log: make use of fmt.Append functionality

Since log is already responsible for managing its own buffers
it is unfortunate that it calls fmt.Sprintf, which allocates,
only to append that intermediate string to another buffer.
Instead, use the new fmt.Append variants and avoid the allocation.

We modify Logger.Output to wrap an internal Logger.output,
which can be configured to use a particular append function.
Logger.output is called from all the other functionality instead.

This has the further advantage of simplifying the isDiscard check,
which occurs to avoid the costly fmt.Print call.
We coalesce all 6 checks as just 1 check in Logger.output.

Also, swap the declaration order of Logger.Print and Logger.Printf
to match the ordering elsewhere in the file.

Performance:
	name               old time/op    new time/op    delta
	Println            188ns ± 2%     172ns ± 4%    -8.39%  (p=0.000 n=10+10)
	PrintlnNoFlags     139ns ± 1%     116ns ± 1%   -16.71%  (p=0.000 n=9+9)

	name               old allocs/op  new allocs/op  delta
	Println             1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
	PrintlnNoFlags      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

Change-Id: I79d0ee404df848beb3626fe863ccc73a3e2eb325
Reviewed-on: https://go-review.googlesource.com/c/go/+/464345
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>

0 files changed, 0 insertions, 0 deletions