This documents the change made in https://golang.org/cl/12864 for
https://golang.org/issue/11925.

Update #11925.

Change-Id: Id09f2a489ea947a725ed12c9cf793e5daef07a06
Reviewed-on: https://go-review.googlesource.com/12866Reviewed-by: David Crawshaw <crawshaw@golang.org>

This only triggers on ARMv7+.
If there are important SMP ARMv6 machines we can reconsider.

Makes TestLFStress tests pass and sync/atomic tests not time out
on Apple iPad Mini 3.

Fixes #7977.
Fixes #10189.

Change-Id: Ie424dea3765176a377d39746be9aa8265d11bec4
Reviewed-on: https://go-review.googlesource.com/12950Reviewed-by: David Crawshaw <crawshaw@golang.org>

For #11949.

Change-Id: I4329604a24efc7f40cf5bf52fb3c9e30916b3cc2
Reviewed-on: https://go-review.googlesource.com/12931Reviewed-by: Alan Donovan <adonovan@google.com>

Fixes #11912.
Fixes #11937.

This reverts commit 1a99ba55.

Change-Id: I32b76053fdabc59f28ca5bedf1b15c0baa8afae1
Reviewed-on: https://go-review.googlesource.com/12893Reviewed-by: Didier Spezia <didier.06@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

Was not allocating space for the frame above sigpanic,
nor was it pushing the LR into the right place.
Because traceback past sigpanic only needs the
LR for faulting leaves, this was not noticed too much.
But it did break the sync/atomic nil deref tests.

Change-Id: Icba53fffa193423aab744c37f21ee893ce2ee3ac
Reviewed-on: https://go-review.googlesource.com/12926Reviewed-by: David Crawshaw <crawshaw@golang.org>

ODOTTYPE should be treated a whole lot like ODOT,
but it was missing completely from the switch in
escwalk and thus escape status did not propagate
to fields.

Since interfaces are required to trigger this bug,
the test was added to escape_iface.go.

Fixes #11931.

Change-Id: Id0383981cc4b1a160f6ad447192a112eed084538
Reviewed-on: https://go-review.googlesource.com/12921
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

There is absolutely no information about how this was failing.
If we reenable the test then at least we can get a build log from
darwin/arm.

There are not even freebsd/arm or netbsd/arm builders,
so not too worried about those. (That is another problem.)

Change-Id: I0e739a4dd2897adbe110aa400d720d8fa02ae65f
Reviewed-on: https://go-review.googlesource.com/12920Reviewed-by: Russ Cox <rsc@golang.org>

Instead of pushing the denominator argument on the stack,
the denominator is now passed in m.

This fixes a variety of bugs related to trying to take stack traces
backwards from the middle of the software div/mod routines.
Some of those bugs have been kludged around in the past,
but others have not. Instead of trying to patch up after breaking
the stack, this CL stops breaking the stack.

This is an update of https://golang.org/cl/19810043,
which was rolled back in https://golang.org/cl/20350043.

The problem in the original CL was that there were divisions
at bad times, when m was not available. These were divisions
by constant denominators, either in C code or in assembly.
The Go compiler knows how to generate division by multiplication
for constant denominators, but the C compiler did not.
There is no longer any C code, so that's taken care of.
There was one problematic DIV in runtime.usleep (assembly)
but https://golang.org/cl/12898 took care of that one.
So now this approach is safe.

Reject DIV/MOD in NOSPLIT functions to keep them from
coming back.

Fixes #6681.
Fixes #6699.
Fixes #10486.

Change-Id: I09a13c76ad08ba75b3bd5d46a3eb78e66a84ab38
Reviewed-on: https://go-review.googlesource.com/12899Reviewed-by: Ian Lance Taylor <iant@golang.org>

In order to fix issue #9401 the compiler was changed to add a padding
byte to any non-empty Go struct that ends in a zero-sized field.  That
causes the Go version of such a C struct to have a different size than
the C struct, which can considerable confusion.  Change cgo so that it
discards any such zero-sized fields, so that the Go and C structs are
the same size.

This is a change from previous releases, in that it used to be
possible to refer to a zero-sized trailing field (by taking its
address), and with this change it no longer is.  That is unfortunate,
but something has to change.  It seems better to visibly break
programs that do this rather than to silently break programs that rely
on the struct sizes being the same.

Update #9401.
Fixes #11925.

Change-Id: I3fba3f02f11265b3c41d68616f79dedb05b81225
Reviewed-on: https://go-review.googlesource.com/12864Reviewed-by: Russ Cox <rsc@golang.org>

We want to adjust the DIV calling convention to use m,
and usleep can be called without an m, so switch to a
multiplication by the reciprocal (and test).

Step toward a fix for #6699 and #10486.

Change-Id: Iccf76a18432d835e48ec64a2fa34a0e4d6d4b955
Reviewed-on: https://go-review.googlesource.com/12898Reviewed-by: Ian Lance Taylor <iant@golang.org>

If a function is large enough to need to flush the constant pool
mid-function, the line number assignment code was forcing the
line numbers not just for the constant pool but for all the instructions
that follow it. This made the line number information completely
wrong for all but the beginning of large functions on arm.

Same problem in code copied into arm64.

This broke runtime/trace's TestTraceSymbolize.

Fixes arm build.

Change-Id: I84d9fb2c798c4085f69b68dc766ab4800c7a6ca4
Reviewed-on: https://go-review.googlesource.com/12894Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

This allows running a cross-compile like
	GOOS=darwin GOARCH=arm go build std
to check that everything builds.

Otherwise there is a redefinition error because both
root_nocgo_darwin.go and root_darwin_armx.go
supply initSystemRoots.

Change-Id: Ic95976b2b698d28c629bfc93d8dac0048b023578
Reviewed-on: https://go-review.googlesource.com/12897Reviewed-by: Ian Lance Taylor <iant@golang.org>

The test expects the dial to take 1.0 seconds
on Windows and allows it to go to 1.095 seconds.
That's far too optimistic.
Recent failures are reporting roughly 1.2 seconds.
Let it have 1.5.

Change-Id: Id69811ccb65bf4b4c159301a2b4767deb6ee8d28
Reviewed-on: https://go-review.googlesource.com/12895Reviewed-by: Ian Lance Taylor <iant@golang.org>

Urge users of math/rand to consider using crypto/rand when doing
security-sensitive work.

Related to issue #11871. While we haven't reached consensus on how
to make the package inherently safer, everyone agrees that the docs
for math/rand can be improved.

Change-Id: I576a312e51b2a3445691da6b277c7b4717173197
Reviewed-on: https://go-review.googlesource.com/12900Reviewed-by: Rob Pike <r@golang.org>

For the android/arm builder.

Change-Id: Iad4881689223cd6479870da9541524a8cc458cce
Reviewed-on: https://go-review.googlesource.com/12859Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

Fixes arm64 builder crash.

The bug is possible on all architectures; you just have to get lucky
and hit a preemption or a stack growth on entry to assertE2I2.
The test stacks the deck.

Change-Id: I8419da909b06249b1ad15830cbb64e386b6aa5f6
Reviewed-on: https://go-review.googlesource.com/12890Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

It says to disable until #7564 is fixed. It was fixed in April 2014.

Change-Id: I9bebfe96802bafdd2d1a0a47591df346d91b000c
Reviewed-on: https://go-review.googlesource.com/12858
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Also make invalidptr control the recently added GC pointer check,
as documented.

Change-Id: Iccfdf49480219d12be8b33b8f03d8312d8ceabed
Reviewed-on: https://go-review.googlesource.com/12857
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

Change-Id: Ie43986d016e5a9fb17ca1393263932bbb56e81ff
Reviewed-on: https://go-review.googlesource.com/12836Reviewed-by: Russ Cox <rsc@golang.org>

The skips added in CL 12579, based on incorrect time stamps,
should be sufficient to identify and exclude all the time-related
flakiness on these systems.

If there is other flakiness, we want to find out.

For #10512.

Change-Id: I5b588ac1585b2e9d1d18143520d2d51686b563e3
Reviewed-on: https://go-review.googlesource.com/12746Reviewed-by: Austin Clements <austin@google.com>

Nearly all the flaky failures we've seen in trace tests have been
due to the use of time stamps to determine relative event ordering.
This is tricky for many reasons, including:
 - different cores might not have exactly synchronized clocks
 - VMs are worse than real hardware
 - non-x86 chips have different timer resolution than x86 chips
 - on fast systems two events can end up with the same time stamp

Stop trying to make time reliable. It's clearly not going to be for Go 1.5.
Instead, record an explicit event sequence number for ordering.
Using our own counter solves all of the above problems.

The trace still contains time stamps, of course. The sequence number
is just used for ordering.

Should alleviate #10554 somewhat. Then tickDiv can be chosen to
be a useful time unit instead of having to be exact for ordering.

Separating ordering and time stamps lets the trace parser diagnose
systems where the time stamp order and actual order do not match
for one reason or another. This CL adds that check to the end of
trace.Parse, after all other sequence order-based checking.
If that error is found, we skip the test instead of failing it.
Putting the check in trace.Parse means that cmd/trace will pick
up the same check, refusing to display a trace where the time stamps
do not match actual ordering.

Using net/http's BenchmarkClientServerParallel4 on various CPU counts,
not tracing vs tracing:

name                      old time/op    new time/op    delta
ClientServerParallel4       50.4µs ± 4%    80.2µs ± 4%  +59.06%        (p=0.000 n=10+10)
ClientServerParallel4-2     33.1µs ± 7%    57.8µs ± 5%  +74.53%        (p=0.000 n=10+10)
ClientServerParallel4-4     18.5µs ± 4%    32.6µs ± 3%  +75.77%        (p=0.000 n=10+10)
ClientServerParallel4-6     12.9µs ± 5%    24.4µs ± 2%  +89.33%        (p=0.000 n=10+10)
ClientServerParallel4-8     11.4µs ± 6%    21.0µs ± 3%  +83.40%        (p=0.000 n=10+10)
ClientServerParallel4-12    14.4µs ± 4%    23.8µs ± 4%  +65.67%        (p=0.000 n=10+10)

Fixes #10512.

Change-Id: I173eecf8191e86feefd728a5aad25bf1bc094b12
Reviewed-on: https://go-review.googlesource.com/12579Reviewed-by: Austin Clements <austin@google.com>

Otherwise the GC may see uninitialized memory there,
which might be old pointers that are retained, or it might
trigger the invalid pointer check.

Fixes #11907.

Change-Id: I67e306384a68468eef45da1a8eb5c9df216a77c0
Reviewed-on: https://go-review.googlesource.com/12852Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

Change-Id: I2f0ecdc02ce275feadf07e402b54f988513e9b49
Reviewed-on: https://go-review.googlesource.com/12855Reviewed-by: Russ Cox <rsc@golang.org>

Change-Id: I3088e17aff72096e3ec2ced49c70564627c982a6
Reviewed-on: https://go-review.googlesource.com/12854Reviewed-by: Russ Cox <rsc@golang.org>

The last time we tried this, linux/arm64 broke.
The series of CLs leading to this one fixes that problem.
Let's try again.

Fixes #9880.

Change-Id: I67bc1d959175ec972d4dcbe4aa6f153790f74251
Reviewed-on: https://go-review.googlesource.com/12849Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

The layout code has to date insisted on stack frames that are 16-aligned
including the saved LR, and it ensured this by growing the frame itself.
This breaks code that refers to values near the top of the frame by positive
offset from SP, and in general it's too magical: if you see TEXT xxx, $N,
you expect that the frame size is actually N, not sometimes N and sometimes N+8.

This led to a serious bug in the compiler where ambiguously live values
were not being zeroed correctly, which in turn triggered an assertion
in the GC about finding only valid pointers. The compiler has been
fixed to always emit aligned frames, and the hand-written assembly
has also been fixed.

Now that everything is aligned, make unaligned an error instead of
something to "fix" silently.

For #9880.

Change-Id: I05f01a9df174d64b37fa19b36a6b6c5f18d5ba2d
Reviewed-on: https://go-review.googlesource.com/12848Reviewed-by: Austin Clements <austin@google.com>

The nosplit stack overflow checks were confused about morestack.
The comment about not having correct SP information at the call
to morestack was true, but that was a real bug, not something to
work around. I fixed that problem in CL 12144. With that fixed,
no need to special-case morestack in the way done here.

This cleanup and simplification of the code was the first step
to fixing a bug that happened when I started working on the
arm64 frame size adjustments, but the cleanup was sufficient
to make the bug go away.

For #9880.

Change-Id: I16b69a5c16b6b8cb4090295d3029c42d606e3b9b
Reviewed-on: https://go-review.googlesource.com/12846Reviewed-by: Austin Clements <austin@google.com>

arm64 requires either no stack frame or a frame with a size that is 8 mod 16
(adding the saved LR will make it 16-aligned).

The cmd/internal/obj/arm64 has been silently aligning frames, but it led to
a terrible bug when the compiler and obj disagreed on the frame size,
and it's just generally confusing, so we're going to make misaligned frames
an error instead of something that is silently changed.

This CL prepares by updating assembly files.
Note that the changes in this CL are already being done silently by
cmd/internal/obj/arm64, so there is no semantic effect here,
just a clarity effect.

For #9880.

Change-Id: Ibd6928dc5fdcd896c2bacd0291bf26b364591e28
Reviewed-on: https://go-review.googlesource.com/12845Reviewed-by: Austin Clements <austin@google.com>

If the compiler doesn't do it, cmd/internal/obj/arm64 will,
and that will break the zeroing of ambiguously live values
done in zerorange, which in turn produces uninitialized
pointer cells that the GC trips over.

For #9880.

Change-Id: Ice97c30bc8b36d06b7b88d778d87fab8e1827fdc
Reviewed-on: https://go-review.googlesource.com/12847Reviewed-by: Austin Clements <austin@google.com>

This adds a GCCPUFraction field to MemStats that reports the
cumulative fraction of the program's execution time spent in the
garbage collector. This is equivalent to the utilization percent shown
in the gctrace output and makes this available programmatically.

This does make one small effect on the gctrace output: we now report
the duration of mark termination up to just before the final
start-the-world, rather than up to just after. However, unlike
stop-the-world, I don't believe there's any way that start-the-world
can block, so it should take negligible time.

While there are many statistics one might want to expose via MemStats,
this is one of the few that will undoubtedly remain meaningful
regardless of future changes to the memory system.

The diff for this change is larger than the actual change. Mostly it
lifts the code for computing the GC CPU utilization out of the
debug.gctrace path.

Updates #10323.

Change-Id: I0f7dc3fdcafe95e8d1233ceb79de606b48acd989
Reviewed-on: https://go-review.googlesource.com/12844Reviewed-by: Russ Cox <rsc@golang.org>

Currently we only capture GC phase transition times if
debug.gctrace>0, but we're about to compute GC CPU utilization
regardless of whether debug.gctrace is set, so we need these
regardless of debug.gctrace.

Change-Id: If3acf16505a43d416e9a99753206f03287180660
Reviewed-on: https://go-review.googlesource.com/12843Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

The following sequence of events can lead to the runtime attempting an
out-of-bounds access on a stack barrier slice:

1. A SIGPROF comes in on a thread while the G on that thread is in
   _Gsyscall. The sigprof handler calls gentraceback, which saves a
   local copy of the G's stkbar slice. Currently the G has no stack
   barriers, so this slice is empty.

2. On another thread, the GC concurrently scans the stack of the
   goroutine being profiled (it considers it stopped because it's in
   _Gsyscall) and installs stack barriers.

3. Back on the sigprof thread, gentraceback comes across a stack
   barrier in the stack and attempts to look it up in its (zero
   length) copy of G's old stkbar slice, which causes an out-of-bounds
   access.

This commit fixes this by adding a simple cas spin to synchronize the
SIGPROF handler with stack barrier insertion.

In general I would prefer that this synchronization be done through
the G status, since that's how stack scans are otherwise synchronized,
but adding a new lock is a much smaller change and G statuses are full
of subtlety.

Fixes #11863.

Change-Id: Ie89614a6238bb9c6a5b1190499b0b48ec759eaf7
Reviewed-on: https://go-review.googlesource.com/12748Reviewed-by: Russ Cox <rsc@golang.org>

The scheduler, work buffer's dispose, and write barriers
can conspire to hide the a pointer from the GC's concurent
mark phase. If this pointer is the only path to a large
amount of marking the STW mark termination phase may take
a lot of time.

Consider the following:
1) dispose places a work buffer on the partial queue
2) the GC is busy so it does not immediately remove and
   process the work buffer
3) the scheduler runs a mutator whose write barrier dequeues the
   work buffer from the partial queue so the GC won't see it
This repeats until the GC reaches the mark termination
phase where the GC finally discovers the pointer along
with a lot of work to do.

This CL fixes the problem by having the mutator
dispose of the buffer to the full queue instead of
the partial queue. Since the write buffer never asks for full
buffers the conspiracy described above is not possible.

Updates #11694.

Change-Id: I2ce832f9657a7570f800e8ce4459cd9e304ef43b
Reviewed-on: https://go-review.googlesource.com/12840Reviewed-by: Austin Clements <austin@google.com>

Change-Id: I45d92fed757fa1866d5b80e53ed1af6712fa6741
Reviewed-on: https://go-review.googlesource.com/12782Reviewed-by: Russ Cox <rsc@golang.org>

Updates #10510.

Change-Id: Ib4d39943969d18517b373292b83d87650d5df12a
Reviewed-on: https://go-review.googlesource.com/12787
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

These are the old assemblers written in C, and now they are
not needed.

Fixes #10510.

Change-Id: Id9337ffc8eccfd93c84b2e23f427fb1a576b543d
Reviewed-on: https://go-review.googlesource.com/12784Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

At this stage, dist is only building go_bootstrap as cmd/compile and
the rest of the Go toolchain has already been built.

Change-Id: I6f99fa00ff1d3585e215f4ce84d49344c4fcb8a5
Reviewed-on: https://go-review.googlesource.com/12779Reviewed-by: Russ Cox <rsc@golang.org>

Fixes #11864.

Change-Id: Ib9d5bd79f3b73ebd32f6585b354aaad556e0fc71
Reviewed-on: https://go-review.googlesource.com/12749Reviewed-by: Rob Pike <r@golang.org>

Currently stackDebug=4 crashes as:

panic: runtime error: index out of range
fatal error: panic on system stack
runtime stack:
runtime.throw(0x607470, 0x15)
	src/runtime/panic.go:527 +0x96
runtime.gopanic(0x5ada00, 0xc82000a1d0)
	src/runtime/panic.go:354 +0xb9
runtime.panicindex()
	src/runtime/panic.go:12 +0x49
runtime.adjustpointers(0xc820065ac8, 0x7ffe58b56100, 0x7ffe58b56318, 0x0)
	src/runtime/stack1.go:428 +0x5fb
runtime.adjustframe(0x7ffe58b56200, 0x7ffe58b56318, 0x1)
	src/runtime/stack1.go:542 +0x780
runtime.gentraceback(0x487760, 0xc820065ac0, 0x0, 0xc820001080, 0x0, 0x0, 0x7fffffff, 0x6341b8, 0x7ffe58b56318, 0x0, ...)
	src/runtime/traceback.go:336 +0xa7e
runtime.copystack(0xc820001080, 0x1000)
	src/runtime/stack1.go:616 +0x3b1
runtime.newstack()
	src/runtime/stack1.go:801 +0xdde

Change-Id: If2d60960231480a9dbe545d87385fe650d6db808
Reviewed-on: https://go-review.googlesource.com/12763Reviewed-by: Russ Cox <rsc@golang.org>

Fixes #11886.

Change-Id: I9392fd2ef5951173ae275b3ab42db4f8bd2e1d7a
Reviewed-on: https://go-review.googlesource.com/12747Reviewed-by: David Crawshaw <crawshaw@golang.org>