cmd/compile: pass stack allocated bucket to makemap inside hmap

name         old time/op    new time/op    delta
NewEmptyMap    53.2ns ± 7%    48.0ns ± 5%  -9.77%  (p=0.000 n=20+20)
NewSmallMap     111ns ± 1%     106ns ± 2%  -3.78%  (p=0.000 n=20+19)

Change-Id: I979d21ab16eae9f6893873becca517db57e054b5
Reviewed-on: https://go-review.googlesource.com/56290
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

4 files changed, 51 insertions, 37 deletions

diff --git a/src/cmd/compile/internal/gc/builtin.go b/src/cmd/compile/internal/gc/builtin.go
index 269f054f0a..05836cc951 100644
--- a/src/cmd/compile/internal/gc/builtin.go
+++ b/src/cmd/compile/internal/gc/builtin.go
@@ -211,7 +211,7 @@ func runtimeTypes() []*types.Type {
 	typs[58] = types.Types[TUNSAFEPTR]
 	typs[59] = functype(nil, []*Node{anonfield(typs[57]), anonfield(typs[58]), anonfield(typs[58])}, []*Node{anonfield(typs[11])})
 	typs[60] = types.NewMap(typs[2], typs[2])
-	typs[61] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[3]), anonfield(typs[3])}, []*Node{anonfield(typs[60])})
+	typs[61] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[3])}, []*Node{anonfield(typs[60])})
 	typs[62] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[60]), anonfield(typs[3])}, []*Node{anonfield(typs[3])})
 	typs[63] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[60]), anonfield(typs[2])}, []*Node{anonfield(typs[3])})
 	typs[64] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[60]), anonfield(typs[3]), anonfield(typs[1])}, []*Node{anonfield(typs[3])})
diff --git a/src/cmd/compile/internal/gc/builtin/runtime.go b/src/cmd/compile/internal/gc/builtin/runtime.go
index bb7a8a9c9e..f69459c549 100644
--- a/src/cmd/compile/internal/gc/builtin/runtime.go
+++ b/src/cmd/compile/internal/gc/builtin/runtime.go
@@ -93,7 +93,7 @@ func ifaceeq(tab *uintptr, x, y unsafe.Pointer) (ret bool)
 func efaceeq(typ *uintptr, x, y unsafe.Pointer) (ret bool)
 
 // *byte is really *runtime.Type
-func makemap(mapType *byte, hint int64, mapbuf *any, bucketbuf *any) (hmap map[any]any)
+func makemap(mapType *byte, hint int64, mapbuf *any) (hmap map[any]any)
 func mapaccess1(mapType *byte, hmap map[any]any, key *any) (val *any)
 func mapaccess1_fast32(mapType *byte, hmap map[any]any, key any) (val *any)
 func mapaccess1_fast64(mapType *byte, hmap map[any]any, key any) (val *any)
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index ac9edca801..dea5893bea 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -1435,32 +1435,49 @@ opswitch:
 
 	case OMAKEMAP:
 		t := n.Type
+		hmapType := hmap(t)
 
-		a := nodnil() // hmap buffer
-		r := nodnil() // bucket buffer
+		// var h *hmap
+		var h *Node
 		if n.Esc == EscNone {
-			// Allocate hmap buffer on stack.
-			var_ := temp(hmap(t))
+			// Allocate hmap and one bucket on stack.
 
-			a = nod(OAS, var_, nil) // zero temp
-			a = typecheck(a, Etop)
-			init.Append(a)
-			a = nod(OADDR, var_, nil)
+			// var hv hmap
+			hv := temp(hmapType)
+			zero := nod(OAS, hv, nil)
+			zero = typecheck(zero, Etop)
+			init.Append(zero)
+			// h = &hv
+			h = nod(OADDR, hv, nil)
 
-			// Allocate one bucket on stack.
+			// Allocate one bucket pointed to by hmap.buckets on stack.
 			// Maximum key/value size is 128 bytes, larger objects
 			// are stored with an indirection. So max bucket size is 2048+eps.
-			var_ = temp(mapbucket(t))
 
-			r = nod(OAS, var_, nil) // zero temp
-			r = typecheck(r, Etop)
-			init.Append(r)
-			r = nod(OADDR, var_, nil)
+			// var bv bmap
+			bv := temp(mapbucket(t))
+
+			zero = nod(OAS, bv, nil)
+			zero = typecheck(zero, Etop)
+			init.Append(zero)
+
+			// b = &bv
+			b := nod(OADDR, bv, nil)
+
+			// h.buckets = b
+			bsym := hmapType.Field(5).Sym // hmap.buckets see reflect.go:hmap
+			na := nod(OAS, nodSym(ODOT, h, bsym), b)
+			na = typecheck(na, Etop)
+			init.Append(na)
+
+		} else {
+			// h = nil
+			h = nodnil()
 		}
 
 		fn := syslook("makemap")
-		fn = substArgTypes(fn, hmap(t), mapbucket(t), t.Key(), t.Val())
-		n = mkcall1(fn, n.Type, init, typename(n.Type), conv(n.Left, types.Types[TINT64]), a, r)
+		fn = substArgTypes(fn, hmapType, t.Key(), t.Val())
+		n = mkcall1(fn, n.Type, init, typename(n.Type), conv(n.Left, types.Types[TINT64]), h)
 
 	case OMAKESLICE:
 		l := n.Left
diff --git a/src/runtime/hashmap.go b/src/runtime/hashmap.go
index efb8a78024..852b65d1db 100644
--- a/src/runtime/hashmap.go
+++ b/src/runtime/hashmap.go
@@ -259,8 +259,8 @@ func (h *hmap) createOverflow() {
 // If the compiler has determined that the map or the first bucket
 // can be created on the stack, h and/or bucket may be non-nil.
 // If h != nil, the map can be created directly in h.
-// If bucket != nil, bucket can be used as the first bucket.
-func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap {
+// If h.buckets != nil, bucket pointed to can be used as the first bucket.
+func makemap(t *maptype, hint int64, h *hmap) *hmap {
 	if sz := unsafe.Sizeof(hmap{}); sz > 48 || sz != t.hmap.size {
 		println("runtime: sizeof(hmap) =", sz, ", t.hmap.size =", t.hmap.size)
 		throw("bad hmap size")
@@ -312,34 +312,31 @@ func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap {
 		throw("bad evacuatedN")
 	}
 
+	// initialize Hmap
+	if h == nil {
+		h = (*hmap)(newobject(t.hmap))
+	}
+	h.hash0 = fastrand()
+
 	// find size parameter which will hold the requested # of elements
 	B := uint8(0)
-	for ; overLoadFactor(hint, B); B++ {
+	for overLoadFactor(hint, B) {
+		B++
 	}
+	h.B = B
 
 	// allocate initial hash table
 	// if B == 0, the buckets field is allocated lazily later (in mapassign)
 	// If hint is large zeroing this memory could take a while.
-	buckets := bucket
-	var extra *mapextra
-	if B != 0 {
+	if h.B != 0 {
 		var nextOverflow *bmap
-		buckets, nextOverflow = makeBucketArray(t, B)
+		h.buckets, nextOverflow = makeBucketArray(t, h.B)
 		if nextOverflow != nil {
-			extra = new(mapextra)
-			extra.nextOverflow = nextOverflow
+			h.extra = new(mapextra)
+			h.extra.nextOverflow = nextOverflow
 		}
 	}
 
-	// initialize Hmap
-	if h == nil {
-		h = (*hmap)(newobject(t.hmap))
-	}
-	h.B = B
-	h.extra = extra
-	h.hash0 = fastrand()
-	h.buckets = buckets
-
 	return h
 }
 
@@ -1171,7 +1168,7 @@ func ismapkey(t *_type) bool {
 
 //go:linkname reflect_makemap reflect.makemap
 func reflect_makemap(t *maptype, cap int) *hmap {
-	return makemap(t, int64(cap), nil, nil)
+	return makemap(t, int64(cap), nil)
 }
 
 //go:linkname reflect_mapaccess reflect.mapaccess