testing: only compute b.N once when passed -count > 1
When running a benchmark multiple times, instead of re-computing the
value of b.N each time, use the value found by the first run.
For
go test -bench=. -benchtime 3s -count 2 p_test.go
on the benchmark in the linked issue; before:
BenchmarkBenchmark-4 500
10180593 ns/op
--- BENCH: BenchmarkBenchmark-4
p_test.go:13: single call took 10.111079ms
p_test.go:13: single call took 1.017298685s
p_test.go:13: single call took 5.090096124s
BenchmarkBenchmark-4 500
10182164 ns/op
--- BENCH: BenchmarkBenchmark-4
p_test.go:13: single call took 10.098169ms
p_test.go:13: single call took 1.017712905s
p_test.go:13: single call took 5.090898517s
PASS
ok command-line-arguments 12.244s
and after:
BenchmarkBenchmark-4 500
10177076 ns/op
--- BENCH: BenchmarkBenchmark-4
p_test.go:13: single call took 10.091301ms
p_test.go:13: single call took 1.016943125s
p_test.go:13: single call took 5.088376028s
BenchmarkBenchmark-4 500
10171497 ns/op
--- BENCH: BenchmarkBenchmark-4
p_test.go:13: single call took 10.140245ms
p_test.go:13: single call took 5.085605921s
PASS
ok command-line-arguments 11.218s
Fixes #23423
Change-Id: Ie66a8c5ac43881eb8741e14105db28745b4d56d3
Reviewed-on: https://go-review.googlesource.com/110775
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>