code review 108870044: cmd/gc: fix &result escaping into result

cmd/gc: fix &result escaping into result

There is a hierarchy of location defined by loop depth:

        -1 = the heap
        0 = function results
        1 = local variables (and parameters)
        2 = local variable declared inside a loop
        3 = local variable declared inside a loop inside a loop
        etc

In general if an address from loopdepth n is assigned to
something in loop depth m < n, that indicates an extended
lifetime of some form that requires a heap allocation.

Function results can be local variables too, though, and so
they don't actually fit into the hierarchy very well.
Treat the address of a function result as level 1 so that
if it is written back into a result, the address is treated
as escaping.

Fixes issue 8185.

[rsc, 2014-06-11 18:01:56]

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

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

[iant, 2014-06-11 18:06:27]

LGTM

https://codereview.appspot.com/108870044/diff/30001/test/escape2.go
File test/escape2.go (right):

https://codereview.appspot.com/108870044/diff/30001/test/escape2.go#newcode1490
test/escape2.go:1490: x = &x // ERROR "&x escapes to heap"
It's astonishing that the many functions that do this have worked to date.

[rsc, 2014-06-11 18:17:09]

On Wed, Jun 11, 2014 at 2:06 PM, <iant@golang.org> wrote:

> https://codereview.appspot.com/108870044/diff/30001/test/
> escape2.go#newcode1490
> test/escape2.go:1490: x = &x // ERROR "&x escapes to heap"
> It's astonishing that the many functions that do this have worked to
> date.
>

Do you know of any?

&input is very common. &output seems like it would be rare.

[rsc, 2014-06-11 18:20:29]

*** Submitted as https://code.google.com/p/go/source/detail?r=331dbd4a6334 ***

cmd/gc: fix &result escaping into result

There is a hierarchy of location defined by loop depth:

        -1 = the heap
        0 = function results
        1 = local variables (and parameters)
        2 = local variable declared inside a loop
        3 = local variable declared inside a loop inside a loop
        etc

In general if an address from loopdepth n is assigned to
something in loop depth m < n, that indicates an extended
lifetime of some form that requires a heap allocation.

Function results can be local variables too, though, and so
they don't actually fit into the hierarchy very well.
Treat the address of a function result as level 1 so that
if it is written back into a result, the address is treated
as escaping.

Fixes issue 8185.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/108870044

[iant, 2014-06-11 18:36:40]

On Wed, Jun 11, 2014 at 11:17 AM, Russ Cox <rsc@golang.org> wrote:
> On Wed, Jun 11, 2014 at 2:06 PM, <iant@golang.org> wrote:
>>
>>
>>
https://codereview.appspot.com/108870044/diff/30001/test/escape2.go#newcode1490
>> test/escape2.go:1490: x = &x // ERROR "&x escapes to heap"
>> It's astonishing that the many functions that do this have worked to
>> date.
>
>
> Do you know of any?
>
> &input is very common. &output seems like it would be rare.

I was joking.  &output is rare, and output = &output is probably non-existent.

Ian