runtime: optimize some hash lookups.

When comparing strings, check these (in order):
- length mismatch => not equal
- string pointer equal => equal
- if length is short:
  - memeq on body
- if length is long:
  - compare first&last few bytes, if different => not equal
  - save entry as a possible match
  - after checking every entry, if there is only one possible
    match, use memeq on that entry.  Otherwise, fallback to hash.

benchmark                 old ns/op    new ns/op    delta
BenchmarkSameLengthMap           43            4  -89.77%

Fixes #5194.
Update #3885.

R=golang-dev, bradfitz, khr, rsc
CC=golang-dev
https://golang.org/cl/12128044

src/pkg/runtime/hashmap.c

@@ -533,48 +533,65 @@ static uint8 empty_value[MAXVALUESIZE];
 #define HASH_LOOKUP2 runtime·mapaccess2_fast32
 #define KEYTYPE uint32
 #define HASHFUNC runtime·algarray[AMEM32].hash
-#define EQFUNC(x,y) ((x) == (y))
-#define EQMAYBE(x,y) ((x) == (y))
-#define HASMAYBE false
-#define QUICKEQ(x) true
+#define FASTKEY(x) true
+#define QUICK_NE(x,y) ((x) != (y))
+#define QUICK_EQ(x,y) true
+#define SLOW_EQ(x,y) true
+#define MAYBE_EQ(x,y) true
 #include "hashmap_fast.c"
 
 #undef HASH_LOOKUP1
 #undef HASH_LOOKUP2
 #undef KEYTYPE
 #undef HASHFUNC
-#undef EQFUNC
-#undef EQMAYBE
-#undef HASMAYBE
-#undef QUICKEQ
+#undef FASTKEY
+#undef QUICK_NE
+#undef QUICK_EQ
+#undef SLOW_EQ
+#undef MAYBE_EQ
 
 #define HASH_LOOKUP1 runtime·mapaccess1_fast64
 #define HASH_LOOKUP2 runtime·mapaccess2_fast64
 #define KEYTYPE uint64
 #define HASHFUNC runtime·algarray[AMEM64].hash
-#define EQFUNC(x,y) ((x) == (y))
-#define EQMAYBE(x,y) ((x) == (y))
-#define HASMAYBE false
-#define QUICKEQ(x) true
+#define FASTKEY(x) true
+#define QUICK_NE(x,y) ((x) != (y))
+#define QUICK_EQ(x,y) true
+#define SLOW_EQ(x,y) true
+#define MAYBE_EQ(x,y) true
 #include "hashmap_fast.c"
 
 #undef HASH_LOOKUP1
 #undef HASH_LOOKUP2
 #undef KEYTYPE
 #undef HASHFUNC
-#undef EQFUNC
-#undef EQMAYBE
-#undef HASMAYBE
-#undef QUICKEQ
+#undef FASTKEY
+#undef QUICK_NE
+#undef QUICK_EQ
+#undef SLOW_EQ
+#undef MAYBE_EQ
+
+#ifdef GOARCH_amd64
+#define CHECKTYPE uint64
+#endif
+#ifdef GOARCH_386
+#define CHECKTYPE uint32
+#endif
+#ifdef GOARCH_arm
+// can't use uint32 on arm because our loads aren't aligned.
+// TODO: use uint32 for arm v6+?
+#define CHECKTYPE uint8
+#endif
 
 #define HASH_LOOKUP1 runtime·mapaccess1_faststr
 #define HASH_LOOKUP2 runtime·mapaccess2_faststr
 #define KEYTYPE String
 #define HASHFUNC runtime·algarray[ASTRING].hash
-#define EQFUNC(x,y) ((x).len == (y).len && ((x).str == (y).str || runtime·memeq((x).str, (y).str, (x).len)))
-#define EQMAYBE(x,y) ((x).len == (y).len)
-#define HASMAYBE true
-#define QUICKEQ(x) ((x).len < 32)
+#define FASTKEY(x) ((x).len < 32)
+#define QUICK_NE(x,y) ((x).len != (y).len)
+#define QUICK_EQ(x,y) ((x).str == (y).str)
+#define SLOW_EQ(x,y) runtime·memeq((x).str, (y).str, (x).len)
+#define MAYBE_EQ(x,y) (*(CHECKTYPE*)(x).str == *(CHECKTYPE*)(y).str && *(CHECKTYPE*)((x).str + (x).len - sizeof(CHECKTYPE)) == *(CHECKTYPE*)((y).str + (x).len - sizeof(CHECKTYPE)))
 #include "hashmap_fast.c"
 
 static void

src/pkg/runtime/hashmap_fast.c

@@ -22,7 +22,6 @@ HASH_LOOKUP1(MapType *t, Hmap *h, KEYTYPE key, byte *value)
 	byte *v;
 	uint8 top;
 	int8 keymaybe;
-	bool quickkey;
 
 	if(debug) {
 		runtime·prints("runtime.mapaccess1_fastXXX: map=");
@@ -43,37 +42,50 @@ HASH_LOOKUP1(MapType *t, Hmap *h, KEYTYPE key, byte *value)
 
 	if(h->B == 0) {
 		// One-bucket table. Don't hash, just check each bucket entry.
-		if(HASMAYBE) {
-			keymaybe = -1;
-		}
-		quickkey = QUICKEQ(key);
 		b = (Bucket*)h->buckets;
-		for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
-			if(b->tophash[i] != 0) {
-				if(quickkey && EQFUNC(key, *k)) {
+		if(FASTKEY(key)) {
+			for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
+				if(b->tophash[i] == 0)
+					continue;
+				if(QUICK_NE(key, *k))
+					continue;
+				if(QUICK_EQ(key, *k) || SLOW_EQ(key, *k)) {
+					value = v;
+					FLUSH(&value);
+					return;
+				}
+			}
+		} else {
+			keymaybe = -1;
+			for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
+				if(b->tophash[i] == 0)
+					continue;
+				if(QUICK_NE(key, *k))
+					continue;
+				if(QUICK_EQ(key, *k)) {
 					value = v;
 					FLUSH(&value);
 					return;
 				}
-				if(HASMAYBE && EQMAYBE(key, *k)) {
-					// TODO: check if key.str matches. Add EQFUNCFAST?
+				if(MAYBE_EQ(key, *k)) {
 					if(keymaybe >= 0) {
 						// Two same-length strings in this bucket.
 						// use slow path.
-						// TODO: keep track of more than just 1. Especially
-						// if doing the TODO above.
+						// TODO: keep track of more than just 1.  We could
+						// afford about 3 equals calls before it would be more
+						// expensive than 1 hash + 1 equals.
 						goto dohash;
 					}
 					keymaybe = i;
 				}
 			}
-		}
-		if(HASMAYBE && keymaybe >= 0) {
-			k = (KEYTYPE*)b->data + keymaybe;
-			if(EQFUNC(key, *k)) {
-				value = (byte*)((KEYTYPE*)b->data + BUCKETSIZE) + keymaybe * h->valuesize;
-				FLUSH(&value);
-				return;
+			if(keymaybe >= 0) {
+				k = (KEYTYPE*)b->data + keymaybe;
+				if(SLOW_EQ(key, *k)) {
+					value = (byte*)((KEYTYPE*)b->data + BUCKETSIZE) + keymaybe * h->valuesize;
+					FLUSH(&value);
+					return;
+				}
 			}
 		}
 	} else {
@@ -95,7 +107,11 @@ dohash:
 		}
 		do {
 			for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
-				if(b->tophash[i] == top && EQFUNC(key, *k)) {
+				if(b->tophash[i] != top)
+					continue;
+				if(QUICK_NE(key, *k))
+					continue;
+				if(QUICK_EQ(key, *k) || SLOW_EQ(key, *k)) {
 					value = v;
 					FLUSH(&value);
 					return;
@@ -118,7 +134,6 @@ HASH_LOOKUP2(MapType *t, Hmap *h, KEYTYPE key, byte *value, bool res)
 	byte *v;
 	uint8 top;
 	int8 keymaybe;
-	bool quickkey;
 
 	if(debug) {
 		runtime·prints("runtime.mapaccess2_fastXXX: map=");
@@ -140,42 +155,57 @@ HASH_LOOKUP2(MapType *t, Hmap *h, KEYTYPE key, byte *value, bool res)
 		check(t, h);
 
 	if(h->B == 0) {
-		// One-bucket table.  Don't hash, just check each bucket entry.
-		if(HASMAYBE) {
-			keymaybe = -1;
-		}
-		quickkey = QUICKEQ(key);
+		// One-bucket table. Don't hash, just check each bucket entry.
 		b = (Bucket*)h->buckets;
-		for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
-			if(b->tophash[i] != 0) {
-				if(quickkey && EQFUNC(key, *k)) {
+		if(FASTKEY(key)) {
+			for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
+				if(b->tophash[i] == 0)
+					continue;
+				if(QUICK_NE(key, *k))
+					continue;
+				if(QUICK_EQ(key, *k) || SLOW_EQ(key, *k)) {
 					value = v;
 					res = true;
 					FLUSH(&value);
 					FLUSH(&res);
 					return;
 				}
-				if(HASMAYBE && EQMAYBE(key, *k)) {
-					// TODO: check if key.str matches. Add EQFUNCFAST?
+			}
+		} else {
+			keymaybe = -1;
+			for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
+				if(b->tophash[i] == 0)
+					continue;
+				if(QUICK_NE(key, *k))
+					continue;
+				if(QUICK_EQ(key, *k)) {
+					value = v;
+					res = true;
+					FLUSH(&value);
+					FLUSH(&res);
+					return;
+				}
+				if(MAYBE_EQ(key, *k)) {
 					if(keymaybe >= 0) {
 						// Two same-length strings in this bucket.
 						// use slow path.
-						// TODO: keep track of more than just 1. Especially
-						// if doing the TODO above.
+						// TODO: keep track of more than just 1.  We could
+						// afford about 3 equals calls before it would be more
+						// expensive than 1 hash + 1 equals.
 						goto dohash;
 					}
 					keymaybe = i;
 				}
 			}
-		}
-		if(HASMAYBE && keymaybe >= 0) {
-			k = (KEYTYPE*)b->data + keymaybe;
-			if(EQFUNC(key, *k)) {
-				value = (byte*)((KEYTYPE*)b->data + BUCKETSIZE) + keymaybe * h->valuesize;
-				res = true;
-				FLUSH(&value);
-				FLUSH(&res);
-				return;
+			if(keymaybe >= 0) {
+				k = (KEYTYPE*)b->data + keymaybe;
+				if(SLOW_EQ(key, *k)) {
+					value = (byte*)((KEYTYPE*)b->data + BUCKETSIZE) + keymaybe * h->valuesize;
+					res = true;
+					FLUSH(&value);
+					FLUSH(&res);
+					return;
+				}
 			}
 		}
 	} else {
@@ -197,7 +227,11 @@ dohash:
 		}
 		do {
 			for(i = 0, k = (KEYTYPE*)b->data, v = (byte*)(k + BUCKETSIZE); i < BUCKETSIZE; i++, k++, v += h->valuesize) {
-				if(b->tophash[i] == top && EQFUNC(key, *k)) {
+				if(b->tophash[i] != top)
+					continue;
+				if(QUICK_NE(key, *k))
+					continue;
+				if(QUICK_EQ(key, *k) || SLOW_EQ(key, *k)) {
 					value = v;
 					res = true;
 					FLUSH(&value);

src/pkg/runtime/mapspeed_test.go

@@ -254,3 +254,17 @@ func BenchmarkMapIterEmpty(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkSameLengthMap(b *testing.B) {
+	// long strings, same length, differ in first few
+	// and last few bytes.
+	m := make(map[string]bool)
+	s1 := "foo" + strings.Repeat("-", 100) + "bar"
+	s2 := "goo" + strings.Repeat("-", 100) + "ber"
+	m[s1] = true
+	m[s2] = true
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = m[s1]
+	}
+}