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# Assembly Tests
The Benchmark library provides a number of functions whose primary
purpose in to affect assembly generation, including `DoNotOptimize`
and `ClobberMemory`. In addition there are other functions,
such as `KeepRunning`, for which generating good assembly is paramount.
For these functions it's important to have tests that verify the
correctness and quality of the implementation. This requires testing
the code generated by the compiler.
This document describes how the Benchmark library tests compiler output,
as well as how to properly write new tests.
## Anatomy of a Test
Writing a test has two steps:
* Write the code you want to generate assembly for.
* Add `// CHECK` lines to match against the verified assembly.
Example:
```c++
// CHECK-LABEL: test_add:
extern "C" int test_add() {
extern int ExternInt;
return ExternInt + 1;
// CHECK: movl ExternInt(%rip), %eax
// CHECK: addl %eax
// CHECK: ret
}
```
#### LLVM Filecheck
[LLVM's Filecheck](https://llvm.org/docs/CommandGuide/FileCheck.html)
is used to test the generated assembly against the `// CHECK` lines
specified in the tests source file. Please see the documentation
linked above for information on how to write `CHECK` directives.
#### Tips and Tricks:
* Tests should match the minimal amount of output required to establish
correctness. `CHECK` directives don't have to match on the exact next line
after the previous match, so tests should omit checks for unimportant
bits of assembly. ([`CHECK-NEXT`](https://llvm.org/docs/CommandGuide/FileCheck.html#the-check-next-directive)
can be used to ensure a match occurs exactly after the previous match).
* The tests are compiled with `-O3 -g0`. So we're only testing the
optimized output.
* The assembly output is further cleaned up using `tools/strip_asm.py`.
This removes comments, assembler directives, and unused labels before
the test is run.
* The generated and stripped assembly file for a test is output under
`<build-directory>/test/<test-name>.s`
* Filecheck supports using [`CHECK` prefixes](https://llvm.org/docs/CommandGuide/FileCheck.html#cmdoption-check-prefixes)
to specify lines that should only match in certain situations.
The Benchmark tests use `CHECK-CLANG` and `CHECK-GNU` for lines that
are only expected to match Clang or GCC's output respectively. Normal
`CHECK` lines match against all compilers. (Note: `CHECK-NOT` and
`CHECK-LABEL` are NOT prefixes. They are versions of non-prefixed
`CHECK` lines)
* Use `extern "C"` to disable name mangling for specific functions. This
makes them easier to name in the `CHECK` lines.
## Problems Writing Portable Tests
Writing tests which check the code generated by a compiler are
inherently non-portable. Different compilers and even different compiler
versions may generate entirely different code. The Benchmark tests
must tolerate this.
LLVM Filecheck provides a number of mechanisms to help write
"more portable" tests; including [matching using regular expressions](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-pattern-matching-syntax),
allowing the creation of [named variables](https://llvm.org/docs/CommandGuide/FileCheck.html#filecheck-variables)
for later matching, and [checking non-sequential matches](https://llvm.org/docs/CommandGuide/FileCheck.html#the-check-dag-directive).
#### Capturing Variables
For example, say GCC stores a variable in a register but Clang stores
it in memory. To write a test that tolerates both cases we "capture"
the destination of the store, and then use the captured expression
to write the remainder of the test.
```c++
// CHECK-LABEL: test_div_no_op_into_shr:
extern "C" void test_div_no_op_into_shr(int value) {
int divisor = 2;
benchmark::DoNotOptimize(divisor); // hide the value from the optimizer
return value / divisor;
// CHECK: movl $2, [[DEST:.*]]
// CHECK: idivl [[DEST]]
// CHECK: ret
}
```
#### Using Regular Expressions to Match Differing Output
Often tests require testing assembly lines which may subtly differ
between compilers or compiler versions. A common example of this
is matching stack frame addresses. In this case regular expressions
can be used to match the differing bits of output. For example:
```c++
int ExternInt;
struct Point { int x, y, z; };
// CHECK-LABEL: test_store_point:
extern "C" void test_store_point() {
Point p{ExternInt, ExternInt, ExternInt};
benchmark::DoNotOptimize(p);
// CHECK: movl ExternInt(%rip), %eax
// CHECK: movl %eax, -{{[0-9]+}}(%rsp)
// CHECK: movl %eax, -{{[0-9]+}}(%rsp)
// CHECK: movl %eax, -{{[0-9]+}}(%rsp)
// CHECK: ret
}
```
## Current Requirements and Limitations
The tests require Filecheck to be installed along the `PATH` of the
build machine. Otherwise the tests will be disabled.
Additionally, as mentioned in the previous section, codegen tests are
inherently non-portable. Currently the tests are limited to:
* x86_64 targets.
* Compiled with GCC or Clang
Further work could be done, at least on a limited basis, to extend the
tests to other architectures and compilers (using `CHECK` prefixes).
Furthermore, the tests fail for builds which specify additional flags
that modify code generation, including `--coverage` or `-fsanitize=`.

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# Build tool dependency policy
To ensure the broadest compatibility when building the benchmark library, but
still allow forward progress, we require any build tooling to be available for:
* Debian stable AND
* The last two Ubuntu LTS releases AND
Currently, this means using build tool versions that are available for Ubuntu
16.04 (Xenial), Ubuntu 18.04 (Bionic), and Debian stretch.
_Note, [travis](.travis.yml) runs under Ubuntu 14.04 (Trusty) for linux builds._
## cmake
The current supported version is cmake 3.5.1 as of 2018-06-06.
_Note, this version is also available for Ubuntu 14.04, the previous Ubuntu LTS
release, as `cmake3`._

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# Benchmark
* [Assembly Tests](AssemblyTests.md)
* [Dependencies](dependencies.md)
* [Perf Counters](perf_counters.md)
* [Platform Specific Build Instructions](platform_specific_build_instructions.md)
* [Random Interleaving](random_interleaving.md)
* [Releasing](releasing.md)
* [Tools](tools.md)
* [User Guide](user_guide.md)

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<a name="perf-counters" />
# User-Requested Performance Counters
When running benchmarks, the user may choose to request collection of
performance counters. This may be useful in investigation scenarios - narrowing
down the cause of a regression; or verifying that the underlying cause of a
performance improvement matches expectations.
This feature is available if:
* The benchmark is run on an architecture featuring a Performance Monitoring
Unit (PMU),
* The benchmark is compiled with support for collecting counters. Currently,
this requires [libpfm](http://perfmon2.sourceforge.net/) be available at build
time
The feature does not require modifying benchmark code. Counter collection is
handled at the boundaries where timer collection is also handled.
To opt-in:
* Install `libpfm4-dev`, e.g. `apt-get install libpfm4-dev`.
* Enable the cmake flag BENCHMARK_ENABLE_LIBPFM.
To use, pass a comma-separated list of counter names through the
`--benchmark_perf_counters` flag. The names are decoded through libpfm - meaning,
they are platform specific, but some (e.g. `CYCLES` or `INSTRUCTIONS`) are
mapped by libpfm to platform-specifics - see libpfm
[documentation](http://perfmon2.sourceforge.net/docs.html) for more details.
The counter values are reported back through the [User Counters](../README.md#custom-counters)
mechanism, meaning, they are available in all the formats (e.g. JSON) supported
by User Counters.

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# Platform Specific Build Instructions
## Building with GCC
When the library is built using GCC it is necessary to link with the pthread
library due to how GCC implements `std::thread`. Failing to link to pthread will
lead to runtime exceptions (unless you're using libc++), not linker errors. See
[issue #67](https://github.com/google/benchmark/issues/67) for more details. You
can link to pthread by adding `-pthread` to your linker command. Note, you can
also use `-lpthread`, but there are potential issues with ordering of command
line parameters if you use that.
On QNX, the pthread library is part of libc and usually included automatically
(see
[`pthread_create()`](https://www.qnx.com/developers/docs/7.1/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/pthread_create.html)).
There's no separate pthread library to link.
## Building with Visual Studio 2015 or 2017
The `shlwapi` library (`-lshlwapi`) is required to support a call to `CPUInfo` which reads the registry. Either add `shlwapi.lib` under `[ Configuration Properties > Linker > Input ]`, or use the following:
```
// Alternatively, can add libraries using linker options.
#ifdef _WIN32
#pragma comment ( lib, "Shlwapi.lib" )
#ifdef _DEBUG
#pragma comment ( lib, "benchmarkd.lib" )
#else
#pragma comment ( lib, "benchmark.lib" )
#endif
#endif
```
Can also use the graphical version of CMake:
* Open `CMake GUI`.
* Under `Where to build the binaries`, same path as source plus `build`.
* Under `CMAKE_INSTALL_PREFIX`, same path as source plus `install`.
* Click `Configure`, `Generate`, `Open Project`.
* If build fails, try deleting entire directory and starting again, or unticking options to build less.
## Building with Intel 2015 Update 1 or Intel System Studio Update 4
See instructions for building with Visual Studio. Once built, right click on the solution and change the build to Intel.
## Building on Solaris
If you're running benchmarks on solaris, you'll want the kstat library linked in
too (`-lkstat`).

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<a name="interleaving" />
# Random Interleaving
[Random Interleaving](https://github.com/google/benchmark/issues/1051) is a
technique to lower run-to-run variance. It randomly interleaves repetitions of a
microbenchmark with repetitions from other microbenchmarks in the same benchmark
test. Data shows it is able to lower run-to-run variance by
[40%](https://github.com/google/benchmark/issues/1051) on average.
To use, you mainly need to set `--benchmark_enable_random_interleaving=true`,
and optionally specify non-zero repetition count `--benchmark_repetitions=9`
and optionally decrease the per-repetition time `--benchmark_min_time=0.1`.

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# How to release
* Make sure you're on main and synced to HEAD
* Ensure the project builds and tests run (sanity check only, obviously)
* `parallel -j0 exec ::: test/*_test` can help ensure everything at least
passes
* Prepare release notes
* `git log $(git describe --abbrev=0 --tags)..HEAD` gives you the list of
commits between the last annotated tag and HEAD
* Pick the most interesting.
* Create one last commit that updates the version saved in `CMakeLists.txt` to the release version you're creating. (This version will be used if benchmark is installed from the archive you'll be creating in the next step.)
```
project (benchmark VERSION 1.5.3 LANGUAGES CXX)
```
* Create a release through github's interface
* Note this will create a lightweight tag.
* Update this to an annotated tag:
* `git pull --tags`
* `git tag -a -f <tag> <tag>`
* `git push --force origin`

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# Benchmark Tools
## compare.py
The `compare.py` can be used to compare the result of benchmarks.
### Dependencies
The utility relies on the [scipy](https://www.scipy.org) package which can be installed using pip:
```bash
pip3 install -r requirements.txt
```
### Displaying aggregates only
The switch `-a` / `--display_aggregates_only` can be used to control the
displayment of the normal iterations vs the aggregates. When passed, it will
be passthrough to the benchmark binaries to be run, and will be accounted for
in the tool itself; only the aggregates will be displayed, but not normal runs.
It only affects the display, the separate runs will still be used to calculate
the U test.
### Modes of operation
There are three modes of operation:
1. Just compare two benchmarks
The program is invoked like:
``` bash
$ compare.py benchmarks <benchmark_baseline> <benchmark_contender> [benchmark options]...
```
Where `<benchmark_baseline>` and `<benchmark_contender>` either specify a benchmark executable file, or a JSON output file. The type of the input file is automatically detected. If a benchmark executable is specified then the benchmark is run to obtain the results. Otherwise the results are simply loaded from the output file.
`[benchmark options]` will be passed to the benchmarks invocations. They can be anything that binary accepts, be it either normal `--benchmark_*` parameters, or some custom parameters your binary takes.
Example output:
```
$ ./compare.py benchmarks ./a.out ./a.out
RUNNING: ./a.out --benchmark_out=/tmp/tmprBT5nW
Run on (8 X 4000 MHz CPU s)
2017-11-07 21:16:44
------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------
BM_memcpy/8 36 ns 36 ns 19101577 211.669MB/s
BM_memcpy/64 76 ns 76 ns 9412571 800.199MB/s
BM_memcpy/512 84 ns 84 ns 8249070 5.64771GB/s
BM_memcpy/1024 116 ns 116 ns 6181763 8.19505GB/s
BM_memcpy/8192 643 ns 643 ns 1062855 11.8636GB/s
BM_copy/8 222 ns 222 ns 3137987 34.3772MB/s
BM_copy/64 1608 ns 1608 ns 432758 37.9501MB/s
BM_copy/512 12589 ns 12589 ns 54806 38.7867MB/s
BM_copy/1024 25169 ns 25169 ns 27713 38.8003MB/s
BM_copy/8192 201165 ns 201112 ns 3486 38.8466MB/s
RUNNING: ./a.out --benchmark_out=/tmp/tmpt1wwG_
Run on (8 X 4000 MHz CPU s)
2017-11-07 21:16:53
------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------
BM_memcpy/8 36 ns 36 ns 19397903 211.255MB/s
BM_memcpy/64 73 ns 73 ns 9691174 839.635MB/s
BM_memcpy/512 85 ns 85 ns 8312329 5.60101GB/s
BM_memcpy/1024 118 ns 118 ns 6438774 8.11608GB/s
BM_memcpy/8192 656 ns 656 ns 1068644 11.6277GB/s
BM_copy/8 223 ns 223 ns 3146977 34.2338MB/s
BM_copy/64 1611 ns 1611 ns 435340 37.8751MB/s
BM_copy/512 12622 ns 12622 ns 54818 38.6844MB/s
BM_copy/1024 25257 ns 25239 ns 27779 38.6927MB/s
BM_copy/8192 205013 ns 205010 ns 3479 38.108MB/s
Comparing ./a.out to ./a.out
Benchmark Time CPU Time Old Time New CPU Old CPU New
------------------------------------------------------------------------------------------------------
BM_memcpy/8 +0.0020 +0.0020 36 36 36 36
BM_memcpy/64 -0.0468 -0.0470 76 73 76 73
BM_memcpy/512 +0.0081 +0.0083 84 85 84 85
BM_memcpy/1024 +0.0098 +0.0097 116 118 116 118
BM_memcpy/8192 +0.0200 +0.0203 643 656 643 656
BM_copy/8 +0.0046 +0.0042 222 223 222 223
BM_copy/64 +0.0020 +0.0020 1608 1611 1608 1611
BM_copy/512 +0.0027 +0.0026 12589 12622 12589 12622
BM_copy/1024 +0.0035 +0.0028 25169 25257 25169 25239
BM_copy/8192 +0.0191 +0.0194 201165 205013 201112 205010
```
What it does is for the every benchmark from the first run it looks for the benchmark with exactly the same name in the second run, and then compares the results. If the names differ, the benchmark is omitted from the diff.
As you can note, the values in `Time` and `CPU` columns are calculated as `(new - old) / |old|`.
2. Compare two different filters of one benchmark
The program is invoked like:
``` bash
$ compare.py filters <benchmark> <filter_baseline> <filter_contender> [benchmark options]...
```
Where `<benchmark>` either specify a benchmark executable file, or a JSON output file. The type of the input file is automatically detected. If a benchmark executable is specified then the benchmark is run to obtain the results. Otherwise the results are simply loaded from the output file.
Where `<filter_baseline>` and `<filter_contender>` are the same regex filters that you would pass to the `[--benchmark_filter=<regex>]` parameter of the benchmark binary.
`[benchmark options]` will be passed to the benchmarks invocations. They can be anything that binary accepts, be it either normal `--benchmark_*` parameters, or some custom parameters your binary takes.
Example output:
```
$ ./compare.py filters ./a.out BM_memcpy BM_copy
RUNNING: ./a.out --benchmark_filter=BM_memcpy --benchmark_out=/tmp/tmpBWKk0k
Run on (8 X 4000 MHz CPU s)
2017-11-07 21:37:28
------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------
BM_memcpy/8 36 ns 36 ns 17891491 211.215MB/s
BM_memcpy/64 74 ns 74 ns 9400999 825.646MB/s
BM_memcpy/512 87 ns 87 ns 8027453 5.46126GB/s
BM_memcpy/1024 111 ns 111 ns 6116853 8.5648GB/s
BM_memcpy/8192 657 ns 656 ns 1064679 11.6247GB/s
RUNNING: ./a.out --benchmark_filter=BM_copy --benchmark_out=/tmp/tmpAvWcOM
Run on (8 X 4000 MHz CPU s)
2017-11-07 21:37:33
----------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------
BM_copy/8 227 ns 227 ns 3038700 33.6264MB/s
BM_copy/64 1640 ns 1640 ns 426893 37.2154MB/s
BM_copy/512 12804 ns 12801 ns 55417 38.1444MB/s
BM_copy/1024 25409 ns 25407 ns 27516 38.4365MB/s
BM_copy/8192 202986 ns 202990 ns 3454 38.4871MB/s
Comparing BM_memcpy to BM_copy (from ./a.out)
Benchmark Time CPU Time Old Time New CPU Old CPU New
--------------------------------------------------------------------------------------------------------------------
[BM_memcpy vs. BM_copy]/8 +5.2829 +5.2812 36 227 36 227
[BM_memcpy vs. BM_copy]/64 +21.1719 +21.1856 74 1640 74 1640
[BM_memcpy vs. BM_copy]/512 +145.6487 +145.6097 87 12804 87 12801
[BM_memcpy vs. BM_copy]/1024 +227.1860 +227.1776 111 25409 111 25407
[BM_memcpy vs. BM_copy]/8192 +308.1664 +308.2898 657 202986 656 202990
```
As you can see, it applies filter to the benchmarks, both when running the benchmark, and before doing the diff. And to make the diff work, the matches are replaced with some common string. Thus, you can compare two different benchmark families within one benchmark binary.
As you can note, the values in `Time` and `CPU` columns are calculated as `(new - old) / |old|`.
3. Compare filter one from benchmark one to filter two from benchmark two:
The program is invoked like:
``` bash
$ compare.py filters <benchmark_baseline> <filter_baseline> <benchmark_contender> <filter_contender> [benchmark options]...
```
Where `<benchmark_baseline>` and `<benchmark_contender>` either specify a benchmark executable file, or a JSON output file. The type of the input file is automatically detected. If a benchmark executable is specified then the benchmark is run to obtain the results. Otherwise the results are simply loaded from the output file.
Where `<filter_baseline>` and `<filter_contender>` are the same regex filters that you would pass to the `[--benchmark_filter=<regex>]` parameter of the benchmark binary.
`[benchmark options]` will be passed to the benchmarks invocations. They can be anything that binary accepts, be it either normal `--benchmark_*` parameters, or some custom parameters your binary takes.
Example output:
```
$ ./compare.py benchmarksfiltered ./a.out BM_memcpy ./a.out BM_copy
RUNNING: ./a.out --benchmark_filter=BM_memcpy --benchmark_out=/tmp/tmp_FvbYg
Run on (8 X 4000 MHz CPU s)
2017-11-07 21:38:27
------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------
BM_memcpy/8 37 ns 37 ns 18953482 204.118MB/s
BM_memcpy/64 74 ns 74 ns 9206578 828.245MB/s
BM_memcpy/512 91 ns 91 ns 8086195 5.25476GB/s
BM_memcpy/1024 120 ns 120 ns 5804513 7.95662GB/s
BM_memcpy/8192 664 ns 664 ns 1028363 11.4948GB/s
RUNNING: ./a.out --benchmark_filter=BM_copy --benchmark_out=/tmp/tmpDfL5iE
Run on (8 X 4000 MHz CPU s)
2017-11-07 21:38:32
----------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------
BM_copy/8 230 ns 230 ns 2985909 33.1161MB/s
BM_copy/64 1654 ns 1653 ns 419408 36.9137MB/s
BM_copy/512 13122 ns 13120 ns 53403 37.2156MB/s
BM_copy/1024 26679 ns 26666 ns 26575 36.6218MB/s
BM_copy/8192 215068 ns 215053 ns 3221 36.3283MB/s
Comparing BM_memcpy (from ./a.out) to BM_copy (from ./a.out)
Benchmark Time CPU Time Old Time New CPU Old CPU New
--------------------------------------------------------------------------------------------------------------------
[BM_memcpy vs. BM_copy]/8 +5.1649 +5.1637 37 230 37 230
[BM_memcpy vs. BM_copy]/64 +21.4352 +21.4374 74 1654 74 1653
[BM_memcpy vs. BM_copy]/512 +143.6022 +143.5865 91 13122 91 13120
[BM_memcpy vs. BM_copy]/1024 +221.5903 +221.4790 120 26679 120 26666
[BM_memcpy vs. BM_copy]/8192 +322.9059 +323.0096 664 215068 664 215053
```
This is a mix of the previous two modes, two (potentially different) benchmark binaries are run, and a different filter is applied to each one.
As you can note, the values in `Time` and `CPU` columns are calculated as `(new - old) / |old|`.
### U test
If there is a sufficient repetition count of the benchmarks, the tool can do
a [U Test](https://en.wikipedia.org/wiki/Mann%E2%80%93Whitney_U_test), of 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.
If the calculated p-value is below this value is lower than the significance
level alpha, then the result is said to be statistically significant and the
null hypothesis is rejected. Which in other words means that the two benchmarks
aren't identical.
**WARNING**: requires **LARGE** (no less than 9) number of repetitions to be
meaningful!

File diff suppressed because it is too large Load Diff