code review 107180044: image/jpeg: unroll the IDCT loop.

image/jpeg: unroll the IDCT loop.

The fundamental gain is bounds check elimination for 6g during array
indexing. The "x7 := src[y8+3]" generated code that used to be:
000000000042d174: MOVQ CX, DX
000000000042d177: ADDQ $0x3, DX
000000000042d17b: CMPQ $0x40, DX
000000000042d17f: JAE 0x42d381
000000000042d185: LEAQ 0(AX)(DX*4), BX
000000000042d189: MOVL 0(BX), DI
000000000042d381: CALL runtime.panicindex(SB)
000000000042d386: UD2

becomes eight separate single-instruction translations of a constant
array offset like:
000000000042dcbe: MOVL 0xcc(AX), DX

In the CPU profile for decoding a baseline JPEG, idct drops from 21%
of the CPU time to 13%.

benchmark                      old ns/op     new ns/op     delta
BenchmarkIDCT                  2124          1319          -37.90%
BenchmarkDecodeBaseline        1220819       1137013       -6.86%
BenchmarkDecodeProgressive     1947365       1857279       -4.63%

benchmark                      old MB/s     new MB/s     speedup
BenchmarkDecodeBaseline        50.62        54.35        1.07x
BenchmarkDecodeProgressive     31.74        33.27        1.05x

[nigeltao, 2014-06-19 05:12:40]

Hello r@golang.org, rsc@golang.org (cc: bradfitz@golang.org,
couchmoney@gmail.com, golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go/

[nigeltao, 2014-06-19 05:19:12]

r, rsc: is the benefit of a 1.06x performance gain worth the complexity cost of
introducing code that generates code?

Or is the fact that bounds-checking occurs with

var src [64]int32
for y := 0; y < 8; y++ {
  src[y*8+1] = etc
}

more a 6g bug, and possibly a bug not worth fixing, in terms of perf benefit /
complexity cost ratio.

If both of you think that this CL is worthwhile, r can do the actual code
review.

There are also a couple other places in the jpeg code where unrolling 8x8 loops
could possibly help performance, not just during the inverse DCT computation,
but IDCT was the obvious place to start.

[nigeltao, 2014-06-27 08:12:54]

*** Abandoned ***