runtime: support runtime.freegc in size-specialized mallocs for noscan objects

This CL is part of a set of CLs that attempt to reduce how much work the
GC must do. See the design in https://go.dev/design/74299-runtime-freegc

This CL updates the smallNoScanStub stub in malloc_stubs.go to reuse
heap objects that have been freed by runtime.freegc calls, and generates
the corresponding size-specialized code in malloc_generated.go.

This CL only adds support in the specialized mallocs for noscan
heap objects (objects without pointers). A later CL handles objects
with pointers.

While we are here, we leave a couple of breadcrumbs in mkmalloc.go on
how to do the generation.

Updates #74299

Change-Id: I2657622601a27211554ee862fce057e101767a70
Reviewed-on: https://go-review.googlesource.com/c/go/+/715761
Reviewed-by: Junyang Shao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: thepudds

src/runtime/_mkmalloc/mkmalloc.go

@@ -254,7 +254,8 @@ func inline(config generatorConfig) []byte {
 	}
 
 	// Write out the package and import declarations.
-	out.WriteString("// Code generated by mkmalloc.go; DO NOT EDIT.\n\n")
+	out.WriteString("// Code generated by mkmalloc.go; DO NOT EDIT.\n")
+	out.WriteString("// See overview in malloc_stubs.go.\n\n")
 	out.WriteString("package " + f.Name.Name + "\n\n")
 	for _, importDecl := range importDecls {
 		out.Write(mustFormatNode(fset, importDecl))

src/runtime/malloc.go

@@ -1094,6 +1094,8 @@ const sizeSpecializedMallocEnabled = goexperiment.SizeSpecializedMalloc && GOOS
 // implementation and the corresponding allocation-related changes: the experiment must be
 // enabled, and none of the memory sanitizers should be enabled. We allow the race detector,
 // in contrast to sizeSpecializedMallocEnabled.
+// TODO(thepudds): it would be nice to check Valgrind integration, though there are some hints
+// there might not be any canned tests in tree for Go's integration with Valgrind.
 const runtimeFreegcEnabled = goexperiment.RuntimeFreegc && !asanenabled && !msanenabled && !valgrindenabled
 
 // Allocate an object of size bytes.
@@ -1966,10 +1968,15 @@ const (
 // or roughly when the liveness analysis of the compiler
 // would otherwise have determined ptr's object is reclaimable by the GC.
 func freegc(ptr unsafe.Pointer, size uintptr, noscan bool) bool {
-	if !runtimeFreegcEnabled || sizeSpecializedMallocEnabled || !reusableSize(size) {
-		// TODO(thepudds): temporarily disable freegc with SizeSpecializedMalloc until we finish integrating.
+	if !runtimeFreegcEnabled || !reusableSize(size) {
 		return false
 	}
+	if sizeSpecializedMallocEnabled && !noscan {
+		// TODO(thepudds): temporarily disable freegc with SizeSpecializedMalloc for pointer types
+		// until we finish integrating.
+		return false
+	}
+
 	if ptr == nil {
 		throw("freegc nil")
 	}