internal/goexperiment: add a sizespecializedmalloc goexperiment setting

Change-Id: I6a6a696497f2a0b0d403bbb11d7502f62edec78b
Reviewed-on: https://go-review.googlesource.com/c/go/+/696535
Auto-Submit: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/goexperiment/exp_sizespecializedmalloc_off.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build !goexperiment.sizespecializedmalloc
+
+package goexperiment
+
+const SizeSpecializedMalloc = false
+const SizeSpecializedMallocInt = 0

src/internal/goexperiment/exp_sizespecializedmalloc_on.go

@@ -0,0 +1,8 @@
+// Code generated by mkconsts.go. DO NOT EDIT.
+
+//go:build goexperiment.sizespecializedmalloc
+
+package goexperiment
+
+const SizeSpecializedMalloc = true
+const SizeSpecializedMallocInt = 1

src/internal/goexperiment/flags.go

@@ -115,4 +115,7 @@ type Flags struct {
 	// RandomizedHeapBase enables heap base address randomization on 64-bit
 	// platforms.
 	RandomizedHeapBase64 bool
+
+	// SizeSpecializedMalloc enables malloc implementations that are specialized per size class.
+	SizeSpecializedMalloc bool
 }