diff options
author | Keith Randall <khr@golang.org> | 2020-09-18 14:19:22 -0700 |
---|---|---|
committer | Keith Randall <khr@golang.org> | 2020-09-18 21:47:28 +0000 |
commit | 8925290cf701fc8f7ec95e4df3d6a8d423b26780 (patch) | |
tree | 199698091a419a71a2133ea2832d47adc0dd70a2 /src/reflect | |
parent | 7e54aa2c25690f5a7f5baad112d231b6ff8d4e5e (diff) | |
download | go-8925290cf701fc8f7ec95e4df3d6a8d423b26780.tar.gz go-8925290cf701fc8f7ec95e4df3d6a8d423b26780.zip |
reflect: use zero buffer to back the Value returned by Zero
In the common case (<1KB types), no allocation is required
by reflect.Zero.
Also use memclr instead of memmove in Set when the source
is known to be zero.
Fixes #33136
Change-Id: Ic66871930fbb53328032e587153ebd12995ccf55
Reviewed-on: https://go-review.googlesource.com/c/go/+/192331
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Diffstat (limited to 'src/reflect')
-rw-r--r-- | src/reflect/all_test.go | 29 | ||||
-rw-r--r-- | src/reflect/value.go | 20 |
2 files changed, 47 insertions, 2 deletions
diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go index 5a12699472..abdfe41908 100644 --- a/src/reflect/all_test.go +++ b/src/reflect/all_test.go @@ -6006,6 +6006,14 @@ func TestReflectMethodTraceback(t *testing.T) { } } +func TestSmallZero(t *testing.T) { + type T [10]byte + typ := TypeOf(T{}) + if allocs := testing.AllocsPerRun(100, func() { Zero(typ) }); allocs > 0 { + t.Errorf("Creating small zero values caused %f allocs, want 0", allocs) + } +} + func TestBigZero(t *testing.T) { const size = 1 << 10 var v [size]byte @@ -6017,6 +6025,27 @@ func TestBigZero(t *testing.T) { } } +func TestZeroSet(t *testing.T) { + type T [16]byte + type S struct { + a uint64 + T T + b uint64 + } + v := S{ + a: 0xaaaaaaaaaaaaaaaa, + T: T{9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9}, + b: 0xbbbbbbbbbbbbbbbb, + } + ValueOf(&v).Elem().Field(1).Set(Zero(TypeOf(T{}))) + if v != (S{ + a: 0xaaaaaaaaaaaaaaaa, + b: 0xbbbbbbbbbbbbbbbb, + }) { + t.Fatalf("Setting a field to a Zero value didn't work") + } +} + func TestFieldByIndexNil(t *testing.T) { type P struct { F int diff --git a/src/reflect/value.go b/src/reflect/value.go index c6f24a5609..a14131e1f8 100644 --- a/src/reflect/value.go +++ b/src/reflect/value.go @@ -1553,7 +1553,11 @@ func (v Value) Set(x Value) { } x = x.assignTo("reflect.Set", v.typ, target) if x.flag&flagIndir != 0 { - typedmemmove(v.typ, v.ptr, x.ptr) + if x.ptr == unsafe.Pointer(&zeroVal[0]) { + typedmemclr(v.typ, v.ptr) + } else { + typedmemmove(v.typ, v.ptr, x.ptr) + } } else { *(*unsafe.Pointer)(v.ptr) = x.ptr } @@ -2360,11 +2364,23 @@ func Zero(typ Type) Value { t := typ.(*rtype) fl := flag(t.Kind()) if ifaceIndir(t) { - return Value{t, unsafe_New(t), fl | flagIndir} + var p unsafe.Pointer + if t.size <= maxZero { + p = unsafe.Pointer(&zeroVal[0]) + } else { + p = unsafe_New(t) + } + return Value{t, p, fl | flagIndir} } return Value{t, nil, fl} } +// must match declarations in runtime/map.go. +const maxZero = 1024 + +//go:linkname zeroVal runtime.zeroVal +var zeroVal [maxZero]byte + // New returns a Value representing a pointer to a new zero value // for the specified type. That is, the returned Value's Type is PtrTo(typ). func New(typ Type) Value { |