Simplify determineLinkMode by calling mustLinkExternal upfront,
then doing a first pass for LinkModeAuto, followed by a second pass
that determines if the link mode is valid.

Change-Id: I9d7668107c159f8fe330b8c05fee035bbe9875fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/195078Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

This change adds an intrinsic for Mul64 on s390x. To achieve that,
a new assembly instruction, MLGR, is introduced in s390x/asmz.go. This assembly
instruction directly uses an existing instruction on Z and supports multiplication
of two 64 bit unsigned integer and stores the result in two separate registers.

In this case, we require the multiplcand to be stored in register R3 and
the output result (the high and low 64 bit of the product) to be stored in
R2 and R3 respectively.

A test case is also added.

Benchmark:
name      old time/op  new time/op  delta
Mul-18    11.1ns ± 0%   1.4ns ± 0%  -87.39%  (p=0.002 n=8+10)
Mul32-18  2.07ns ± 0%  2.07ns ± 0%     ~     (all equal)
Mul64-18  11.1ns ± 1%   1.4ns ± 0%  -87.42%  (p=0.000 n=10+10)

Change-Id: Ieca6ad1f61fff9a48a31d50bbd3f3c6d9e6675c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/194572Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

This reverts CL 194381

Reason for revert: break tests like add2line.

Change-Id: I9e858c7ada340a842bd0cad719616ad30fae4aaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/195137Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Provide the initial framework for the riscv64 assembler. For now this
only supports raw WORD instructions, but at least allows for basic
testing. Additional functionality will be added in separate changes.

Based on the riscv-go port.

Updates #27532

Change-Id: I181ffb2d37a34764a3e91eded177d13a89c69f9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/194117Reviewed-by: Cherry Zhang <cherryyz@google.com>

Updates #32789

Change-Id: Ie5e8e3b7b6a923aa9068c8af3ac8f081bd92c830
Reviewed-on: https://go-review.googlesource.com/c/go/+/190838Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Fixes #33808

Change-Id: If1f30bc80004093ffdf9121768464dfb3a6e1f63
Reviewed-on: https://go-review.googlesource.com/c/go/+/194381
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Since the cache has been required since Go 1.12, also remove redundant
checks for a nil cache.

Updates #29127
Updates #29667

Change-Id: Ibc59b659306a4eef2d4f0e3d0b651986d6cf84ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/188021
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

No changes in compilebench, pass toolstash-check.

Change-Id: I2688f7b45af0eaa0cf3b38e726bce6e68c20f69c
Reviewed-on: https://go-review.googlesource.com/c/go/+/195077
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

This skeleton follows the model of CL 174521,
with additions from 'relnote -html'.

Updates #33738
Updates #30439

Change-Id: I0f88ff7f231b7728dca0695143f2f2eda74d60c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/195058
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

This change updates the status of gccgo 8 and gccgo 9, which are now
released.

It also replaces every instance of two-spaces with one space in text
paragraphs, which is the preferred style in Go documentation.

Fixes #34167

Change-Id: I94a4d85c06281f2623d39a68db7b6c95b5867999
Reviewed-on: https://go-review.googlesource.com/c/go/+/193842Reviewed-by: Ian Lance Taylor <iant@golang.org>

TestWriteFileModeAppliesUmask and TestVectoredHandlerDontCrashOnLibrary
could both leak /tmp/go-build-* directories which isn't very friendly.

Change-Id: Ibee9c33d49ad48958fae4df73853b82d92314bf0
GitHub-Last-Rev: 814e2fa4bb4e4fe9c00b6d465313ce35c7ab4e32
GitHub-Pull-Request: golang/go#34253
Reviewed-on: https://go-review.googlesource.com/c/go/+/194880
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Originally, we default to use load and store instruction with 20 bit displacement.
However, that is not necessary. Some instructions have a displacement smaller
than 12 bit. This CL allows the usage of 12 bit load and store instruction when
that happens.

This change also reduces the size of .text section in go binary by 19 KB.

Some tests are also added to verify the functionality of the change.

Change-Id: I13edea06ca653d4b9ffeaefe8d010bc2f065c2ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/194857Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Jay suggested this in CL 189780, and it seems semantically correct.

As far as I can tell this has no impact one way or the other right
now, but might prevent confusion (or at least give us more experience
with error handling!) in future changes.

Updates #30748
Updates #28459
Updates #30322

Change-Id: I5d7e9a08ea141628ed6a8fd03c62d0d3c2edf2bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/194817
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

For the server response to be displayed, the response must be served
as type text/plain with charset us-ascii or utf-8, and must consist of
only graphic characters and whitespace.

We truncate the server response at the first blank line or after 8
lines or a fixed number of characters, and tab-indent (if multiple
lines) to ensure that the response is offset from ordinary go command
output.

Fixes #30748

Change-Id: I0bc1d734737e456e3251aee2252463b6355e8c97
Reviewed-on: https://go-review.googlesource.com/c/go/+/189783
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Updates #30748

Change-Id: I38a6cdc9c9b488fec579e6362a4284e26e0f526e
Reviewed-on: https://go-review.googlesource.com/c/go/+/189782
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

VersionError wraps the given error in a ModuleError struct.

If the given error is already a ModuleError for the same path and
version, we now return it directly instead of wrapping.
This makes it safer to call VersionError if we don't know whether
a given error is already wrapped.

Updates #30748

Change-Id: I41b23f6c3ead0ec382e848696da51f478da1ad35
Reviewed-on: https://go-review.googlesource.com/c/go/+/189781
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Using go/types to get rid of all unused variables in CL 189798 was a
neat idea, but it was pretty expensive. go/types is a full typechecker,
which does a lot more work than we actually need. Moreover, we had to
run it multiple times, to catch variables that became unused after
removing existing unused variables.

Instead, write our own little detector for unused imports and variables.
It doesn't use ast.Walk, as we need to know what fields we're
inspecting. For example, in "foo := bar", "foo" is declared, and "bar"
is used, yet they both appear as simple *ast.Ident cases under ast.Walk.

The code is documented to explain how unused variables are detected in a
single syntax tree pass. Since this happens after we've generated a
complete go/ast.File, we don't need to worry about our own simplified
node types.

The generated code is the same, but rulegen is much faster and uses less
memory at its peak, so it should scale better with time.

With 'benchcmd Rulegen go run *.go' on perflock, we get:

	name     old time/op         new time/op         delta
	Rulegen          4.00s ± 0%          3.41s ± 1%  -14.70%  (p=0.008 n=5+5)

	name     old user-time/op    new user-time/op    delta
	Rulegen          14.1s ± 1%          10.6s ± 1%  -24.62%  (p=0.008 n=5+5)

	name     old sys-time/op     new sys-time/op     delta
	Rulegen          318ms ±26%          263ms ± 9%     ~     (p=0.056 n=5+5)

	name     old peak-RSS-bytes  new peak-RSS-bytes  delta
	Rulegen          231MB ± 4%          181MB ± 3%  -21.69%  (p=0.008 n=5+5)

Change-Id: I8387d52818f6131357868ad348dac8c96d926191
Reviewed-on: https://go-review.googlesource.com/c/go/+/191782
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

The listed invariant, while technically true, was misleading, and the
invariant can be tightened. We never actually get to (d.hi ==
d.overflow), due to the "d.hi--" line in the decoder.decode method.

This is a comment-only commit, changing the comment to match the code.

A follow-up commit could restore the comment, changing the code to match
the original intented invariant. But the first step is to have the
comment and the code say the same thing.

Change-Id: Ifc9f78d5060454fc107af9be298026bf3043d400
Reviewed-on: https://go-review.googlesource.com/c/go/+/191358Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

After CL 192979, it is safe now to optimize isfat slightly to handle
1-field structs and 1-element arrays.

Change-Id: Ie3bc30299abbcef36eee7a0681997cc2f88ed6a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/192980
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Fixes #22344

Change-Id: I7c400d9d4ebcab279d08a8c190508d82cbd20899
Reviewed-on: https://go-review.googlesource.com/c/go/+/194717
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

When reaching const declaration, Checker override context iota to use
correct iota value, but does not restore the old value when exit, and
always set context iota to nil. It ends up with undefined iota after
const declaration.

To fix it, preserve the original iota value and restore it after const
declaration.

Fixes #34228

Change-Id: I42d5efb55a57e5ddc369bb72d31f1f039c92361c
Reviewed-on: https://go-review.googlesource.com/c/go/+/194737
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

Part of the general trend of moving yyerror calls out of walk and into
typecheck.

Notably, this requires splitting test/typeswitch2.go into two files,
because now some of the errors are reported during typecheck and
others are still reported during walk; and if there were any errors
during typecheck, then cmd/compile exits without invoking walk.

Passes toolstash-check.

Change-Id: I05ee0c00b99af659ee1eef098d342d0d736cf31e
Reviewed-on: https://go-review.googlesource.com/c/go/+/194659Reviewed-by: Robert Griesemer <gri@golang.org>

While superficially type and expression switch handling seem similar
and that it would be worthwhile to unify typechecking them, it turns
out they're actually different enough that separately handling them is
fewer lines of code and easier to understand as well.

Passes toolstash-check.

Change-Id: I357d6912dd580639b6001bccdb2e227ed83c6fe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/194566
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

Revert "cmd/go/internal/modconv: use modules to examine instead of using only direct source control entries"

This reverts CL 191218.

Reason for revert: broke cmd/go/internal/modconv in the 'linux-amd64-longtest' builder.
(https://build.golang.org/log/e28011d0f918d4b4c503ab47e479d9e76c769abd)

Change-Id: I0d260b0a5ad510d3d304c8aac8286fcab921d2fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/194797
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

The go/ast package uses and guarantees nil slices for optional
elements that weren't present in the parsed source code, such as the
list of return values of a function. Packages using go/ast rely on
this attribute and check for nils explicitly.

One such package is go/printer. In the presence of empty slices
instead of nil slices, it generates invalid code, such as "case :"
instead of "default:". The issues that this CL fixes are all
manifestations of that problem, each for a different syntactic
element.

Fixes #33103
Fixes #33104
Fixes #33105

Change-Id: I219f95a7da820eaf697a4ee227d458ab6e4a80bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/187917Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

'go get' accepts arguments of the form path@version, and it passes
them through search.CleanPatterns before querying proxies. With this
change, CleanPatterns preserves text after '@' and will strip trailing
slashes from the patn.

Previously, we did not strip trailing slashes when a version was
present, which caused proxy base URL validation to fail. Module paths
that end with ".go" (for example, github.com/nats-io/nats.go) use
trailing slashes to prevent 'go build' and other commands from
interpreting packages as source file names, so this caused unnecessary
problems for them.

Updates #32483

Change-Id: Id3730c52089e52f1cac446617c20132a3021a808
Reviewed-on: https://go-review.googlesource.com/c/go/+/194600
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Since modules now support parsing multiple forms of versions (including
commit hash and source control tag), I think modconv.ConvertLegacyConfig
no longer needs modfetch.ImportRepoRev. So I suggest that we use modules
to convert legacy config instead of using VCS directly. By doing this,
we can make the module proxy participate in the conversion process and
benefit from it (such as speeding up "go mod init" or breaking through
the firewall).

And since modconv.ConvertLegacyConfig is the only caller of
modfetch.ImportRepoRev, I think modfetch.ImportRepoRev can be removed.

Fixes #33767

Change-Id: Ic79b14fa805ed297ca1735a8498cfed2a5ddeec2
Reviewed-on: https://go-review.googlesource.com/c/go/+/191218
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Fixes #34235.

Change-Id: Ia3795fd18860530fa6a4b171545f525e784ffdcb
GitHub-Last-Rev: 1a319c452857818f7aaf22ef46823b43ca9b2276
GitHub-Pull-Request: golang/go#34238
Reviewed-on: https://go-review.googlesource.com/c/go/+/194642Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Some of the instructions were incorrectly grouped - untangle this and
separate the RV64I instructions, which are under separate sections of
the RISC-V specification.

Change-Id: I232962ab4054bf0b4745887506f51e74ea73f73d
Reviewed-on: https://go-review.googlesource.com/c/go/+/194238Reviewed-by: Cherry Zhang <cherryyz@google.com>

Updates #30748
Updates #28459

Change-Id: I1c34b3dae0bf9361dba0dae66bb868901ecafe29
Reviewed-on: https://go-review.googlesource.com/c/go/+/189780
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Updates #30748

Change-Id: Ic93c68c1c4b2728f383edfdb06371ecc79a6f7b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/189779
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

The signature of parseMetaGoImports implies that it can return an error,
but it has not done so since CL 119675. Restore the missing error check,
and remove the named return-values to avoid reintroducing this bug in the
future.

Updates #30748
Updates #21291

Change-Id: Iab19ade5b1c23c282f3c385a55ed277465526515
Reviewed-on: https://go-review.googlesource.com/c/go/+/189778
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

The doc comment for charsetReader claims that it supports UTF-8,
but in practice it does not: instead, it is never invoked for UTF-8.

We could update the comment to clarify that fact, but it seems simpler
to change the implementation to match the comment.

Change-Id: I39b11395ccef3feff96480b9294e8f2a232728dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/189777
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Incoming URLs may omit the scheme to indicate “either HTTP or HTTPS”.
For such URLs, log the scheme actually used instead of leaving it out.

(This issue was noticed while triaging #34075.)

Updates #34075

Change-Id: I39e5ca83543dd780258d41d5c2c4ba907cd20e5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/193262
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Currently, when running the "CC=clang go run -msan misc/cgo/
testsanitizers/testdata/msan.go" command on arm64, it will
report an error and the error is reported by llvm/compiler-rt/
lib/msan and it is "Make sure to compile with -fPIE and to link
with -pie".

This CL fixes this issue, using PIE link mode when using MSAN
on arm64.

This CL also updates the related document and go build help message.

Fixes #33712

Change-Id: I0cc9d95f3fa264d6c042c27a40ccbb82826922fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/190482
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

If a go-import directive refers to a nonexistent repository, today we
treat that as an error fetching a module that actually exists.
That makes the HTTP server responsible for determining which
repositories do or do not exist, which may in general depend on
the user's separately-stored credentials, and imposes significant
complexity on such a server, which can otherwise be very simple.

Instead, check the repository URL and/or error message to try to
determine whether the repository exists at all. If the repo does not
exist, treat its absence as a “not found” error — as if the server had
not returned it in the first place.

Updates #34094

Change-Id: I142619ff43b96d0de428cdd0b01cca828c9ba234
Reviewed-on: https://go-review.googlesource.com/c/go/+/194561Reviewed-by: Jay Conrod <jayconrod@google.com>

Since Go 1.2 every string can be marshaled to JSON without error even if it
contains invalid UTF-8 byte sequences. Therefore there is no need to use
Marshal again for the only reason of enclosing the string in double quotes.
Not using Marshal here also removes the error check as there has not been a
way for Marshal to fail anyway.

name          old time/op    new time/op    delta
Issue34127-4     360ns ± 3%     200ns ± 3%  -44.56%  (p=0.008 n=5+5)

name          old alloc/op   new alloc/op   delta
Issue34127-4     56.0B ± 0%     40.0B ± 0%  -28.57%  (p=0.008 n=5+5)

name          old allocs/op  new allocs/op  delta
Issue34127-4      3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.008 n=5+5)

Fixes #34154

Change-Id: Ib60dc11980f9b20d8bef2982de7168943d632263
GitHub-Last-Rev: 9b0ac1d4c5318b6bf9ed7930320f2bd755f9939c
GitHub-Pull-Request: golang/go#34127
Reviewed-on: https://go-review.googlesource.com/c/go/+/193604Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Added a new instruction, NOPH, with the encoding [0x0700](i.e: bcr 0, 0) and
replace the current 4-byte nop that was encoded using the WORD instruction.

This reduces the size of .text section in go binary by around 17KB and make
generated code easier to read.

Change-Id: I6a756df39e93c4415ea6d038ba4af001b8ccb286
Reviewed-on: https://go-review.googlesource.com/c/go/+/194344Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

This allows the use of CLONE_VFORK and CLONE_VM for fork/exec, preventing
"fork/exec ...: cannot allocate memory" failures from occuring when attempting
to execute commands from a Go process that has a large memory footprint.
Additionally, this should reduce the latency of fork/exec on linux/arm64.

With CLONE_VM the child process shares the same memory with the parent
process. On its own this would lead to conflicting use of the same
memory, so CLONE_VFORK is used to suspend the parent process until the
child releases the memory when switching to the new program binary
via the exec syscall. When the parent process continues to run, one
has to consider the changes to memory that the child process did,
namely the return address of the syscall function needs to be restored
from a register.

exec.Command() callers can start in a faster manner, as child process who
do exec commands job can be cloned faster via vfork than via fork on arm64.

The same problem was addressed on linux/amd64 via issue #5838.

Updates #31936
Contributed by Howard Zhang <howard.zhang@arm.com> and Bin Lu <bin.lu@arm.com>

Change-Id: Ia99d81d877f564ec60d19f17e596276836576eaf
Reviewed-on: https://go-review.googlesource.com/c/go/+/189418
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

As discussed in #32912, a crash occurs when go runtime calls a VDSO function (say
__vdso_clock_gettime) and a signal arrives to that thread.
Since VDSO functions temporarily destroy the G register (R10),
Go functions asynchronously executed in that thread (i.e. Go's signal
handler) can try to load data from the destroyed G, which causes
segmentation fault.

To fix the issue a guard is inserted in front of sigtrampgo, so that the control escapes from
signal handlers without touching G in case the signal occurred in the VDSO context.
The test case included in the patch is take from discussion in a relevant thread on github:
https://github.com/golang/go/issues/32912#issuecomment-517874531.
This patch not only fixes the issue on AArch64 but also that on 32bit ARM.

Fixes #32912

Change-Id: I657472e54b7aa3c617fabc5019ce63aa4105624a
GitHub-Last-Rev: 28ce42c4a02a060f08c1b0dd1c9a392123fd2ee9
GitHub-Pull-Request: golang/go#34030
Reviewed-on: https://go-review.googlesource.com/c/go/+/192937
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>