This CL also adds support for marking the likelyness of IF nodes in the AST being true. This feature is being used here to mark the slow path as unlikely.

src/pkg/runtime:
benchmark                  old ns/op    new ns/op    delta
BenchmarkConvT2IUintptr           16            1  -91.63%

test/bench/go1:
benchmark                 old ns/op    new ns/op    delta
BenchmarkBinaryTree17    5416917000   5461355000   +0.82%
BenchmarkFannkuch11      3810355000   3842609000   +0.85%
BenchmarkGobDecode         19950950     19855420   -0.48%
BenchmarkGobEncode         11301220     11308530   +0.06%
BenchmarkGzip             548119600    546869200   -0.23%
BenchmarkGunzip           176145400    180208300   +2.31%
BenchmarkJSONEncode        93117400     70163100  -24.65%
BenchmarkJSONDecode       406626800    409999200   +0.83%
BenchmarkMandelbrot200      6300992      6317866   +0.27%
BenchmarkParse              7664396      7451625   -2.78%
BenchmarkRevcomp         1189424000   1412332000  +18.74%
BenchmarkTemplate         491308400    458654200   -6.65%

benchmark                  old MB/s     new MB/s  speedup
BenchmarkGobDecode            38.47        38.66    1.00x
BenchmarkGobEncode            67.92        67.87    1.00x
BenchmarkGzip                 35.40        35.48    1.00x
BenchmarkGunzip              110.16       107.68    0.98x
BenchmarkJSONEncode           20.84        27.66    1.33x
BenchmarkJSONDecode            4.77         4.73    0.99x
BenchmarkParse                 7.56         7.77    1.03x
BenchmarkRevcomp             213.69       179.96    0.84x
BenchmarkTemplate              3.95         4.23    1.07x

R=rsc, dave, nigeltao
CC=golang-dev
https://golang.org/cl/6351090

Given the following Go program:

func sum(s []int) int {
        ret := 0
        for _, x := range s {
                ret += x
        }
        return ret
}

6g would previously generate:

--- prog list "sum" ---
0000 (main.go:3) TEXT    sum+0(SB),$0-24
0001 (main.go:5) MOVQ    s+0(FP),CX
0002 (main.go:5) MOVL    s+8(FP),DI
0003 (main.go:5) MOVL    s+12(FP),BX
0004 (main.go:4) MOVL    $0,SI
0005 (main.go:5) MOVL    $0,AX
0006 (main.go:5) JMP     ,8
0007 (main.go:5) INCL    ,AX
0008 (main.go:5) CMPL    AX,DI
0009 (main.go:5) JGE     $0,16
0010 (main.go:5) MOVL    (CX),DX
0011 (main.go:5) MOVQ    $4,BX
0012 (main.go:5) ADDQ    CX,BX
0013 (main.go:5) MOVQ    BX,CX
0014 (main.go:6) ADDL    DX,SI
0015 (main.go:5) JMP     ,7
0016 (main.go:8) MOVL    SI,.noname+16(FP)
0017 (main.go:8) RET     ,

and now generates:

--- prog list "sum" ---
0000 (main.go:3) TEXT    sum+0(SB),$0-24
0001 (main.go:5) MOVQ    s+0(FP),CX
0002 (main.go:5) MOVL    s+8(FP),DI
0003 (main.go:5) MOVL    s+12(FP),BX
0004 (main.go:4) MOVL    $0,SI
0005 (main.go:5) MOVL    $0,AX
0006 (main.go:5) JMP     ,8
0007 (main.go:5) INCL    ,AX
0008 (main.go:5) CMPL    AX,DI
0009 (main.go:5) JGE     $0,14
0010 (main.go:5) MOVL    (CX),BP
0011 (main.go:5) ADDQ    $4,CX
0012 (main.go:6) ADDL    BP,SI
0013 (main.go:5) JMP     ,7
0014 (main.go:8) MOVL    SI,.noname+16(FP)
0015 (main.go:8) RET     ,

The key difference is that
0011 (main.go:5) MOVQ    $4,BX
0012 (main.go:5) ADDQ    CX,BX
0013 (main.go:5) MOVQ    BX,CX
has changed to
0011 (main.go:5) ADDQ    $4,CX

R=rsc, dave, remyoudompheng
CC=golang-dev
https://golang.org/cl/6506089

R=golang-dev, dsymonds, campoy
CC=golang-dev
https://golang.org/cl/6490102

It is enough to load directly the data word and the itab word
from memory, so we save a LEA instruction for each method call,
and allow elimination of some extra temporaries.

Update #1914.

R=daniel.morsing, rsc
CC=golang-dev, remy
https://golang.org/cl/6501110

Removes an extra LEAL/LEAQ instructions there and usually saves
a useless temporary in the idiom
    if err := foo(); err != nil {...}

Generated code is also less involved:
    MOVQ err+n(SP), AX
    CMPQ AX, $0
(potentially CMPQ n(SP), $0) instead of
    LEAQ err+n(SP), AX
    CMPQ (AX), $0

Update #1914.

R=daniel.morsing, nigeltao, rsc
CC=golang-dev, remy
https://golang.org/cl/6493099

This verifies existing behavior. Some replacements are arguably wrong
(these are marked with TODO) but changing behavior is left for a
follow-up CL.

Also fix that BenchmarkGenericMatch wasn't actually matching anything.

R=rsc, eric.d.eisner
CC=bradfitz, golang-dev
https://golang.org/cl/6488110

R=iant, nigeltao
CC=golang-dev
https://golang.org/cl/6492102

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6506096

Fixes #3069.

R=golang-dev, n13m3y3r
CC=golang-dev
https://golang.org/cl/6460044

Rationale: for large screens, the convenience of not having to scroll
to the top of the page to do a search outweighs having less vertical
space.
Tested with Chrome, Firefox, Safari with various window and text sizes.

R=adg
CC=gobot, golang-dev
https://golang.org/cl/6493071

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6496103

R=rsc, dave
CC=bradfitz, gobot, golang-dev
https://golang.org/cl/6497044

A race between
        a = "hello, world"
and
        print(a)
is not guaranteed to print either "hello, world" or "".
Its behaviour is undefined.

Fixes #4039.

R=rsc
CC=dvyukov, gobot, golang-dev, r
https://golang.org/cl/6489075

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6489056

First set of type checker files for review.
The primary concern here is the typechecker
API (types.go).

R=rsc, adonovan, r, rogpeppe
CC=golang-dev
https://golang.org/cl/6490089

Fixes #4040.

R=rsc
CC=golang-dev
https://golang.org/cl/6497098

Broke tests on 386.

««« original CL description
6l/8l: emit correct opcodes to F(SUB|DIV)R?D.

When the destination was not F0, 6l and 8l swapped FSUBD/FSUBRD and
FDIVD/FDIVRD.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6498092
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/6492100

When the destination was not F0, 6l and 8l swapped FSUBD/FSUBRD and
FDIVD/FDIVRD.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6498092

Also, clear Content-Type and Content-Length on Not Modified
responses before server.go strips them and spams the logs with
warnings.

R=rsc
CC=golang-dev
https://golang.org/cl/6503090

No longer necessary after https://golang.org/cl/6497073/
removed the `if(n5.op != OXXX) { regfree(&n5); }`.

R=remy, r
CC=golang-dev, rsc
https://golang.org/cl/6498101

R=golang-dev
CC=golang-dev
https://golang.org/cl/6497106

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/6496099

This makes the compilers code more similar and improves
code generation a lot.

The number of LEAL instructions generated for cmd/go drops
by 60%.

% GOARCH=386 go build -gcflags -S -a cmd/go | grep LEAL | wc -l
Before:       89774
After:        47548

benchmark                              old ns/op    new ns/op    delta
BenchmarkAppendFloatDecimal                  540          444  -17.78%
BenchmarkAppendFloat                        1160         1035  -10.78%
BenchmarkAppendFloatExp                     1060          922  -13.02%
BenchmarkAppendFloatNegExp                  1053          920  -12.63%
BenchmarkAppendFloatBig                     1773         1558  -12.13%
BenchmarkFormatInt                         13065        12481   -4.47%
BenchmarkAppendInt                         10981         9900   -9.84%
BenchmarkFormatUint                         3804         3650   -4.05%
BenchmarkAppendUint                         3506         3303   -5.79%
BenchmarkUnquoteEasy                         714          683   -4.34%
BenchmarkUnquoteHard                        5117         2915  -43.03%

Update #1914.

R=nigeltao, rsc, golang-dev
CC=golang-dev, remy
https://golang.org/cl/6489067

It might be meaningful, and stable than others.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/6499091

Refactored build + buildTrie into build + buildOrdering.
Note that since the tailoring code is not checked in yet, all tailorings are identical
to root.  The table therefore should not and does not grow at this point.

R=r
CC=golang-dev
https://golang.org/cl/6500087

R=r
CC=golang-dev
https://golang.org/cl/6499079

R=r
CC=golang-dev
https://golang.org/cl/6493097

R=r
CC=golang-dev
https://golang.org/cl/6498106

R=golang-dev, 0xjnml, iant, adonovan, aram
CC=golang-dev
https://golang.org/cl/6500092

R=r
CC=golang-dev
https://golang.org/cl/6490095

Fixes #4052.

R=golang-dev, bradfitz, iant, rsc
CC=golang-dev
https://golang.org/cl/6490092

This test was previously removed in 087c6e15702e.

R=bradfitz, rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/6506061

After further deliberation, let's back down to the Unicode proposal.
Ignoring aBOMinations anywhere means that things like
        grep unsafe *.go
might fail because there's a BOM in the middle: unBOMsafe.

R=golang-dev, rsc, 0xjnml, gri, bradfitz
CC=golang-dev
https://golang.org/cl/6490091

R=r
CC=dsymonds, gobot, golang-dev
https://golang.org/cl/6495085

Fixes #4037.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6488093

It's easier just to declare a new variable.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6501103

The main case where it happens is when evaluating &s[i] without
bounds checking, which usually happens during range loops (i=0).

This allows registerization of the corresponding variables,
saving 16 bytes of stack frame for each such range loop and a
LEAQ instruction.

R=golang-dev, rsc, dave
CC=golang-dev, remy
https://golang.org/cl/6497073

Fixes #3911.

Requires CL 6449127.

dfc@qnap:~$ ./runtime.test
runtime: this CPU has no floating point hardware, so it cannot run
this GOARM=7 binary. Recompile using GOARM=5.

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6442109

When generating enums use the debug data section instead of the
DWARF debug info, if it is available in the ELF file. This allows
mkerrors.sh to work correctly on OpenBSD/386 and NetBSD/386.

Fixes #2470.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6495090

The parser depends on it but the client might not import it, so make sure it's there.
Fixes #4038.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6497094