Re: PATCH: tree-ssa-sink breaks stack layout

[Mark Mitchell <mark at codesourcery dot com>]

Richard Guenther wrote:

>> Do we?  Again, that's not a rhetorical question; I don't know the
>> answer.  Do we have other test-cases showing this problem with other
>> passes?
> 
> I have created tens of testcases showing PTA analysis bugs, I think
> it would be not difficult to create testcases that break with tree
> loop invariant/store motion and with PRE (just to name the two
> obvious candidates).

All right.  I'm not surprised, but I hadn't seen the problem come up
with these optimizations.

> I suggest to merge all overlapping scopes, not just detect if variable
> accesses have been migrated outside their scope (that's not easy - if
> you just move a stmt its lexical block does not change, instead if
> you scan stmts looking for out-of-order (or unknown) blocks for stmts
> then you know sth bad may have happened)

That sounds plausible.  This is just checking GIMPLE_STMT_BLOCK, right?

> True.  But there are a lot of existing wrong-code bugs in released compilers
> (with small testcases).  It's not that we can fix everything, but providing
> a reasonable workaround like -fno-strict-aliasing is something we should
> make sure exists (no, I don't really consider -O0 an acceptable workaround).

I agree; we can't fix everything, and work-arounds are good.  But, if we
can do something like what you're suggesting above to avoid the problem,
that's even better.

Thanks,

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713