runtime: don't acquirem on malloc fast path

Mallocgc must be atomic wrt GC, but for performance reasons
don't acquirem/releasem on fast path. The code does not have
split stack checks, so it can't be preempted by GC.
Functions like roundup/add are inlined. And onM/racemalloc are nosplit.
Also add debug code that checks these assumptions.

benchmark                     old ns/op     new ns/op     delta
BenchmarkMalloc8              20.5          17.2          -16.10%
BenchmarkMalloc16             29.5          27.0          -8.47%
BenchmarkMallocTypeInfo8      31.5          27.6          -12.38%
BenchmarkMallocTypeInfo16     34.7          30.9          -10.95%

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/123100043

1 files changed, 4 insertions, 1 deletions

diff --git a/src/cmd/api/goapi.go b/src/cmd/api/goapi.go
index 38bf9592f2..e0d17c9564 100644
--- a/src/cmd/api/goapi.go
+++ b/src/cmd/api/goapi.go
@@ -378,7 +378,10 @@ func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
 	}
 	if w.context != nil && file == fmt.Sprintf("zruntime_defs_%s_%s.go", w.context.GOOS, w.context.GOARCH) {
 		// Just enough to keep the api checker happy.
-		src := "package runtime; type maptype struct{}; type _type struct{}; type alg struct{}; type mspan struct{}; type m struct{}; type lock struct{}; type slicetype struct{}; type iface struct{}; type eface struct{}; type interfacetype struct{}; type itab struct{}"
+		src := "package runtime; type maptype struct{}; type _type struct{}; type alg struct{};" +
+			" type mspan struct{}; type m struct{}; type lock struct{}; type slicetype struct{};" +
+			" type iface struct{}; type eface struct{}; type interfacetype struct{}; type itab struct{};" +
+			" type mcache struct{}"
 		f, err = parser.ParseFile(fset, filename, src, 0)
 		if err != nil {
 			log.Fatalf("incorrect generated file: %s", err)