go/printer: reuse go/token.FileSet.PositionFor calls in setPos
setPos is called for most nodes, and in a number of cases,
the position is already the same.
PositionFor is a relatively expensive call,
as it needs to "unpack" a token.Pos into a token.Position.
If we can tell that the position is the same in a cheap way,
we can then avoid calling setPos and PositionFor.
Luckily, we can get the position's offset within the file,
and it doesn't involve the relatively expensive unpacking.
name old time/op new time/op delta
PrintFile-16 4.79ms ± 1% 4.36ms ± 1% -8.88% (p=0.008 n=5+5)
name old speed new speed delta
PrintFile-16 10.8MB/s ± 1% 11.9MB/s ± 1% +9.73% (p=0.008 n=5+5)
name old alloc/op new alloc/op delta
PrintFile-16 106kB ± 1% 106kB ± 1% ~ (p=0.167 n=5+5)
name old allocs/op new allocs/op delta
PrintFile-16 2.42k ± 0% 2.42k ± 0% ~ (all equal)
This does assume that the positions of a node being printed are all
within a file, as go/token.Position.Offset is relative to each file.
This seems like a perfectly fine assumption to make right now,
as the largest node which can be printed is an *ast.File.
Change-Id: I2ae55f507ba8ba9f280898c9c8e01c994d9b2a26
Reviewed-on: https://go-review.googlesource.com/c/go/+/461739 Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>