runtime: add type algorithms for zero-sized types

BenchmarkChanSem old=127ns new=78.6ns

R=golang-dev, bradfitz, sameer, rsc
CC=golang-dev
https://golang.org/cl/5558049

src/cmd/gc/go.h

@@ -40,12 +40,14 @@ enum
 	// These values are known by runtime.
 	// The MEMx and NOEQx values must run in parallel.  See algtype.
 	AMEM		= 0,
+	AMEM0,
 	AMEM8,
 	AMEM16,
 	AMEM32,
 	AMEM64,
 	AMEM128,
 	ANOEQ,
+	ANOEQ0,
 	ANOEQ8,
 	ANOEQ16,
 	ANOEQ32,

src/cmd/gc/subr.c

@@ -586,6 +586,8 @@ algtype(Type *t)
 		if(isslice(t))
 			return ASLICE;
 		switch(t->width) {
+		case 0:
+			return a + AMEM0 - AMEM;
 		case 1:
 			return a + AMEM8 - AMEM;
 		case 2:

src/pkg/runtime/alg.c

@@ -88,6 +88,23 @@ runtime·memcopy(uintptr s, void *a, void *b)
 	runtime·memmove(a, b, s);
 }
 
+void
+runtime·memequal0(bool *eq, uintptr s, void *a, void *b)
+{
+	USED(s);
+	USED(a);
+	USED(b);
+	*eq = true;
+}
+
+void
+runtime·memcopy0(uintptr s, void *a, void *b)
+{
+	USED(s);
+	USED(a);
+	USED(b);
+}
+
 void
 runtime·memequal8(bool *eq, uintptr s, void *a, void *b)
 {
@@ -332,11 +349,13 @@ runtime·algarray[] =
 [AINTER]	{ runtime·interhash, runtime·interequal, runtime·interprint, runtime·intercopy },
 [ANILINTER]	{ runtime·nilinterhash, runtime·nilinterequal, runtime·nilinterprint, runtime·nilintercopy },
 [ASLICE]	{ runtime·nohash, runtime·noequal, runtime·memprint, runtime·slicecopy },
+[AMEM0]		{ runtime·memhash, runtime·memequal0, runtime·memprint, runtime·memcopy0 },
 [AMEM8]		{ runtime·memhash, runtime·memequal8, runtime·memprint, runtime·memcopy8 },
 [AMEM16]	{ runtime·memhash, runtime·memequal16, runtime·memprint, runtime·memcopy16 },
 [AMEM32]	{ runtime·memhash, runtime·memequal32, runtime·memprint, runtime·memcopy32 },
 [AMEM64]	{ runtime·memhash, runtime·memequal64, runtime·memprint, runtime·memcopy64 },
 [AMEM128]	{ runtime·memhash, runtime·memequal128, runtime·memprint, runtime·memcopy128 },
+[ANOEQ0]	{ runtime·nohash, runtime·noequal, runtime·memprint, runtime·memcopy0 },
 [ANOEQ8]	{ runtime·nohash, runtime·noequal, runtime·memprint, runtime·memcopy8 },
 [ANOEQ16]	{ runtime·nohash, runtime·noequal, runtime·memprint, runtime·memcopy16 },
 [ANOEQ32]	{ runtime·nohash, runtime·noequal, runtime·memprint, runtime·memcopy32 },

src/pkg/runtime/chan_test.go

@@ -371,3 +371,12 @@ func BenchmarkChanCreation(b *testing.B) {
 		<-c
 	}
 }
+
+func BenchmarkChanSem(b *testing.B) {
+	type Empty struct{}
+	c := make(chan Empty, 1)
+	for i := 0; i < b.N; i++ {
+		c <- Empty{}
+		<-c
+	}
+}

src/pkg/runtime/runtime.h

@@ -358,12 +358,14 @@ enum {
 enum
 {
 	AMEM,
+	AMEM0,
 	AMEM8,
 	AMEM16,
 	AMEM32,
 	AMEM64,
 	AMEM128,
 	ANOEQ,
+	ANOEQ0,
 	ANOEQ8,
 	ANOEQ16,
 	ANOEQ32,