I should have done this a year ago in:
changeset: 5137:686b18098944
user: Russ Cox <rsc@golang.org>
date: Thu Mar 25 14:05:54 2010 -0700
files: src/cmd/8c/swt.c
description:
make alignment rules match 8g, just like 6c matches 6g.
R=ken2
CC=golang-dev
https://golang.org/cl/760042
R=ken2
CC=golang-dev
https://golang.org/cl/4437054
* Reduces malloc counts during gob encoder/decoder test from 6/6 to 3/5.
The current reflect uses Set to mean two subtly different things.
(1) If you have a reflect.Value v, it might just represent
itself (as in v = reflect.NewValue(42)), in which case calling
v.Set only changed v, not any other data in the program.
(2) If you have a reflect Value v derived from a pointer
or a slice (as in x := []int{42}; v = reflect.NewValue(x).Index(0)),
v represents the value held there. Changing x[0] affects the
value returned by v.Int(), and calling v.Set affects x[0].
This was not really by design; it just happened that way.
The motivation for the new reflect implementation was
to remove mallocs. The use case (1) has an implicit malloc
inside it. If you can do:
v := reflect.NewValue(0)
v.Set(42)
i := v.Int() // i = 42
then that implies that v is referring to some underlying
chunk of memory in order to remember the 42; that is,
NewValue must have allocated some memory.
Almost all the time you are using reflect the goal is to
inspect or to change other data, not to manipulate data
stored solely inside a reflect.Value.
This CL removes use case (1), so that an assignable
reflect.Value must always refer to some other piece of data
in the program. Put another way, removing this case would
make
v := reflect.NewValue(0)
v.Set(42)
as illegal as
0 = 42.
It would also make this illegal:
x := 0
v := reflect.NewValue(x)
v.Set(42)
for the same reason. (Note that right now, v.Set(42) "succeeds"
but does not change the value of x.)
If you really wanted to make v refer to x, you'd start with &x
and dereference it:
x := 0
v := reflect.NewValue(&x).Elem() // v = *&x
v.Set(42)
It's pretty rare, except in tests, to want to use NewValue and then
call Set to change the Value itself instead of some other piece of
data in the program. I haven't seen it happen once yet while
making the tree build with this change.
For the same reasons, reflect.Zero (formerly reflect.MakeZero)
would also return an unassignable, unaddressable value.
This invalidates the (awkward) idiom:
pv := ... some Ptr Value we have ...
v := reflect.Zero(pv.Type().Elem())
pv.PointTo(v)
which, when the API changed, turned into:
pv := ... some Ptr Value we have ...
v := reflect.Zero(pv.Type().Elem())
pv.Set(v.Addr())
In both, it is far from clear what the code is trying to do. Now that
it is possible, this CL adds reflect.New(Type) Value that does the
obvious thing (same as Go's new), so this code would be replaced by:
pv := ... some Ptr Value we have ...
pv.Set(reflect.New(pv.Type().Elem()))
The changes just described can be confusing to think about,
but I believe it is because the old API was confusing - it was
conflating two different kinds of Values - and that the new API
by itself is pretty simple: you can only Set (or call Addr on)
a Value if it actually addresses some real piece of data; that is,
only if it is the result of dereferencing a Ptr or indexing a Slice.
If you really want the old behavior, you'd get it by translating:
v := reflect.NewValue(x)
into
v := reflect.New(reflect.Typeof(x)).Elem()
v.Set(reflect.NewValue(x))
Gofix will not be able to help with this, because whether
and how to change the code depends on whether the original
code meant use (1) or use (2), so the developer has to read
and think about the code.
You can see the effect on packages in the tree in
https://golang.org/cl/4423043/.
R=r
CC=golang-dev
https://golang.org/cl/4435042
Interesting comparisons between old and new machine,
and relationship between gccgo and gc.
R=golang-dev, rsc1
CC=golang-dev
https://golang.org/cl/4430045
Uses placeholder attribute instead of changing the value of search
field on browsers that support it. On other browsers, the fake
placeholder text is restored when the empty box loses focus.
R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4441041
NewRequest will save a lot of boilerplate code.
This also updates some docs on Request.Write and
adds some tests.
R=rsc, petar-m, r
CC=golang-dev
https://golang.org/cl/4406047
Don't use the rewrite rule from a previous test
for the next test if there is no rewrite rule
provided.
R=r, r2
CC=golang-dev
https://golang.org/cl/4419045
The new reflection API makes it an error to call value.Set(x)
if x is invalid. Guard for it.
Added corresponding test case.
Fixes#1696.
R=rsc, r
CC=golang-dev
https://golang.org/cl/4398047
Ubuntu and/or GNOME have some bug that likes
to set the "http_proxy" environment variable
and forgets to unset it. This is annoying
to debug. Be clear in the error message that
a proxy was in use.
R=rsc
CC=golang-dev
https://golang.org/cl/4409045
