Implement the design described in:
https://docs.google.com/document/d/1v4Oqa0WwHunqlb8C3ObL_uNQw3DfSY-ztoA-4wWbKcg/pub

Summary of the changes:
GC uses "2-bits per word" pointer type info embed directly into bitmap.
Scanning of stacks/data/heap is unified.
The old spans types go away.
Compiler generates "sparse" 4-bits type info for GC (directly for GC bitmap).
Linker generates "dense" 2-bits type info for data/bss (the same as stacks use).

Summary of results:
-1680 lines of code total (-1000+ in mgc0.c only)
-25% memory consumption
-3-7% binary size
-15% GC pause reduction
-7% run time reduction

LGTM=khr
R=golang-codereviews, rsc, christoph, khr
CC=golang-codereviews, rlh
https://golang.org/cl/106260045

benchmark                        old ns/op     new ns/op     delta
BenchmarkPaeth                   5.06          6.02          +18.97%
BenchmarkDecodeGray              1010551       956911        -5.31%
BenchmarkDecodeNRGBAGradient     3877813       3754160       -3.19%
BenchmarkDecodeNRGBAOpaque       3194058       3079094       -3.60%
BenchmarkDecodePaletted          699243        700211        +0.14%
BenchmarkDecodeRGB               2835733       2692120       -5.06%
BenchmarkDecodeInterlacing       3651805       3563124       -2.43%
BenchmarkEncodeGray              4399183       4404113       +0.11%
BenchmarkEncodeNRGBOpaque        13323627      13306485      -0.13%
BenchmarkEncodeNRGBA             15840092      15751188      -0.56%
BenchmarkEncodePaletted          4396622       4404373       +0.18%
BenchmarkEncodeRGBOpaque         13320475      13279189      -0.31%
BenchmarkEncodeRGBA              36898392      36781002      -0.32%

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/117290043

Because the reference time is the reference time but beginners seem
to think otherwise, make it clearer you can't choose the reference time.

LGTM=josharian, dave
R=golang-codereviews, josharian, dave
CC=golang-codereviews
https://golang.org/cl/117250044

This change causes a TLS client and server to verify that received
elliptic curve points are on the expected curve. This isn't actually
necessary in the Go TLS stack, but Watson Ladd has convinced me that
it's worthwhile because it's pretty cheap and it removes the
possibility that some change in the future (e.g. tls-unique) will
depend on it without the author checking that precondition.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/115290046

ASN.1 elements can be optional, and can have a default value.
Traditionally, Go has omitted elements that are optional and that have
the zero value. I believe that's a bug (see [1]).

This change causes an optional element with a default value to only be
omitted when it has that default value. The previous behaviour of
omitting optional, zero elements with no default is retained because
it's used quite a lot and will break things if changed.

[1] https://groups.google.com/d/msg/Golang-nuts/9Ss6o9CW-Yo/KL_V7hFlyOAJ

Fixes #7780.

R=bradfitz

LGTM=bradfitz
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews, r
https://golang.org/cl/86960045

R=golang-codereviews, bradfitz, agl
CC=golang-codereviews
https://golang.org/cl/117180043

Fixes #8367

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/116300044

While we're here, make it lookup the tlsfallback symbol only once.

LGTM=crawshaw
R=golang-codereviews, crawshaw, dave
CC=golang-codereviews
https://golang.org/cl/107430044

It is already called by linknew().

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/120760043

LGTM=bradfitz, dave
R=rsc, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/115070043

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/116330043

Unused. cmd/dist will generate enams as liblink/anames[568].c.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/119940043

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/116320043

glibc devs have apparently decided _BSD_SOURCE will be
deprecated on Linux, and issue a preprocessor warning if
declaring _BSD_SOURCE without _DEFAULT_SOURCE.

https://sourceware.org/glibc/wiki/Release/2.20

Fixes #8397.

LGTM=iant
R=dave, gobot, iant
CC=golang-codereviews
https://golang.org/cl/112530043

selv is created with temp() which calls tempname, which marks
the new n with EscNever, so there is no need to explicitly set
EscNone on the select descriptor.

Fixes #8396.

LGTM=dvyukov
R=golang-codereviews, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/112520043

LGTM=chines
R=chines
CC=golang-codereviews
https://golang.org/cl/118160044

golang.org now serves HTTPS with a valid cert, so it's reasonable
that users should click through to the HTTPS versions of *.golang.org
and other known sites.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/112650043

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/120150043

Sweepone may be running while a new span is allocating.  It
must not see the state updated while the sweepgen is unset.

Fixes #8399

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/118050043

Following CL 68150047, the goos and goarch
variables are not currently set when the GOOS
and GOARCH environment variables are not set.

This made the content of the build tag to be
ignored in this case.

This CL sets goos and goarch to runtime.GOOS
and runtime.GOARCH when the GOOS and GOARCH
environments variables are not set.

LGTM=aram, bradfitz
R=golang-codereviews, aram, gobot, rsc, dave, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/112490043

LGTM=josharian, adg
R=golang-codereviews, josharian, adg
CC=golang-codereviews
https://golang.org/cl/119110043

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/117060043

This is bad for 2 reasons:
1. if the code under lock ever grows stack,
it will deadlock as stack growing acquires mheap lock.
2. It currently deadlocks with SetCPUProfileRate:
scavenger locks mheap, receives prof signal and tries to lock prof lock;
meanwhile SetCPUProfileRate locks prof lock and tries to grow stack
(presumably in runtime.unlock->futexwakeup). Boom.
Let's assume that it
Fixes #8407.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://golang.org/cl/112640043

With cl/112640043 TestCgoDeadlockCrash episodically print:
unexpected return pc for runtime.newstackcall
After adding debug output I see the following trace:

runtime: unexpected return pc for runtime.newstackcall called from 0xc208011b00
runtime.throw(0x414da86)
        src/pkg/runtime/panic.c:523 +0x77
runtime.gentraceback(0x40165fc, 0xba440c28, 0x0, 0xc208d15200, 0xc200000000, 0xc208ddfd20, 0x20, 0x0, 0x0, 0x300)
	src/pkg/runtime/traceback_x86.c:185 +0xca4
runtime.callers(0x1, 0xc208ddfd20, 0x20)
	src/pkg/runtime/traceback_x86.c:438 +0x98
mcommoninit(0xc208ddfc00)
	src/pkg/runtime/proc.c:369 +0x5c
runtime.allocm(0xc208052000)
	src/pkg/runtime/proc.c:686 +0xa6
newm(0x4017850, 0xc208052000)
	src/pkg/runtime/proc.c:933 +0x27
startm(0xc208052000, 0x100000001)
	src/pkg/runtime/proc.c:1011 +0xba
wakep()
	src/pkg/runtime/proc.c:1071 +0x57
resetspinning()
	src/pkg/runtime/proc.c:1297 +0xa1
schedule()
	src/pkg/runtime/proc.c:1366 +0x14b
runtime.gosched0(0xc20808e240)
	src/pkg/runtime/proc.c:1465 +0x5b
runtime.newstack()
	src/pkg/runtime/stack.c:891 +0x44d
runtime: unexpected return pc for runtime.newstackcall called from 0xc208011b00
runtime.newstackcall(0x4000cbd, 0x4000b80)
	src/pkg/runtime/asm_amd64.s:278 +0x6f

I suspect that it can happen on any stack split.
So don't unwind g0 stack.
Also, that comment is lying -- we can traceback w/o mcache,
CPU profiler does that.

LGTM=rsc
R=golang-codereviews
CC=golang-codereviews, khr, rsc
https://golang.org/cl/120040043

There are fields in the Addr that do not matter for the
purpose of deciding that the same word is already
in the current literal pool. Copy only the fields that
do matter.

This came up when comparing against the Go version
because the way it is invoked doesn't copy a few fields
(like node) that are never directly used by liblink itself.

Also remove a stray print that is not well-defined in
the new liblink. (Cannot use %D outside of %P, because
%D needs the outer Prog*.)

LGTM=minux
R=minux
CC=golang-codereviews
https://golang.org/cl/119000043

This matches Go's fmt.Printf instead of ANSI C's dumb rules.
It makes the -S output from C liblink match Go's liblink.

LGTM=minux
R=minux
CC=golang-codereviews
https://golang.org/cl/112600043

They do not, but pretend that they do.
The immediate need is that it breaks the new GC because
these are weird symbols as if with pointers but not necessary
pointer aligned.

LGTM=rsc
R=golang-codereviews, dave, josharian, khr, rsc
CC=golang-codereviews, iant, khr, rlh
https://golang.org/cl/116060043

R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/111560043

I've found this very useful for generating
good test case lists for -short mode for
the disassemblers.

Fixes #7959.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/98150043

Fixes #8398.

LGTM=chines
R=golang-codereviews, chines, josharian
CC=golang-codereviews
https://golang.org/cl/114140043

As pointed out by Elias Naur, the convention for Go runtime functions means this function should be named gothrow.

Discussion: https://golang.org/cl/115860045/#msg6

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/118120043

Currently they are scanned conservatively.
But there is no reason to scan them. C world must not contain
pointers into Go heap. Moreover, we don't have enough information
to emit write barriers nor update pointers there in future.
The immediate need is that it breaks the new GC because
these are weird symbols as if with pointers but not necessary
pointer aligned.

LGTM=rsc
R=golang-codereviews, rlh, rsc
CC=golang-codereviews, iant, khr
https://golang.org/cl/117000043

CL 117950044 updates vet to fix LEA* issue.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/112440043

In the runtime, we want to control where allocations happen.
In particular, we don't want the code implementing malloc to
itself trigger a malloc.  This change prevents the compiler
from inserting mallocs on our behalf (due to escaping declarations).

This check does not trigger on the current runtime code.

Note: Composite literals are still allowed.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/105280047

So we can tell from a binary which version of
Go built it.

LGTM=minux, rsc
R=golang-codereviews, minux, khr, rsc, dave
CC=golang-codereviews
https://golang.org/cl/117040043

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/112420044

This is more useful than panicking, since otherwise every caller needs
to do the length check before calling; some will forget, and have a
potential submarine crasher as a result. Other implementations of this
functionality do a length check.

This is backward compatible, except if someone has written code that
relies on this panicking with different length args. However, that was
not the case before Go 1.3 either.

Updates #7304.

LGTM=agl
R=agl, minux, hanwen
CC=golang-codereviews
https://golang.org/cl/118750043

In both cases we lie to malloc about the actual size that we need.
In panic we ask for less memory than we are going to use.
In slice we ask for more memory than we are going to use
(potentially asking for a fractional number of elements).
This breaks the new GC.

LGTM=khr
R=golang-codereviews, dave, khr
CC=golang-codereviews, rsc
https://golang.org/cl/116940043

LGTM=khr
R=golang-codereviews, dave, khr
CC=golang-codereviews, rsc
https://golang.org/cl/116950043

Rewrite gotos that violate Go's stricter rules.
Use uchar* instead of char* in a few places that aren't strings.
Remove dead opcross code from asm5.c.
Declare regstr (in both list6 and list8) static.

LGTM=minux, dave
R=minux, dave
CC=golang-codereviews
https://golang.org/cl/113230043