code review 5677092: runtime: fix tiny memory leak

runtime: fix tiny memory leak

The m->cret word holds the C return value when returning
across a stack split boundary.  It was not being cleared after
use, which means that the return value (if a C function)
or else the value of AX/R0 at the time of the last stack unsplit
was being kept alive longer than necessary.  Clear it.

I think the effect here should be very small, but worth fixing
anyway.

[rsc, 2012-02-19 05:21:46]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go/

[bradfitz, 2012-02-19 05:23:58]

LGTM

On Sat, Feb 18, 2012 at 9:21 PM, <rsc@golang.org> wrote:

> Reviewers: golang-dev_googlegroups.com,
>
> Message:
> Hello golang-dev@googlegroups.com,
>
> I'd like you to review this change to
> https://code.google.com/p/go/
>
>
> Description:
> runtime: fix tiny memory leak
>
> The m->cret word holds the C return value when returning
> across a stack split boundary.  It was not being cleared after
> use, which means that the return value (if a C function)
> or else the value of AX/R0 at the time of the last stack unsplit
> was being kept alive longer than necessary.  Clear it.
>
> I think the effect here should be very small, but worth fixing
> anyway.
>
> Please review this at
http://codereview.appspot.com/**5677092/<http://codereview.appspot.com/5677092/>
>
> Affected files:
>  M src/pkg/runtime/proc.c
>
>
> Index: src/pkg/runtime/proc.c
> ==============================**==============================**=======
> --- a/src/pkg/runtime/proc.c
> +++ b/src/pkg/runtime/proc.c
> @@ -1011,6 +1011,7 @@
>  {
>        Stktop *top, old;
>        uint32 argsize;
> +       uintptr cret;
>        byte *sp;
>        G *g1;
>        int32 goid;
> @@ -1034,7 +1035,9 @@
>        g1->stackbase = old.stackbase;
>        g1->stackguard = old.stackguard;
>
> -       runtime·gogo(&old.gobuf, m->cret);
> +       cret = m->cret;
> +       m->cret = 0;  // drop reference
> +       runtime·gogo(&old.gobuf, cret);
>  }
>
>  // Called from reflect·call or from runtime·morestack when a new
>
>
>

[rsc, 2012-02-19 05:26:37]

*** Submitted as b65103673706 ***

runtime: fix tiny memory leak

The m->cret word holds the C return value when returning
across a stack split boundary.  It was not being cleared after
use, which means that the return value (if a C function)
or else the value of AX/R0 at the time of the last stack unsplit
was being kept alive longer than necessary.  Clear it.

I think the effect here should be very small, but worth fixing
anyway.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5677092