[y8+3]
+		x0 := (s[0] << 11) + 128
+		x1 := s[4] << 11
+		x2 := s[6]
+		x3 := s[2]
+		x4 := s[1]
+		x5 := s[7]
+		x6 := s[5]
+		x7 := s[3]
 
 		// Stage 1.
 		x8 := w7 * (x4 + x5)
@@ -126,14 +127,14 @@ func idct(src *block) {
 		x4 = (r2*(x4-x5) + 128) >> 8
 
 		// Stage 4.
-		src[y8+0] = (x7 + x1) >> 8
-		src[y8+1] = (x3 + x2) >> 8
-		src[y8+2] = (x0 + x4) >> 8
-		src[y8+3] = (x8 + x6) >> 8
-		src[y8+4] = (x8 - x6) >> 8
-		src[y8+5] = (x0 - x4) >> 8
-		src[y8+6] = (x3 - x2) >> 8
-		src[y8+7] = (x7 - x1) >> 8
+		s[0] = (x7 + x1) >> 8
+		s[1] = (x3 + x2) >> 8
+		s[2] = (x0 + x4) >> 8
+		s[3] = (x8 + x6) >> 8
+		s[4] = (x8 - x6) >> 8
+		s[5] = (x0 - x4) >> 8
+		s[6] = (x3 - x2) >> 8
+		s[7] = (x7 - x1) >> 8
 	}
 
 	// Vertical 1-D IDCT.
@@ -141,16 +142,17 @@ func idct(src *block) {
 		// Similar to the horizontal 1-D IDCT case, if all the AC components are zero, then the IDCT is trivial.
 		// However, after performing the horizontal 1-D IDCT, there are typically non-zero AC components, so
 		// we do not bother to check for the all-zero case.
+		s := src[x : x+57 : x+57] // Small cap improves performance, see https://golang.org/issue/27857
 
 		// Prescale.
-		y0 := (src[8*0+x] << 8) + 8192
-		y1 := src[8*4+x] << 8
-		y2 := src[8*6+x]
-		y3 := src[8*2+x]
-		y4 := src[8*1+x]
-		y5 := src[8*7+x]
-		y6 := src[8*5+x]
-		y7 := src[8*3+x]
+		y0 := (s[8*0] << 8) + 8192
+		y1 := s[8*4] << 8
+		y2 := s[8*6]
+		y3 := s[8*2]
+		y4 := s[8*1]
+		y5 := s[8*7]
+		y6 := s[8*5]
+		y7 := s[8*3]
 
 		// Stage 1.
 		y8 := w7*(y4+y5) + 4
@@ -180,13 +182,13 @@ func idct(src *block) {
 		y4 = (r2*(y4-y5) + 128) >> 8
 
 		// Stage 4.
-		src[8*0+x] = (y7 + y1) >> 14
-		src[8*1+x] = (y3 + y2) >> 14
-		src[8*2+x] = (y0 + y4) >> 14
-		src[8*3+x] = (y8 + y6) >> 14
-		src[8*4+x] = (y8 - y6) >> 14
-		src[8*5+x] = (y0 - y4) >> 14
-		src[8*6+x] = (y3 - y2) >> 14
-		src[8*7+x] = (y7 - y1) >> 14
+		s[8*0] = (y7 + y1) >> 14
+		s[8*1] = (y3 + y2) >> 14
+		s[8*2] = (y0 + y4) >> 14
+		s[8*3] = (y8 + y6) >> 14
+		s[8*4] = (y8 - y6) >> 14
+		s[8*5] = (y0 - y4) >> 14
+		s[8*6] = (y3 - y2) >> 14
+		s[8*7] = (y7 - y1) >> 14
 	}
 }