They are basically proto-version of custom user counters.
It does not seem that they do anything that custom user counters
don't do. And having two similar entities is not good for generalization.

Migration plan:
* ```
  SetItemsProcessed(<val>)
    =>
  state.counters.insert({
    {"<Name>", benchmark::Counter(<val>, benchmark::Counter::kIsRate)},
    ...
  });
  ```
* ```
  SetBytesProcessed(<val>)
    =>
  state.counters.insert({
    {"<Name>", benchmark::Counter(<val>, benchmark::Counter::kIsRate,
                                  benchmark::Counter::OneK::kIs1024)},
    ...
  });
  ```
* ```
  <Name>_processed()
    =>
  state.counters["<Name>"]
  ```

One thing the custom user counters miss is better support
for units of measurement.

Refs. https://github.com/google/benchmark/issues/627

* Counter(): add 'one thousand' param.

Needed for https://github.com/google/benchmark/pull/654

Custom user counters are quite custom. It is not guaranteed
that the user *always* expects for these to have 1k == 1000.
If the counter represents bytes/memory/etc, 1k should be 1024.

Some bikeshedding points:
1. Is this sufficient, or do we really want to go full on
   into custom types with names?
   I think just the '1000' is sufficient for now.
2. Should there be a helper benchmark::Counter::Counter{1000,1024}()
   static 'constructor' functions, since these two, by far,
   will be the most used?
3. In the future, we should be somehow encoding this info into JSON.

* Counter(): use std::pair<> to represent 'one thousand'

* Counter(): just use a new enum with two values 1000 vs 1024.

Simpler is better. If someone comes up with a real reason
to need something more advanced, it can be added later on.

* Counter: just store the 1000 or 1024 in the One_K values directly

* Counter: s/One_K/OneK/

There are two destinations:
* display (console, terminal) and
* file.

And each of the destinations can be poplulated with one of the reporters:
* console - human-friendly table-like display
* json
* csv (deprecated)

So using the name console_reporter is confusing.
Is it talking about the console reporter in the sense of
table-like reporter, or in the sense of display destination?

This is *only* exposed in the JSON. Not in CSV, which is deprecated.

This *only* supposed to track these two states.
An additional field could later track which aggregate this is,
specifically (statistic name, rms, bigo, ...)

The motivation is that we already have ReportAggregatesOnly,
but it affects the entire reports, both the display,
and the reporters (json files), which isn't ideal.

It would be very useful to have a 'display aggregates only' option,
both in the library's console reporter, and the python tooling,
This will be especially needed for the 'store separate iterations'.

This only specifically represents handling of reporting of aggregates.
Not of anything else. Making it more specific makes the name less generic.

This is an issue because i want to add "iteration report mode",
so the naming would be conflicting.

found while working on reproducible builds for openSUSE

To reproduce there
osc checkout openSUSE:Factory/benchmark && cd $_
osc build -j1 --vm-type=kvm

As discussed in IRC, time to deduplicate.

* Remove redundant default which causes failures

* Fix old GCC warnings caused by poor analysis

* Use __builtin_unreachable

* Use BENCHMARK_UNREACHABLE()

* Pull __has_builtin to benchmark.h too

* Also move compiler identification macro to main header

* Move custom compiler identification macro back

* Clarifications and cleaning of the core documentation.

* Introduce memory manager interface

* Add memory stats to JSON reporter and a test

* Add comments and switch json output test to int

Adding myself to AUTHORS and CONTRIBUTORS according to guidelines.

* Set -Wno-deprecated-declarations for Intel

Intel compiler silently ignores -Wno-deprecated-declarations
so warning no. 1786 must be explicitly suppressed.

* Make std::int64_t → double casts explicit

While std::int64_t → double is a perfectly conformant
implicit conversion, Intel compiler warns about it.
Make them explicit via static_cast<double>.

* Make std::int64_t → int casts explicit

Intel compiler warns about emplacing an std::int64_t
into an int container. Just make the conversion explicit
via static_cast<int>.

* Cleanup Intel -Wno-deprecated-declarations workaround logic

Inspired by these [two](https://github.com/darktable-org/rawspeed/commit/a1ebe07bea5738f8607b48a7596c172be249590e) [bugs](https://github.com/darktable-org/rawspeed/commit/0891555be56b24f9f4af716604cedfa0da1efc6b) in my code due to the lack of those i have found fixed in my code:
* `kIsIterationInvariant` - `* state.iterations()`
  The value is constant for every iteration, and needs to be **multiplied** by the iteration count.
* `kAvgIterations` - `/ state.iterations()`
  The is global over all the iterations, and needs to be **divided** by the iteration count.

They play nice with `kIsRate`:
* `kIsIterationInvariantRate`
* `kAvgIterationsRate`.

I'm not sure how  meaningful they are when combined with `kAvgThreads`.
I guess the `kIsThreadInvariant` can be added, too, for symmetry with `kAvgThreads`.

* Use EXPECT_DOUBLE_EQ when comparing doubles in tests.

Fixes #623

* disable 'float-equal' warning

As previously discussed, let's flip the switch ^^.

This exposes the problem that it will now be run
for everyone, even if one did not read the help
about the recommended repetition count.

This is not good. So i think we can do the smart thing:
```
$ ./compare.py benchmarks gbench/Inputs/test3_run{0,1}.json
Comparing gbench/Inputs/test3_run0.json to gbench/Inputs/test3_run1.json
Benchmark                   Time             CPU      Time Old      Time New       CPU Old       CPU New
--------------------------------------------------------------------------------------------------------
BM_One                   -0.1000         +0.1000            10             9           100           110
BM_Two                   +0.1111         -0.0111             9            10            90            89
BM_Two                   +0.2500         +0.1125             8            10            80            89
BM_Two_pvalue             0.2207          0.6831      U Test, Repetitions: 2. WARNING: Results unreliable! 9+ repetitions recommended.
BM_Two_stat              +0.0000         +0.0000             8             8            80            80
```
(old screenshot)
![image](https://user-images.githubusercontent.com/88600/41502182-ea25d872-71bc-11e8-9842-8aa049509b14.png)

Or, in the good case (noise omitted):
```
s$ ./compare.py benchmarks /tmp/run{0,1}.json
Comparing /tmp/run0.json to /tmp/run1.json
Benchmark                                            Time             CPU      Time Old      Time New       CPU Old       CPU New
---------------------------------------------------------------------------------------------------------------------------------
<99 more rows like this>
./_T012014.RW2/threads:8/real_time                +0.0160         +0.0596            46            47            10            10
./_T012014.RW2/threads:8/real_time_pvalue          0.0000          0.0000      U Test, Repetitions: 100
./_T012014.RW2/threads:8/real_time_mean           +0.0094         +0.0609            46            47            10            10
./_T012014.RW2/threads:8/real_time_median         +0.0104         +0.0613            46            46            10            10
./_T012014.RW2/threads:8/real_time_stddev         -0.1160         -0.1807             1             1             0             0
```
(old screenshot)
![image](https://user-images.githubusercontent.com/88600/41502185-fb8193f4-71bc-11e8-85fa-cbba83e39db4.png)

Fixes #608

list(FILTER ...) is a CMake 3.6 feature, but benchmark targets CMake 2.8.12

* Fix compilation on Android with GNU STL

GNU STL in Android NDK lacks string conversion functions from C++11, including std::stoul, std::stoi, and std::stod.
This patch reimplements these functions in benchmark:: namespace using C-style equivalents from C++03.

* Avoid use of log2 which doesn't exist in Android GNU STL

GNU STL in Android NDK lacks log2 function from C99/C++11.
This patch replaces their use in the code with double log(double) function.

* format all documents according to contributor guidelines and specifications
use clang-format on/off to stop formatting when it makes excessively poor decisions

* format all tests as well, and mark blocks which change too much

Specifically some iOS targets.

As @dominichamon and I have discussed, the current reporter interface
is poor at best. And something should be done to fix it.

I strongly suspect such a fix will require an entire reimagining
of the API, and therefore breaking backwards compatibility fully.

For that reason we should start deprecating and removing parts
that we don't intend to replace. One of these parts, I argue,
is the CSVReporter. I propose that the new reporter interface
should choose a single output format (JSON) and traffic entirely
in that. If somebody really wanted to replace the functionality
of the CSVReporter they would do so as an external tool which
transforms the JSON.

For these reasons I propose deprecating the CSVReporter.

The first problem you have to solve yourself. The second one can be aided.
The benchmark library can compute some statistics over the repetitions,
which helps with grasping the results somewhat.

But that is only for the one set of results. It does not really help to compare
the two benchmark results, which is the interesting bit. Thankfully, there are
these bundled `tools/compare.py` and `tools/compare_bench.py` scripts.

They can provide a diff between two benchmarking results. Yay!
Except not really, it's just a diff, while it is very informative and better than
nothing, it does not really help answer The Question - am i just looking at the noise?
It's like not having these per-benchmark statistics...

Roughly, we can formulate the question as:
> Are these two benchmarks the same?
> Did my change actually change anything, or is the difference below the noise level?

Well, this really sounds like a [null hypothesis](https://en.wikipedia.org/wiki/Null_hypothesis), does it not?
So maybe we can use statistics here, and solve all our problems?
lol, no, it won't solve all the problems. But maybe it will act as a tool,
to better understand the output, just like the usual statistics on the repetitions...

I'm making an assumption here that most of the people care about the change
of average value, not the standard deviation. Thus i believe we can use T-Test,
be it either [Student's t-test](https://en.wikipedia.org/wiki/Student%27s_t-test), or [Welch's t-test](https://en.wikipedia.org/wiki/Welch%27s_t-test).
**EDIT**: however, after @dominichamon review, it was decided that it is better
to use more robust [Mann–Whitney U test](https://en.wikipedia.org/wiki/Mann–Whitney_U_test)
I'm using [scipy.stats.mannwhitneyu](https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.mannwhitneyu.html#scipy.stats.mannwhitneyu).

There are two new user-facing knobs:
```
$ ./compare.py --help
usage: compare.py [-h] [-u] [--alpha UTEST_ALPHA]
                  {benchmarks,filters,benchmarksfiltered} ...

versatile benchmark output compare tool
<...>
optional arguments:
  -h, --help            show this help message and exit

  -u, --utest           Do a two-tailed Mann-Whitney U test with the null
                        hypothesis that it is equally likely that a randomly
                        selected value from one sample will be less than or
                        greater than a randomly selected value from a second
                        sample. WARNING: requires **LARGE** (9 or more)
                        number of repetitions to be meaningful!
  --alpha UTEST_ALPHA   significance level alpha. if the calculated p-value is
                        below this value, then the result is said to be
                        statistically significant and the null hypothesis is
                        rejected. (default: 0.0500)
```

Example output:
![screenshot_20180512_175517](https://user-images.githubusercontent.com/88600/39958581-ae897924-560d-11e8-81b9-806db6c3e691.png)
As you can guess, the alpha does affect anything but the coloring of the computed p-values.
If it is green, then the change in the average values is statistically-significant.

I'm detecting the repetitions by matching name. This way, no changes to the json are _needed_.
Caveats:
* This won't work if the json is not in the same order as outputted by the benchmark,
   or if the parsing does not retain the ordering.
* This won't work if after the grouped repetitions there isn't at least one row with
  different name (e.g. statistic). Since there isn't a knob to disable printing of statistics
  (only the other way around), i'm not too worried about this.
* **The results will be wrong if the repetition count is different between the two benchmarks being compared.**
* Even though i have added (hopefully full) test coverage, the code of these python tools is staring
  to look a bit jumbled.
* So far i have added this only to the `tools/compare.py`.
  Should i add it to `tools/compare_bench.py` too?
  Or should we deduplicate them (by removing the latter one)?

* Add benchmark_main library with support for Bazel.

* fix newline at end of file

* Add CMake support for benchmark_main.

* Mention optionally using benchmark_main in README.

* Correct/clarify build/install instructions

GTest is google test, don't obsfucate needlessly for newcomers.
Adding google test into installation guide helps newcomers.
Third option under this  line: "Note that Google Benchmark requires Google Test to build and run the tests. This
dependency can be provided three ways:"
Was not true (did not occur). If there is a further option that needs to be specified in order for that functionality to work it needs to be specified.

* Add prerequisite knowledge section

A lot of assumptions are made about the reader in the documentation. This is unfortunate.

* Removal of abbreviations for google test

* Return a reasonable value from State::iterations() even before starting a benchmark

* Optimize State::iterations() for started case.

* Update AUTHORS and CONTRIBUTORS

* split_list is not defined for assembly tests