cmd/compile: avoid string allocations when map key is struct or array literal

x = map[string(byteslice)] is already optimized by the compiler to avoid a
string allocation. This CL generalizes this optimization to:

x = map[T1{ ... Tn{..., string(byteslice), ...} ... }]
where T1 to Tn is a nesting of struct and array literals.

Found in a hot code path that used a struct of strings made from []byte
slices to make a map lookup.

There are no uses of the more generalized optimization in the standard library.
Passes toolstash -cmp.

MapStringConversion/32/simple    21.9ns ± 2%    21.9ns ± 3%      ~     (p=0.995 n=17+20)
MapStringConversion/32/struct    28.8ns ± 3%    22.0ns ± 2%   -23.80%  (p=0.000 n=20+20)
MapStringConversion/32/array     28.5ns ± 2%    21.9ns ± 2%   -23.14%  (p=0.000 n=19+16)
MapStringConversion/64/simple    21.0ns ± 2%    21.1ns ± 3%      ~     (p=0.072 n=19+18)
MapStringConversion/64/struct    72.4ns ± 3%    21.3ns ± 2%   -70.53%  (p=0.000 n=20+20)
MapStringConversion/64/array     72.8ns ± 1%    21.0ns ± 2%   -71.13%  (p=0.000 n=17+19)

name                           old allocs/op  new allocs/op  delta
MapStringConversion/32/simple      0.00           0.00           ~     (all equal)
MapStringConversion/32/struct      0.00           0.00           ~     (all equal)
MapStringConversion/32/array       0.00           0.00           ~     (all equal)
MapStringConversion/64/simple      0.00           0.00           ~     (all equal)
MapStringConversion/64/struct      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+20)
MapStringConversion/64/array       1.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+20)

Change-Id: I483b4d84d8d74b1025b62c954da9a365e79b7a3a
Reviewed-on: https://go-review.googlesource.com/c/116275
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/test/codegen/maps.go b/test/codegen/maps.go
index d167715898..8dd22ed5ca 100644
--- a/test/codegen/maps.go
+++ b/test/codegen/maps.go
@@ -38,6 +38,35 @@ func AccessString2(m map[string]int) bool {
 }
 
 // ------------------- //
+//  String Conversion  //
+// ------------------- //
+
+func LookupStringConversionSimple(m map[string]int, bytes []byte) int {
+	// amd64:-`.*runtime\.slicebytetostring\(`
+	return m[string(bytes)]
+}
+
+func LookupStringConversionStructLit(m map[struct{ string }]int, bytes []byte) int {
+	// amd64:-`.*runtime\.slicebytetostring\(`
+	return m[struct{ string }{string(bytes)}]
+}
+
+func LookupStringConversionArrayLit(m map[[2]string]int, bytes []byte) int {
+	// amd64:-`.*runtime\.slicebytetostring\(`
+	return m[[2]string{string(bytes), string(bytes)}]
+}
+
+func LookupStringConversionNestedLit(m map[[1]struct{ s [1]string }]int, bytes []byte) int {
+	// amd64:-`.*runtime\.slicebytetostring\(`
+	return m[[1]struct{ s [1]string }{struct{ s [1]string }{s: [1]string{string(bytes)}}}]
+}
+
+func LookupStringConversionKeyedArrayLit(m map[[2]string]int, bytes []byte) int {
+	// amd64:-`.*runtime\.slicebytetostring\(`
+	return m[[2]string{0: string(bytes)}]
+}
+
+// ------------------- //
 //     Map Clear       //
 // ------------------- //