name                                       time/op
HandshakeServer/RSA-4                      1.10ms ± 0%
HandshakeServer/ECDHE-P256-RSA-4           1.23ms ± 1%
HandshakeServer/ECDHE-P256-ECDSA-P256-4     178µs ± 1%
HandshakeServer/ECDHE-X25519-ECDSA-P256-4   180µs ± 2%
HandshakeServer/ECDHE-P521-ECDSA-P521-4    19.8ms ± 1%

Change-Id: I6b2c79392995d259cfdfc5199be44cc7cc40e155
Reviewed-on: https://go-review.googlesource.com/44730Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

The processClientKeyExchange and processServerKeyExchange functions unmarshal an
encoded EC point and explicitly check whether the point is on the curve. The explicit
check can be omitted because elliptic.Unmarshal fails if the point is not on the curve
and the returned error would always be the same.

Fixes #20496

Change-Id: I5231a655eace79acee2737dd036a0c255ed42dbb
Reviewed-on: https://go-review.googlesource.com/44311Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Run-TryBot: Adam Langley <agl@golang.org>

asn1.NullRawValue was used according to RFC 3279 2.2.1. Without this tag,
the output didn't match openssl.

Fixes #19972

Change-Id: Ia52ddb810888837f913dbd65c4e1328f6c8084bb
Reviewed-on: https://go-review.googlesource.com/40730Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

Change-Id: I1b42fca2107b06e6fc95728f7bf3d08d005c4cb4
Reviewed-on: https://go-review.googlesource.com/55810Reviewed-by: Ian Lance Taylor <iant@golang.org>

golang.org/cl/55130 added utimensat for Solaris but didn't use it in
UtimesNano (despite indicating otherwise in the commit message). Fix
this by also using utimensat for UtimesNano on Solaris.

Because all versions of Solaris suppported by Go support utimensat,
there is no need for the fallback logic and utimensat can be called
unconditionally.

This issue was pointed out by Shawn Walker-Salas.

Updates #16480

Change-Id: I114338113a6da3cfcb8bca950674bdc8f5a7a9e5
Reviewed-on: https://go-review.googlesource.com/55141
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Some IDEs (e.g. intellij IDE based ones) create the .idea folder
to store project specific settings. This is irrelevant to Go project
that does not assume any specific IDEs, but interferes with git.

Change-Id: I0c93d9a3f7edff095fbe0c7a53b06c92b391c970
Reviewed-on: https://go-review.googlesource.com/53770Reviewed-by: Ian Lance Taylor <iant@golang.org>

The destination slice does not need to be created at all. The source
slice itself can be used as the destination because the decode loop
increments by one and then the 'seen' byte is not used anymore. Therefore
the decoded byte can be stored in that index of the source slice itself.

This trick cannot be applied to EncodeString() because in that case,
the destination slice is large than the source. And for a single byte
in the source slice, two bytes in the destination slice is written.

func BenchmarkDecodeString(b *testing.B) {
  for i := 0; i < b.N; i++ {
    DecodeString("0123456789abcdef")
  }
}

name          old time/op    new time/op    delta
DecodeString    71.0ns ± 6%    58.0ns ± 0%  -18.28%  (p=0.008 n=5+5)

name          old alloc/op   new alloc/op   delta
DecodeString     16.0B ± 0%      8.0B ± 0%  -50.00%  (p=0.008 n=5+5)

name          old allocs/op  new allocs/op  delta
DecodeString      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Change-Id: Id98db4e712444557a804155457a4dd8d1b8b416d
Reviewed-on: https://go-review.googlesource.com/55611Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

CL golang.org/cl/55130 messed up the definition of _AT_FDCWD on
dragonfly.

This fixes the following test failure on dragonfly/amd64:

  --- FAIL: TestPackageMainTestImportsArchiveNotBinary (0.00s)
  	  go_test.go:192: chtimes ./testdata/src/main_test/m.go: bad file descriptor

Change-Id: I4c96983769e6b02d714859dc838875c3c0f1be50
Reviewed-on: https://go-review.googlesource.com/55690
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

The stxr/stxrw/stxrb/stxrh instructions belong to STLXR-like instructions
set and they require special handling. The current code has no special
handling for those instructions.

The fix adds the special handling for those instructions.

Uncomment stxr/stxrw/stxrb/stxrh test cases.

Fixes #21397
Change-Id: I31cee29dd6b30b1c25badd5c7574dda7a01bf016
Reviewed-on: https://go-review.googlesource.com/54951
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Improve static branch prediction in arm64 wrapper prologue
by making the unusual case branch forwards. (Most other
architectures implement this optimization.)

Additionally, replace a CMP+BNE pair with a CBNZ
to save one instruction.

Change-Id: Id970038b34b4aaec18c101d62e2ee00f3e32a761
Reviewed-on: https://go-review.googlesource.com/54070
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Otherwise, some test flags don't work.

Change-Id: Iacf3930d0eec28e4d690cd382adbb2ecf866a0e2
Reviewed-on: https://go-review.googlesource.com/55615Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Add support for generating TBZ/TBNZ instructions.

The bit-test-and-branch pattern shows up in a number of
important places, including the runtime (gc bitmaps).

Before this change, there were 3 TB[N]?Z instructions in the Go tool,
all of which were in hand-written assembly. After this change, there
are 285. Also, the go1 benchmark binary gets about 4.5kB smaller.

Fixes #21361

Change-Id: I170c138b852754b9b8df149966ca5e62e6dfa771
Reviewed-on: https://go-review.googlesource.com/54470
Run-TryBot: Philip Hofer <phofer@umich.edu>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

TestSizes has been added in CL 55551. This test is
failing on Plan 9 because executables don't have
a DWARF symbol table.

Fixes #21453.

Change-Id: I560611b49aea5417e8c5ac0cec6c7882bd9f8335
Reviewed-on: https://go-review.googlesource.com/55692
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

struct32 and struct40 structs are already declared, remove them to
make runtime tests build.

Change-Id: I3814f2b850dcb15c4002a3aa22e2a9326e5a5e53
Reviewed-on: https://go-review.googlesource.com/55614Reviewed-by: Martin Möhrmann <moehrmann@google.com>

Where possible generate calls to runtime makechan with int arguments
during compile time instead of makechan with int64 arguments.

This eliminates converting arguments for calls to makechan with
int64 arguments for platforms where int64 values do not fit into
arguments of type int.

A similar optimization for makeslice was introduced in CL
golang.org/cl/27851.

386:
name                old time/op  new time/op  delta
MakeChan/Byte       52.4ns ± 6%  45.0ns ± 1%  -14.14%  (p=0.000 n=10+10)
MakeChan/Int        54.5ns ± 1%  49.1ns ± 1%   -9.87%  (p=0.000 n=10+10)
MakeChan/Ptr         150ns ± 1%   143ns ± 0%   -4.38%  (p=0.000 n=9+7)
MakeChan/Struct/0   49.2ns ± 2%  43.2ns ± 2%  -12.27%  (p=0.000 n=10+10)
MakeChan/Struct/32  81.7ns ± 2%  76.2ns ± 1%   -6.71%  (p=0.000 n=10+10)
MakeChan/Struct/40  88.4ns ± 2%  82.5ns ± 2%   -6.60%  (p=0.000 n=10+10)

AMD64:
name                old time/op  new time/op  delta
MakeChan/Byte       83.4ns ± 8%  80.8ns ± 3%    ~     (p=0.171 n=10+10)
MakeChan/Int         101ns ± 3%   101ns ± 2%    ~     (p=0.412 n=10+10)
MakeChan/Ptr         128ns ± 1%   128ns ± 1%    ~     (p=0.191 n=10+10)
MakeChan/Struct/0   67.6ns ± 3%  68.7ns ± 4%    ~     (p=0.224 n=10+10)
MakeChan/Struct/32   138ns ± 1%   139ns ± 1%    ~     (p=0.185 n=10+9)
MakeChan/Struct/40   154ns ± 1%   154ns ± 1%  -0.55%  (p=0.027 n=10+9)

Change-Id: Ie854cb066007232c5e9f71ea7d6fe27e81a9c050
Reviewed-on: https://go-review.googlesource.com/55140
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

The logic for USTAR was disabled because a previous implementation of
Writer had a wrong understanding of the differences between USTAR and GNU,
causing the prefix field is incorrectly be populated in GNU files.

Now that this issue has been fixed, we can re-enable the logic for USTAR
path splitting, which allows Writer to use the USTAR for a wider range
of possible inputs.

Updates #9683
Updates #12594
Updates #17630

Change-Id: I9fe34e5df63f99c6dd56fee3a7e7e4d6ec3995c9
Reviewed-on: https://go-review.googlesource.com/55574
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Move all sentinel errors to common.go since some of them are
returned by both the reader and writer and remove errInvalidHeader
since it not used.

Also, consistently use the "tar: " prefix for errors.

Change-Id: I0afb185bbf3db80dfd9595321603924454a4c2f9
Reviewed-on: https://go-review.googlesource.com/55650Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Move variable declarations closer to their first uses.

Use an additional string variable s0 for error reporting that references
the original input string. This allows the variable s to be modified.

Change-Id: I4725152490ca1dc10c1161ad8ad2f4ae8933493f
Reviewed-on: https://go-review.googlesource.com/55138Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Instead of comparing if the number of elements will
not fit into memory check if the memory size of the
slices backing memory is higher then the memory limit.

This avoids a division or maxElems lookup.

With et.size > 0:
   uintptr(newcap)                > maxSliceCap(et.size)
-> uintptr(int(capmem / et.size)) > _MaxMem  /  et.size
->             capmem / et.size   > _MaxMem  /  et.size
->             capmem             > _MaxMem

Note that due to integer division from capmem > _MaxMem
it does not follow that uintptr(newcap) > maxSliceCap(et.size).

Consolidated runtime GrowSlice benchmarks by using sub-benchmarks and
added more struct sizes to show performance improvement when division
is avoided for element sizes larger than 32 bytes.

AMD64:
GrowSlice/Byte       38.9ns ± 2%  38.9ns ± 1%    ~     (p=0.974 n=20+20)
GrowSlice/Int        58.3ns ± 3%  58.0ns ± 2%    ~     (p=0.154 n=20+19)
GrowSlice/Ptr        95.7ns ± 2%  95.1ns ± 2%  -0.60%  (p=0.034 n=20+20)
GrowSlice/Struct/24  95.4ns ± 1%  93.9ns ± 1%  -1.54%  (p=0.000 n=19+19)
GrowSlice/Struct/32   110ns ± 1%   108ns ± 1%  -1.76%  (p=0.000 n=19+20)
GrowSlice/Struct/40   138ns ± 1%   128ns ± 1%  -7.09%  (p=0.000 n=20+20)

Change-Id: I1c37857c74ea809da373e668791caffb6a5cbbd3
Reviewed-on: https://go-review.googlesource.com/53471
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

Change-Id: I1d7bd5cff7350a4e0f78b8efc8406e79c74732d1
Reviewed-on: https://go-review.googlesource.com/55370Reviewed-by: Ian Lance Taylor <iant@golang.org>

Both the GNU and PAX formats support atime and ctime fields.
The implementation is trivial now that we have:
* support for formatting PAX records for timestamps
* dedicated methods that only handle one format (e.g., GNU)

Fixes #17876

Change-Id: I0c604fce14a47d722098afc966399cca2037395d
Reviewed-on: https://go-review.googlesource.com/55570
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

We forbid empty keys or keys with '=' because it leads to ambiguous parsing.
Relevent PAX specification:
<<<
A keyword shall not include an <equals-sign>.

>
> Also, we forbid the writer from encoding records with an empty value.
> While, this is a valid record syntactically, the semantics of an empty
> value is that previous records with that key should be deleted.
> Since we have no support (and probably never will) for global PAX records,
> deletion is a non-sensible operation.
> <<<
> If the <value> field is zero length,
> it shall delete any header block field,
> previously entered extended header value,
> or global extended header value of the same name.


Fixes #20698
Fixes #15567

Change-Id: Ia29c5c6ef2e36cd9e6d7f6cff10e92b96a62f0d1
Reviewed-on: https://go-review.googlesource.com/55571Reviewed-by: Ian Lance Taylor <iant@golang.org>

Add support for PAX subsecond resolution times. Since the parser
supports negative timestamps, the formatter also handles negative
timestamps.

The relevant PAX specification is:
<<<
Portable file timestamps cannot be negative. If pax encounters a
file with a negative timestamp in copy or write mode, it can reject
the file, substitute a non-negative timestamp, or generate a
non-portable timestamp with a leading '-'.

>
> <<<
> All of these time records shall be formatted as a decimal
> representation of the time in seconds since the Epoch.
> If a <period> ( '.' ) decimal point character is present,
> the digits to the right of the point shall represent the units of
> a subsecond timing granularity, where the first digit is tenths of
> a second and each subsequent digit is a tenth of the previous digit.


Fixes #11171

Change-Id: Ied108f3d2654390bc1b0ddd66a4081c2b83e490b
Reviewed-on: https://go-review.googlesource.com/55552
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

We weren't initializing this field for dynamically-generated itabs.
Turns out it doesn't matter, as any time we use this field we also
generate a static itab for the interface type / concrete type pair.
But we should initialize it anyway, just to be safe.

Performance on the benchmarks in CL 44339:
benchmark               old ns/op     new ns/op     delta
BenchmarkItabFew-12     1040585       26466         -97.46%
BenchmarkItabAll-12     228873499     4287696       -98.13%

Change-Id: I58ed2b31e6c98b584122bdaf844fee7268b58295
Reviewed-on: https://go-review.googlesource.com/44475Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

We don't use it any more, remove it.

Change-Id: I76ce1a4c2e7048fdd13a37d3718b5abf39ed9d26
Reviewed-on: https://go-review.googlesource.com/44474Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

Just use fun[0]==0 to indicate a bad itab.

Change-Id: I28ecb2d2d857090c1ecc40b1d1866ac24a844848
Reviewed-on: https://go-review.googlesource.com/44473Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

Keep itabs in a growable hash table.
Use a simple open-addressable hash table, quadratic probing, power
of two sized.
Synchronization gets a bit more tricky. The common read path now
has two atomic reads, one to get the table pointer and one to read
the entry out of the table.

I set the max load factor to 75%, kind of arbitrarily. There's a
space-speed tradeoff here, and I'm not sure where we should land.

Because we use open addressing the itab.link field is no longer needed.
I'll remove it in a separate CL.

Fixes #20505

Change-Id: Ifb3d9a337512d6cf968c1fceb1eeaf89559afebf
Reviewed-on: https://go-review.googlesource.com/44472
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

Certain special type-flags, specifically 1, 2, 3, 4, 5, 6,
do not have a data section. Thus, regardless of what the size field
says, we should not attempt to write any data for these special types.

The relevant PAX and USTAR specification says:
<<<
If the typeflag field is set to specify a file to be of type 1 (a link)
or 2 (a symbolic link), the size field shall be specified as zero.
If the typeflag field is set to specify a file of type 5 (directory),
the size field shall be interpreted as described under the definition
of that record type. No data logical records are stored for types 1, 2, or 5.
If the typeflag field is set to 3 (character special file),
4 (block special file), or 6 (FIFO), the meaning of the size field is
unspecified by this volume of POSIX.1-2008, and no data logical records shall
be stored on the medium.
Additionally, for type 6, the size field shall be ignored when reading.
If the typeflag field is set to any other value, the number of logical
records written following the header shall be (size+511)/512, ignoring
any fraction in the result of the division.
>>>

Fixes #15565

Change-Id: Id11886b723b3b13deb15221dca51c25cd778a6b5
Reviewed-on: https://go-review.googlesource.com/55553Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Both GNU and BSD tar do not care if the devmajor and devminor values are
set on entries (like regular files) that aren't character or block devices.

While this is non-sensible, it is more consistent with the Writer to actually
read these fields always. In a vast majority of the cases these will still
be zero. In the rare situation where someone actually cares about these,
at least information was not silently lost.

Change-Id: I6e4ba01cd897a1b13c28b1837e102a4fdeb420ba
Reviewed-on: https://go-review.googlesource.com/55572Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Fixes #21437

Change-Id: I55fbf5114ae1bb7f4aa1a20450e8d5309756cd5b
Reviewed-on: https://go-review.googlesource.com/55430
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Last runtime use was removed in https://golang.org/cl/133700043,
September 2014.

Replace plan9 syscall uses with plan9-specific variable.

Change-Id: Ifb910c021c1419a7c782959f90b054ed600d9e19
Reviewed-on: https://go-review.googlesource.com/55450Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ld.SymKind and objabi.RelocType have string representations,
which is human friendly. Prefer to use it.

Change-Id: I458ee0ca5866be0db8462c36cd053561a8206c95
Reviewed-on: https://go-review.googlesource.com/55253Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

No functional changes; tophash is inlined.

Change-Id: Ic8ce95b3622eafbddcfbc97f8c630ab8c5bfe7ad
Reviewed-on: https://go-review.googlesource.com/55233
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

The preceding cleanup made it clear that two cases
(have golden data, unreachable key) are handled identically.
Simplify the control flow to reflect that.

Simplifies the code and generates shorter machine code.

Change-Id: Id612e0da6679813e855506f47222c58ea6497d70
Reviewed-on: https://go-review.googlesource.com/55093
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

Shaves a few instructions off.

Change-Id: I39f1b01ae7e770d632d5e77a6aa4b5a1f123b41a
Reviewed-on: https://go-review.googlesource.com/55090
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

This fixes the -x output so that when it reports environment variables they
are correctly quoted for later execution by the shell.
Also fix -x output to use the right path to the pack tool, and note when
we are touching a file.

Fixes #21427

Change-Id: I323ef4edf9905b08bc26944b94183d8da2fa9675
Reviewed-on: https://go-review.googlesource.com/55350Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Fixes #21436

Change-Id: I56f43e2852696c28edbcc772a54125a9a9c32497
Reviewed-on: https://go-review.googlesource.com/55262Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Only set MH_NOUNDEFS if there are no undefined symbols.
Doesn't seem to matter, but may as well do it right.

Change-Id: I6c472e000578346c28cf0e10f24f870e3a0de628
Reviewed-on: https://go-review.googlesource.com/55310Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

They are currently not given a size, which makes the DWARF reader
very confused. Particularly things like [4]func() get a size of -4, not 32.

Fixes #21097

Change-Id: I01e754134d82fbbe6567e3c7847a4843792a3776
Reviewed-on: https://go-review.googlesource.com/55551Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

This change unifies the x and y cases.

It shrinks evacuate's machine code by ~25% and its stack size by ~15%.

It also eliminates a critical branch.
Whether an entry should go to x or y is designed to be unpredictable.
As a result, half of the branch predictions for useX were wrong.
Mispredicting that branch can easily incur an expensive cache miss.
Switching to an xy array allows elimination of that branch,
which in turn reduces cache misses.

Change-Id: Ie9cef53744b96c724c377ac0985b487fc50b49b1
Reviewed-on: https://go-review.googlesource.com/54653
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>