runtime: use multiplication with overflow check for makemap

This improves performance for maps with a bucket size
(key+value*8 bytes) larger than 32 bytes and removes loading
a value from the maxElems array for smaller bucket sizes.

name                old time/op  new time/op  delta
MakeMap/[Byte]Byte  93.5ns ± 1%  91.8ns ± 1%  -1.83%  (p=0.000 n=10+10)
MakeMap/[Int]Int     134ns ± 1%   127ns ± 2%  -5.61%  (p=0.000 n=9+10)

Updates #21588

Change-Id: I53f77186769c4bd0f2b90f3c6c17df643b060e39
Reviewed-on: https://go-review.googlesource.com/c/143797
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/runtime/map_benchmark_test.go b/src/runtime/map_benchmark_test.go
index 1d9d09c698..5681d5eeb8 100644
--- a/src/runtime/map_benchmark_test.go
+++ b/src/runtime/map_benchmark_test.go
@@ -228,6 +228,23 @@ func benchmarkRepeatedLookup(b *testing.B, lookupKeySize int) {
 func BenchmarkRepeatedLookupStrMapKey32(b *testing.B) { benchmarkRepeatedLookup(b, 32) }
 func BenchmarkRepeatedLookupStrMapKey1M(b *testing.B) { benchmarkRepeatedLookup(b, 1<<20) }
 
+func BenchmarkMakeMap(b *testing.B) {
+	b.Run("[Byte]Byte", func(b *testing.B) {
+		var m map[byte]byte
+		for i := 0; i < b.N; i++ {
+			m = make(map[byte]byte, 10)
+		}
+		hugeSink = m
+	})
+	b.Run("[Int]Int", func(b *testing.B) {
+		var m map[int]int
+		for i := 0; i < b.N; i++ {
+			m = make(map[int]int, 10)
+		}
+		hugeSink = m
+	})
+}
+
 func BenchmarkNewEmptyMap(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {