Rewrite of the decoder (#263)
New code is smaller (in both source size and compiled size) and faster.
# Speed
The decoder speeds up on all machines I tested, though the amount of speedup varies. I was only able to test Intel CPUs.
### Linux Desktop
```
CPU: Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
OS: Linux
name old time/op new time/op delta
CreateArena 4.72ns ± 0% 4.93ns ± 0% +4.47% (p=0.000 n=11+11)
ParseDescriptor 12.4µs ± 1% 9.1µs ± 1% -26.65% (p=0.000 n=11+11)
```
### Mac Laptop
```
CPU: Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
OS: macOS
name old time/op new time/op delta
CreateArena 5.33ns ± 3% 5.58ns ± 2% +4.69% (p=0.000 n=12+12)
ParseDescriptor 15.0µs ± 2% 11.9µs ± 2% -20.20% (p=0.000 n=12+12)
```
### Linux Workstation
```
CPU: Intel(R) Xeon(R) Gold 6154 CPU @ 3.00GHz
OS: Linux
name old time/op new time/op delta
CreateArena 5.29ns ± 0% 5.52ns ± 0% +4.37% (p=0.000 n=10+12)
ParseDescriptor 18.6µs ± 0% 16.4µs ± 0% -11.54% (p=0.000 n=12+12)
```
# Size
A few source files grow marginally because of some arena functionality moved inline. But `upb/decode.c` shrinks by 30% on Linux:
```
VM SIZE
--------------
+2.1% +283 upb/json_decode.c
+24% +205 upb/msg.c
+8.4% +115 upb/upb.c
+0.9% +28 upb/reflection.c
[ = ] 0 upb/def.c
[ = ] 0 upb/encode.c
[ = ] 0 upb/json_encode.c
[ = ] 0 upb/table.c
-30.3% -1.51Ki upb/decode.c
-0.7% -738 TOTAL
```diff --git a/benchmark.py b/benchmark.py
new file mode 100755
index 0000000..9c59674
--- /dev/null
+++ b/benchmark.py
@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+
+import json
+import subprocess
+import re
+
+def Run(cmd):
+ subprocess.check_call(cmd, shell=True)
+
+def RunAgainstBranch(branch, outfile, runs=12):
+ tmpfile = "/tmp/bench-output.json"
+ Run("rm -rf {}".format(tmpfile))
+ Run("git checkout {}".format(branch))
+ Run("bazel build -c opt :benchmark")
+
+ Run("./bazel-bin/benchmark --benchmark_out_format=json --benchmark_out={} --benchmark_repetitions={}".format(tmpfile, runs))
+
+ with open(tmpfile) as f:
+ bench_json = json.load(f)
+
+ with open(outfile, "w") as f:
+ for run in bench_json["benchmarks"]:
+ name = re.sub(r'^BM_', 'Benchmark', run["name"])
+ if name.endswith("_mean") or name.endswith("_median") or name.endswith("_stddev"):
+ continue
+ values = (name, run["iterations"], run["cpu_time"])
+ print("{} {} {} ns/op".format(*values), file=f)
+
+RunAgainstBranch("master", "/tmp/old.txt")
+RunAgainstBranch("decoder", "/tmp/new.txt")
+
+Run("~/go/bin/benchstat /tmp/old.txt /tmp/new.txt")
