cmd/compile/internal/ssa: expand runtime.memequal for length {3,5,6,7}

This CL slightly speeds up strings.HasPrefix when testing constant
prefixes of length {3,5,6,7}.

goos: linux
goarch: amd64
cpu: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
                │      old     │                 new                 │
                │    sec/op    │   sec/op     vs base                │
StringPrefix3-8   11.125n ± 2%   8.539n ± 1%  -23.25% (p=0.000 n=20)
StringPrefix5-8   11.170n ± 2%   8.700n ± 1%  -22.11% (p=0.000 n=20)
StringPrefix6-8   11.190n ± 2%   8.655n ± 1%  -22.65% (p=0.000 n=20)
StringPrefix7-8   11.095n ± 1%   8.878n ± 1%  -19.98% (p=0.000 n=20)

Change-Id: I510a80d59cf78680b57d68780d35d212d24030e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/700816
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Keith Randall <khr@golang.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/test/codegen/strings.go b/test/codegen/strings.go
index fbcef57cd1..498c3d398f 100644
--- a/test/codegen/strings.go
+++ b/test/codegen/strings.go
@@ -6,6 +6,8 @@
 
 package codegen
 
+import "strings"
+
 // This file contains code generation tests related to the handling of
 // string types.
 
@@ -89,3 +91,23 @@ func NotEqualSelf(s string) bool {
 }
 
 var bsink []byte
+
+func HasPrefix3(s string) bool {
+	// amd64:-`.*memequal.*`
+	return strings.HasPrefix(s, "str")
+}
+
+func HasPrefix5(s string) bool {
+	// amd64:-`.*memequal.*`
+	return strings.HasPrefix(s, "strin")
+}
+
+func HasPrefix6(s string) bool {
+	// amd64:-`.*memequal.*`
+	return strings.HasPrefix(s, "string")
+}
+
+func HasPrefix7(s string) bool {
+	// amd64:-`.*memequal.*`
+	return strings.HasPrefix(s, "strings")
+}