cmd/gc: disable precise stack collection overhead

[rsc]

CL 12616045 enables zeroing of pointers so that garbage collection of Go frames is
completely precise wrt pointers and so that it can be made completely precise wrt
interface values. It also incurs a significant slowdown. Before Go 1.2, we must remove
that slowdown. The CL description has more details.

[rsc]

Comment 1:

Precise collection will not be ready for Go 1.2. The fix for this issue, then, is to
disable most of the precise code. This bug now tracks that.

[bradfitz]

Comment 2:

What remains?  I missed some discussion.

[rsc]

Comment 3:

The zeroing was intended as a partial step, on the way toward call site-specific bitmaps
saying which pointers were live. Once those bitmaps were in, much of the zeroing would
be deleted. But the call site-specific bitmaps are not ready, and the current large
amount of zeroing causes serious performance degradation (see CL 12616045 description),
so we have to back it out for the release.

[rsc]

Comment 4:

Labels changed: added releaseblocker.

[rsc]

Comment 5:

This issue was closed by revision 30ecb4c.

Status changed to Fixed.