reflect: avoid zeroing memory that will be overwritten

Avoid new'ing memory that will be overwritten by assignment.

name              old time/op    new time/op    delta
Call-4               160ns ± 4%     155ns ± 2%  -3.19%        (p=0.003 n=10+10)
FieldByName1-4      94.5ns ± 2%    95.2ns ± 1%  +0.65%          (p=0.026 n=9+9)
FieldByName2-4      3.09µs ± 4%    3.13µs ± 2%    ~           (p=0.165 n=10+10)
FieldByName3-4      19.8µs ± 1%    19.9µs ± 1%    ~            (p=0.395 n=10+8)
InterfaceBig-4      11.6ns ± 0%    11.7ns ± 0%  +0.86%          (p=0.000 n=8+9)
InterfaceSmall-4    11.7ns ± 0%    11.7ns ± 0%    ~     (all samples are equal)
New-4               26.6ns ± 0%    26.4ns ± 0%  -0.64%         (p=0.000 n=10+9)

name              old alloc/op   new alloc/op   delta
Call-4              0.00B ±NaN%    0.00B ±NaN%    ~     (all samples are equal)

name              old allocs/op  new allocs/op  delta
Call-4               0.00 ±NaN%     0.00 ±NaN%    ~     (all samples are equal)

Change-Id: I12c85d4e65245598669dd6f66beb0744ec9b9d6d
Reviewed-on: https://go-review.googlesource.com/28011
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/type.go

@@ -1456,25 +1456,24 @@ func (t *rtype) ptrTo() *rtype {
 
 	// Create a new ptrType starting with the description
 	// of an *unsafe.Pointer.
-	p = new(ptrType)
 	var iptr interface{} = (*unsafe.Pointer)(nil)
 	prototype := *(**ptrType)(unsafe.Pointer(&iptr))
-	*p = *prototype
+	pp := *prototype
 
-	p.str = resolveReflectName(newName(s, "", "", false))
+	pp.str = resolveReflectName(newName(s, "", "", false))
 
 	// For the type structures linked into the binary, the
 	// compiler provides a good hash of the string.
 	// Create a good hash for the new string by using
 	// the FNV-1 hash's mixing function to combine the
 	// old hash and the new "*".
-	p.hash = fnv1(t.hash, '*')
+	pp.hash = fnv1(t.hash, '*')
 
-	p.elem = t
+	pp.elem = t
 
-	ptrMap.m[t] = p
+	ptrMap.m[t] = &pp
 	ptrMap.Unlock()
-	return &p.rtype
+	return &pp.rtype
 }
 
 // fnv1 incorporates the list of bytes into the hash x using the FNV-1 hash function.
@@ -1852,8 +1851,7 @@ func ChanOf(dir ChanDir, t Type) Type {
 	// Make a channel type.
 	var ichan interface{} = (chan unsafe.Pointer)(nil)
 	prototype := *(**chanType)(unsafe.Pointer(&ichan))
-	ch := new(chanType)
+	ch := *prototype
-	*ch = *prototype
 	ch.tflag = 0
 	ch.dir = uintptr(dir)
 	ch.str = resolveReflectName(newName(s, "", "", false))
@@ -1896,8 +1894,7 @@ func MapOf(key, elem Type) Type {
 
 	// Make a map type.
 	var imap interface{} = (map[unsafe.Pointer]unsafe.Pointer)(nil)
-	mt := new(mapType)
+	mt := **(**mapType)(unsafe.Pointer(&imap))
-	*mt = **(**mapType)(unsafe.Pointer(&imap))
 	mt.str = resolveReflectName(newName(s, "", "", false))
 	mt.tflag = 0
 	mt.hash = fnv1(etyp.hash, 'm', byte(ktyp.hash>>24), byte(ktyp.hash>>16), byte(ktyp.hash>>8), byte(ktyp.hash))
@@ -2248,15 +2245,16 @@ func bucketOf(ktyp, etyp *rtype) *rtype {
 		}
 	}
 
-	b := new(rtype)
+	b := &rtype{
-	b.align = ptrSize
+		align:   ptrSize,
+		size:    size,
+		kind:    kind,
+		ptrdata: ptrdata,
+		gcdata:  gcdata,
+	}
 	if overflowPad > 0 {
 		b.align = 8
 	}
-	b.size = size
-	b.ptrdata = ptrdata
-	b.kind = kind
-	b.gcdata = gcdata
 	s := "bucket(" + ktyp.String() + "," + etyp.String() + ")"
 	b.str = resolveReflectName(newName(s, "", "", false))
 	return b
@@ -2285,8 +2283,7 @@ func SliceOf(t Type) Type {
 	// Make a slice type.
 	var islice interface{} = ([]unsafe.Pointer)(nil)
 	prototype := *(**sliceType)(unsafe.Pointer(&islice))
-	slice := new(sliceType)
+	slice := *prototype
-	*slice = *prototype
 	slice.tflag = 0
 	slice.str = resolveReflectName(newName(s, "", "", false))
 	slice.hash = fnv1(typ.hash, '[')
@@ -2830,8 +2827,7 @@ func ArrayOf(count int, elem Type) Type {
 	// Make an array type.
 	var iarray interface{} = [1]unsafe.Pointer{}
 	prototype := *(**arrayType)(unsafe.Pointer(&iarray))
-	array := new(arrayType)
+	array := *prototype
-	*array = *prototype
 	array.str = resolveReflectName(newName(s, "", "", false))
 	array.hash = fnv1(typ.hash, '[')
 	for n := uint32(count); n > 0; n >>= 8 {
@@ -3071,13 +3067,14 @@ func funcLayout(t *rtype, rcvr *rtype) (frametype *rtype, argSize, retOffset uin
 	offset += -offset & (ptrSize - 1)
 
 	// build dummy rtype holding gc program
-	x := new(rtype)
+	x := &rtype{
-	x.align = ptrSize
+		align:   ptrSize,
+		size:    offset,
+		ptrdata: uintptr(ptrmap.n) * ptrSize,
+	}
 	if runtime.GOARCH == "amd64p32" {
 		x.align = 8
 	}
-	x.size = offset
-	x.ptrdata = uintptr(ptrmap.n) * ptrSize
 	if ptrmap.n > 0 {
 		x.gcdata = &ptrmap.data[0]
 	} else {