code review 8368044: runtime: replace union in MHeap with a struct

runtime: replace union in MHeap with a struct
Unions break precise GC.
Update issue 5193.

[dvyukov, 2013-04-07 01:34:22]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://dvyukov%40google.com@code.google.com/p/go/

[iant, 2013-04-07 03:00:21]

LGTM

[dvyukov, 2013-04-07 03:02:07]

*** Submitted as https://code.google.com/p/go/source/detail?r=b127aaab80eb ***

runtime: replace union in MHeap with a struct
Unions break precise GC.
Update issue 5193.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/8368044

[bradfitz, 2013-04-07 03:31:23]

https://codereview.appspot.com/8368044/diff/12001/src/pkg/runtime/malloc.h
File src/pkg/runtime/malloc.h (right):

https://codereview.appspot.com/8368044/diff/12001/src/pkg/runtime/malloc.h#ne...
src/pkg/runtime/malloc.h:424: byte pad[CacheLineSize];
why don't you adjust for sizeof(MCentral) like you did elsewhere, now that it's
a struct instead of a union?

[dvyukov, 2013-04-07 03:37:17]

https://codereview.appspot.com/8368044/diff/12001/src/pkg/runtime/malloc.h
File src/pkg/runtime/malloc.h (right):

https://codereview.appspot.com/8368044/diff/12001/src/pkg/runtime/malloc.h#ne...
src/pkg/runtime/malloc.h:424: byte pad[CacheLineSize];
On 2013/04/07 03:31:23, bradfitz wrote:
> why don't you adjust for sizeof(MCentral) like you did elsewhere, now that
it's
> a struct instead of a union?

Actually this version is correct and all others are not (but they were incorrect
before, so it's fine :)). If you use pad[CacheLineSize-sizeof(X)], then it only
works if the array starts at cache line boundary. If it is not, then end one
element is collocated in the same cache line with beginning of the next.
If the array is not aligned to cache line boundary then you need to use
pad[CacheLineSize] to separate them for sure.