code review 5375093: runtime: avoid allocation for make([]T, 0)

runtime: avoid allocation for make([]T, 0)

[rsc, 2011-11-14 21:27:46]

Hello gri, iant (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://go.googlecode.com/hg

[gri, 2011-11-14 21:37:16]

LGTM

http://codereview.appspot.com/5375093/diff/1002/src/pkg/runtime/slice.c
File src/pkg/runtime/slice.c (right):

http://codereview.appspot.com/5375093/diff/1002/src/pkg/runtime/slice.c#newco...
src/pkg/runtime/slice.c:51: ret->array = (byte*)&zerobase;
couldn't this just be ret->array = 0 ? (or (byte*)0) ?

[iant2, 2011-11-14 21:38:37]

LGTM

rsc@golang.org writes:

> Reviewers: gri, iant,
>
> Message:
> Hello gri, iant (cc: golang-dev@googlegroups.com),
>
> I'd like you to review this change to
> https://go.googlecode.com/hg
>
>
> Description:
> runtime: avoid allocation for make([]T, 0)
>
> Please review this at http://codereview.appspot.com/5375093/
>
> Affected files:
>   M src/pkg/runtime/slice.c
>
>
> Index: src/pkg/runtime/slice.c
> ===================================================================
> --- a/src/pkg/runtime/slice.c
> +++ b/src/pkg/runtime/slice.c
> @@ -32,6 +32,11 @@
>  	}
>  }
>
> +// Dummy word to use as base pointer for make([]T, 0).
> +// Since you cannot take the address of such a slice,
> +// you can't tell that they all have the same base pointer.
> +static uintptr zerobase;
> +
>  static void
>  makeslice1(SliceType *t, int32 len, int32 cap, Slice *ret)
>  {
> @@ -42,7 +47,9 @@
>  	ret->len = len;
>  	ret->cap = cap;
>
> -	if((t->elem->kind&KindNoPointers))
> +	if(cap == 0)
> +		ret->array = (byte*)&zerobase;
> +	else if((t->elem->kind&KindNoPointers))
>  		ret->array = runtime·mallocgc(size, FlagNoPointers, 1, 1);
>  	else
>  		ret->array = runtime·mal(size);

[rsc, 2011-11-14 21:44:52]

On Mon, Nov 14, 2011 at 16:37,  <gri@golang.org> wrote:
> couldn't this just be ret->array = 0 ? (or (byte*)0) ?

No: then make([]T, 0) would return nil.

[rsc, 2011-11-15 17:05:33]

*** Submitted as http://code.google.com/p/go/source/detail?r=14a037cbc719 ***

runtime: avoid allocation for make([]T, 0)

R=gri, iant, iant
CC=golang-dev
http://codereview.appspot.com/5375093