Add compare_bench.py documentation. Fixes #309 (#318)

README.md

@@ -11,6 +11,8 @@ IRC channel: https://freenode.net #googlebenchmark
 
 [Known issues and common problems](#known-issues)
 
+[Additional Tooling Documentation](docs/tools.md)
+
 ## Example usage
 ### Basic usage
 Define a function that executes the code to be measured.

docs/tools.md

+# Benchmark Tools
+
+## compare_bench.py
+
+The `compare_bench.py` utility which can be used to compare the result of benchmarks.
+The program is invoked like:
+
+``` bash
+$ compare_bench.py <old-benchmark> <new-benchmark> [benchmark options]...
+```
+
+Where `<old-benchmark>` and `<new-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.
+
+The sample output using the JSON test files under `Inputs/` gives:
+
+``` bash
+$ ./compare_bench.py ./gbench/Inputs/test1_run1.json ./gbench/Inputs/test1_run2.json
+Comparing ./gbench/Inputs/test1_run1.json to ./gbench/Inputs/test1_run2.json
+Benchmark                   Time           CPU
+----------------------------------------------
+BM_SameTimes               +0.00         +0.00
+BM_2xFaster                -0.50         -0.50
+BM_2xSlower                +1.00         +1.00
+BM_10PercentFaster         -0.10         -0.10
+BM_10PercentSlower         +0.10         +0.10
+```
+
+When a benchmark executable is run, the raw output from the benchmark is printed in real time to stdout. The sample output using `benchmark/basic_test` for both arguments looks like:
+
+```
+./compare_bench.py  test/basic_test test/basic_test  --benchmark_filter=BM_empty.*
+RUNNING: test/basic_test --benchmark_filter=BM_empty.*
+Run on (4 X 4228.32 MHz CPU s)
+2016-08-02 19:21:33
+Benchmark                              Time           CPU Iterations
+--------------------------------------------------------------------
+BM_empty                               9 ns          9 ns   79545455
+BM_empty/threads:4                     4 ns          9 ns   75268816
+BM_empty_stop_start                    8 ns          8 ns   83333333
+BM_empty_stop_start/threads:4          3 ns          8 ns   83333332
+RUNNING: test/basic_test --benchmark_filter=BM_empty.*
+Run on (4 X 4228.32 MHz CPU s)
+2016-08-02 19:21:35
+Benchmark                              Time           CPU Iterations
+--------------------------------------------------------------------
+BM_empty                               9 ns          9 ns   76086957
+BM_empty/threads:4                     4 ns          9 ns   76086956
+BM_empty_stop_start                    8 ns          8 ns   87500000
+BM_empty_stop_start/threads:4          3 ns          8 ns   88607596
+Comparing test/basic_test to test/basic_test
+Benchmark                              Time           CPU
+---------------------------------------------------------
+BM_empty                              +0.00         +0.00
+BM_empty/threads:4                    +0.00         +0.00
+BM_empty_stop_start                   +0.00         +0.00
+BM_empty_stop_start/threads:4         +0.00         +0.00
+```
+
+Obviously this example doesn't give any useful output, but it's intended to show the output format when 'compare_bench.py' needs to run benchmarks.