runtime: special case copy, equal for one-word interface values

Based on the observation that a great number of the types that
are copied or compared in interfaces, maps, and channels are
word-sized, this uses specialized copy and equality functions
for them that use a word instead of 4 or 8 bytes. Seems to yield
0-6% improvements in performance in the benchmarks I've run.
For example, with the regexp benchmarks:

Before:
regexp.BenchmarkLiteral   500000       3.26 µs/op
regexp.BenchmarkNotLiteral    100000      13.67 µs/op
regexp.BenchmarkMatchClass    100000      18.72 µs/op
regexp.BenchmarkMatchClass_InRange    100000      20.04 µs/op
regexp.BenchmarkReplaceAll    100000      27.85 µs/op

After:
regexp.BenchmarkLiteral   500000       3.11 µs/op
regexp.BenchmarkNotLiteral    200000      13.29 µs/op
regexp.BenchmarkMatchClass    100000      17.65 µs/op
regexp.BenchmarkMatchClass_InRange    100000      18.49 µs/op
regexp.BenchmarkReplaceAll    100000      26.34 µs/op

R=rsc
CC=golang-dev
https://golang.org/cl/1967047

src/cmd/gc/go.h

@@ -38,6 +38,7 @@ enum
 	ASTRING,
 	AINTER,
 	ANILINTER,
+	AMEMWORD,
 
 	BADWIDTH	= -1000000000,
 	MAXWIDTH        = 1<<30

src/cmd/gc/subr.c

@@ -476,9 +476,13 @@ algtype(Type *t)
 	int a;
 
 	if(issimple[t->etype] || isptr[t->etype] || iscomplex[t->etype] ||
-	   t->etype == TCHAN || t->etype == TFUNC || t->etype == TMAP)
-		a = AMEM;	// just bytes (int, ptr, etc)
-	else if(t->etype == TSTRING)
+		t->etype == TCHAN || t->etype == TFUNC || t->etype == TMAP) {
+		if (t->width == widthptr) {
+			a = AMEMWORD;
+		} else {
+			a = AMEM;	// just bytes (int, ptr, etc)
+		}
+	} else if(t->etype == TSTRING)
 		a = ASTRING;	// string
 	else if(isnilinter(t))
 		a = ANILINTER;	// nil interface

src/pkg/runtime/runtime.c

@@ -361,6 +361,24 @@ memcopy(uint32 s, void *a, void *b)
 		ba[i] = bb[i];
 }
 
+static uint32
+memwordequal(uint32 s, void *a, void *b)
+{
+	USED(s);
+	return *(uintptr*)(a) == *(uintptr*)(b);
+}
+
+static void
+memwordcopy(uint32 s, void *a, void *b)
+{
+	USED(s);
+	if (b == nil) {
+		*(uintptr*)(a) = 0;
+		return;
+	}
+	*(uintptr*)(a) = *(uintptr*)(b);
+}
+
 static uintptr
 strhash(uint32 s, String *a)
 {
@@ -451,6 +469,7 @@ algarray[] =
 [ASTRING]	{ strhash, strequal, strprint, memcopy },
 [AINTER]		{ interhash, interequal, interprint, memcopy },
 [ANILINTER]	{ nilinterhash, nilinterequal, nilinterprint, memcopy },
+[AMEMWORD] { memhash, memwordequal, memprint, memwordcopy },
 };
 
 #pragma textflag 7

src/pkg/runtime/runtime.h

@@ -307,6 +307,7 @@ enum
 	ASTRING,
 	AINTER,
 	ANILINTER,
+	AMEMWORD,
 	Amax
 };