unicode/utf8: fix docs for DecodeRune(empty) and friends.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/157080043
cmd/gc: simplify compiled code for explicit zeroing

Among other things, *x = T{} does not need a write barrier.
The changes here avoid an unnecessary copy even when
no pointers are involved, so it may have larger effects.

In 6g and 8g, avoid manually repeated STOSQ in favor of
writing explicit MOVs, under the theory that the MOVs
should have fewer dependencies and pipeline better.

Benchmarks compare best of 5 on a 2012 MacBook Pro Core i5
with TurboBoost disabled. Most improvements can be explained
by the changes in this CL.

The effect in Revcomp is real but harder to explain: none of
the instructions in the inner loop changed. I suspect loop
alignment but really have no idea.

benchmark                       old         new         delta
BenchmarkBinaryTree17           3809027371  3819907076  +0.29%
BenchmarkFannkuch11             3607547556  3686983012  +2.20%
BenchmarkFmtFprintfEmpty        118         103         -12.71%
BenchmarkFmtFprintfString       289         277         -4.15%
BenchmarkFmtFprintfInt          304         290         -4.61%
BenchmarkFmtFprintfIntInt       507         458         -9.66%
BenchmarkFmtFprintfPrefixedInt  425         408         -4.00%
BenchmarkFmtFprintfFloat        555         555         +0.00%
BenchmarkFmtManyArgs            1835        1733        -5.56%
BenchmarkGobDecode              14738209    14639331    -0.67%
BenchmarkGobEncode              14239039    13703571    -3.76%
BenchmarkGzip                   538211054   538701315   +0.09%
BenchmarkGunzip                 135430877   134818459   -0.45%
BenchmarkHTTPClientServer       116488      116618      +0.11%
BenchmarkJSONEncode             28923406    29294334    +1.28%
BenchmarkJSONDecode             105779820   104289543   -1.41%
BenchmarkMandelbrot200          5791758     5771964     -0.34%
BenchmarkGoParse                5376642     5310943     -1.22%
BenchmarkRegexpMatchEasy0_32    195         190         -2.56%
BenchmarkRegexpMatchEasy0_1K    477         455         -4.61%
BenchmarkRegexpMatchEasy1_32    170         165         -2.94%
BenchmarkRegexpMatchEasy1_1K    1410        1394        -1.13%
BenchmarkRegexpMatchMedium_32   336         329         -2.08%
BenchmarkRegexpMatchMedium_1K   108979      106328      -2.43%
BenchmarkRegexpMatchHard_32     5854        5821        -0.56%
BenchmarkRegexpMatchHard_1K     185089      182838      -1.22%
BenchmarkRevcomp                834920364   780202624   -6.55%
BenchmarkTemplate               137046937   129728756   -5.34%
BenchmarkTimeParse              600         594         -1.00%
BenchmarkTimeFormat             559         539         -3.58%

LGTM=r
R=r
CC=golang-codereviews, iant, khr, rlh
https://codereview.appspot.com/157910047
crypto/tls: support TLS_FALLBACK_SCSV as a server.

A new attack on CBC padding in SSLv3 was released yesterday[1]. Go only
supports SSLv3 as a server, not as a client. An easy fix is to change
the default minimum version to TLS 1.0 but that seems a little much
this late in the 1.4 process as it may break some things.

Thus this patch adds server support for TLS_FALLBACK_SCSV[2] -- a
mechanism for solving the fallback problem overall. Chrome has
implemented this since February and Google has urged others to do so in
light of yesterday's news.

With this change, clients can indicate that they are doing a fallback
connection and Go servers will be able to correctly reject them.

[1] http://googleonlinesecurity.blogspot.com/2014/10/this-poodle-bites-exploiting-ssl-30.html
[2] https://tools.ietf.org/html/draft-ietf-tls-downgrade-scsv-00

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/157090043
cmd/gc: elide write barrier for x = x[0:y] and x = append(x, ...)

Both of these forms can avoid writing to the base pointer in x
(in the slice, always, and in the append, most of the time).

For Go 1.5, will need to change the compilation of x = x[0:y]
to avoid writing to the base pointer, so that the elision is safe,
and will need to change the compilation of x = append(x, ...)
to write to the base pointer (through a barrier) only when
growing the underlying array, so that the general elision is safe.

For Go 1.4, elide the write barrier always, a change that should
have equivalent performance characteristics but is much
simpler and therefore safer.

benchmark                       old ns/op   new ns/op   delta
BenchmarkBinaryTree17           3910526122  3918802545  +0.21%
BenchmarkFannkuch11             3747650699  3732600693  -0.40%
BenchmarkFmtFprintfEmpty        106         98.7        -6.89%
BenchmarkFmtFprintfString       280         269         -3.93%
BenchmarkFmtFprintfInt          296         282         -4.73%
BenchmarkFmtFprintfIntInt       467         470         +0.64%
BenchmarkFmtFprintfPrefixedInt  418         398         -4.78%
BenchmarkFmtFprintfFloat        574         535         -6.79%
BenchmarkFmtManyArgs            1768        1818        +2.83%
BenchmarkGobDecode              14916799    14925182    +0.06%
BenchmarkGobEncode              14110076    13358298    -5.33%
BenchmarkGzip                   546609795   542630402   -0.73%
BenchmarkGunzip                 136270657   136496277   +0.17%
BenchmarkHTTPClientServer       126574      125245      -1.05%
BenchmarkJSONEncode             30006238    27862354    -7.14%
BenchmarkJSONDecode             106020889   102664600   -3.17%
BenchmarkMandelbrot200          5793550     5818320     +0.43%
BenchmarkGoParse                5437608     5463962     +0.48%
BenchmarkRegexpMatchEasy0_32    192         179         -6.77%
BenchmarkRegexpMatchEasy0_1K    462         460         -0.43%
BenchmarkRegexpMatchEasy1_32    168         153         -8.93%
BenchmarkRegexpMatchEasy1_1K    1420        1280        -9.86%
BenchmarkRegexpMatchMedium_32   338         286         -15.38%
BenchmarkRegexpMatchMedium_1K   107435      98027       -8.76%
BenchmarkRegexpMatchHard_32     5941        4846        -18.43%
BenchmarkRegexpMatchHard_1K     185965      153830      -17.28%
BenchmarkRevcomp                795497458   798447829   +0.37%
BenchmarkTemplate               132091559   134938425   +2.16%
BenchmarkTimeParse              604         608         +0.66%
BenchmarkTimeFormat             551         548         -0.54%

LGTM=r
R=r, dave
CC=golang-codereviews, iant, khr, rlh
https://codereview.appspot.com/159960043
runtime: fix memory profiler
There are 3 issues:
1. Skip argument of callers is off by 3,
so that all allocations are deep inside of memory profiler.
2. Memory profiling statistics are not updated after runtime.GC.
3. Testing package does not update memory profiling statistics
before capturing the profile.
Also add an end-to-end test.
Fixes issue 8867.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/148710043
runtime: make pprof a little nicer

Update issue 8942

This does not fully address issue 8942 but it does make
the profiles much more useful, until that issue can be
fixed completely.

LGTM=dvyukov
R=r, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/159990043
runtime/pprof: disable new memory test

It cannot run 'go tool pprof'. There is no guarantee that's installed.
It needs to build a temporary pprof binary and run that.
It also needs to skip the test on systems that can't build and
run binaries, namely android and nacl.

See src/cmd/nm/nm_test.go's TestNM for a template.

Update issue 8867
Status: Accepted

TBR=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/153710043
syscall: don't cache environment variables on Plan 9

Fixes issue 8849.

LGTM=bradfitz, aram
R=bradfitz, rsc, aram
CC=golang-codereviews
https://codereview.appspot.com/158970045
CONTRIBUTORS: add Damien Neil

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/159920044

Committer: Rob Pike <r@golang.org>
reflect: fix struct size calculation to include terminal padding

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/160920045

Committer: Rob Pike <r@golang.org>
spec: define "variable"

Fixes issue 8496.

LGTM=rsc, r, iant
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/148580043
undo CL 159990043 / 421fadcef39a

Dmitriy believes this broke Windows.
It looks like build.golang.org stopped before that,
but it's worth a shot.

««« original CL description
runtime: make pprof a little nicer

Update issue 8942

This does not fully address issue 8942 but it does make
the profiles much more useful, until that issue can be
fixed completely.

LGTM=dvyukov
R=r, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/159990043
»»»

TBR=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/160030043
CONTRIBUTORS: add James Robinson (Google CLA)

LGTM=iant
R=golang-codereviews, iant
CC=dvyukov, golang-codereviews, jamesr, nigeltao
https://codereview.appspot.com/155530043
runtime: remove comment that leaked into CL 153710043

This doesn't actually do anything. Maybe it will some day,
but maybe not.

TBR=r
CC=golang-codereviews
https://codereview.appspot.com/155490043
runtime/pprof: fix test

gogo called from GC is okay
for the same reasons that
gogo called from System or ExternalCode is okay.
All three are fake stack traces.

Fixes issue 8408.

LGTM=dvyukov, r
R=r, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/152580043
encoding/gob: speed up encoding of arrays and slices

We borrow a trick from the fmt package and avoid reflection
to walk the elements when possible. We could push further with
unsafe (and we may) but this is a good start.
Decode can benefit similarly; it will be done separately.

Use go generate (engen.go) to produce the helper functions
(enc_helpers.go).

benchmark                            old ns/op     new ns/op     delta
BenchmarkEndToEndPipe                6593          6482          -1.68%
BenchmarkEndToEndByteBuffer          3662          3684          +0.60%
BenchmarkEndToEndSliceByteBuffer     350306        351693        +0.40%
BenchmarkComplex128Slice             96347         80045         -16.92%
BenchmarkInt32Slice                  42484         26008         -38.78%
BenchmarkFloat64Slice                51143         36265         -29.09%
BenchmarkStringSlice                 53402         35077         -34.32%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/156310043
reflect: a few microoptimizations

Replace i < 0 || i >= x with uint(i) >= uint(x).
Shorten a few other code sequences.
Move the kind bits to the bottom of the flag word, to avoid shifts.

LGTM=r
R=r, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/159020043
cmd/gc: don't use static init to initialize small structs, fields

Better to avoid the memory loads and just use immediate constants.
This especially applies to zeroing, which was being done by
copying zeros from elsewhere in the binary, even if the value
was going to be completely initialized with non-zero values.
The zero writes were optimized away but the zero loads from
the data segment were not.

LGTM=r
R=r, bradfitz, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/152700045
runtime/pprof: fix memory profiler test
Don't use cmd/pprof as it is not necessary installed
and does not work on nacl and plan9.
Instead just look at the raw profile.

LGTM=crawshaw, rsc
R=golang-codereviews, crawshaw, 0intro, rsc
CC=golang-codereviews
https://codereview.appspot.com/159010043
encoding/gob: custom array/slice decoders
Use go generate to write better loops for decoding arrays,
just as we did for encoding. It doesn't help as much,
relatively speaking, but it's still noticeable.

benchmark                          old ns/op     new ns/op     delta
BenchmarkDecodeComplex128Slice     202348        184529        -8.81%
BenchmarkDecodeFloat64Slice        135800        120979        -10.91%
BenchmarkDecodeInt32Slice          121200        105149        -13.24%
BenchmarkDecodeStringSlice         288129        278214        -3.44%

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/154420044