cmd/compile: inline memequal(x, const, sz) for small sizes

This CL adds late expanded memequal(x, const, sz) inlining for 2, 4, 8
bytes size. This PoC is using the same method as CL 248404.
This optimization fires about 100 times in Go compiler (1675 occurrences
reduced to 1574, so -6%).
Also, added unit-tests to codegen/comparisions.go file.

Updates #37275

Change-Id: Ia52808d573cb706d1da8166c5746ede26f46c5da
Reviewed-on: https://go-review.googlesource.com/c/go/+/328291
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Trust: David Chase <drchase@google.com>

1 files changed, 62 insertions, 0 deletions

diff --git a/test/codegen/comparisons.go b/test/codegen/comparisons.go
index 17dcd94ae1..35a181f83b 100644
--- a/test/codegen/comparisons.go
+++ b/test/codegen/comparisons.go
@@ -538,3 +538,65 @@ func CmpToOneU_ex2(a uint8, b uint16, c uint32, d uint64) int {
 	}
 	return 0
 }
+
+// Check that small memequals are replaced with eq instructions
+
+func equalConstString1() bool {
+	a := string("A")
+	b := string("Z")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a == b
+}
+
+func equalVarString1(a string) bool {
+	b := string("Z")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a[:1] == b
+}
+
+func equalConstString2() bool {
+	a := string("AA")
+	b := string("ZZ")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a == b
+}
+
+func equalVarString2(a string) bool {
+	b := string("ZZ")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a[:2] == b
+}
+
+func equalConstString4() bool {
+	a := string("AAAA")
+	b := string("ZZZZ")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a == b
+}
+
+func equalVarString4(a string) bool {
+	b := string("ZZZZ")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a[:4] == b
+}
+
+func equalConstString8() bool {
+	a := string("AAAAAAAA")
+	b := string("ZZZZZZZZ")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a == b
+}
+
+func equalVarString8(a string) bool {
+	b := string("ZZZZZZZZ")
+	// amd64:-".*memequal"
+	// arm64:-".*memequal"
+	return a[:8] == b
+}