Re: [PATCH] fix alias cache breakage

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi -

On Mon, Jul 14, 2008 at 11:13:58AM -0500, James Bottomley wrote:
> PR6739: speed up decl-alias cache by avoiding its recomputation;
> move to per-module/cu

> Actually completely broke the alias cache.  [...later...]  I also
> updated the test to make it actually work (the reason why this
> breakage was only spotted running a script, not running the
> tests). [...]

Right, that's why we need good test cases.

> The reason is that we have to cross CUs to do resolution of
> DW_AT_declaration.  If it could be resolved within the CU, the
> compiler wouldn't have stubbed the declaration.

OK.

> It seems that the intent of this change (although never discussed on
> the mailing list)

The problem was summarized on the mailing list.
(http://sourceware.org/ml/systemtap/2008-q3/msg00148.html)

> was to only run the cache addition once per CU in the interests of
> efficiency.  This update actually fixes the alias cache again, while
> preserving that goal.

I see what you're getting at, but having a single "global" array is
still wrong; it needs to be at least module-indexed.  Further, it is
undesirable to speculatively tabulate all those definitions even if we
don't actually encounter unresolved decls, which is why my version
moved the traversal to the moment of a "cache miss".

So, it seems that the declaration_resolve() function is still the
right place to trigger the search; and that it should iterate over
CU's too and compute a union of found name->die entries.

- FChE