Fallout: PR84340

[Paolo Bonzini]

On 13/02/2018 14:35, Jakub Jelinek wrote:
> On Tue, Feb 13, 2018 at 12:21:55PM +0100, Jakub Jelinek wrote:
>> On Tue, Feb 13, 2018 at 12:15:36PM +0100, Paolo Bonzini wrote:
>>> The issue is that the ASAN_CHECK doesn't exist at early DSE time, and
>>> thus causes the store to disappear.
>>
>> If it was DSE removing the stores before asan pass, then it would FAIL
>> before as well.
> 
> Sorry, while ASAN_CHECK is introduced late, ASAN_MARK is present there
> already from the gimplification.

Yeah, and it keeps everything alive.

> For use after scope, I guess a lot of the stores after end of scope
> (rather than reads) are something DSE could consider removing.
> So, shall we just disable DSE on vars where their address "escapes"
> through ASAN_MARK when -fsanitize-address-use-after-scope?

But the stores _are_ dead; it's only the ASAN_CHECK that isn't.  Hence
the idea of doing the entire instrumentation very early.

> Generally, dead stores could be eliminable when stored before the
> corresponding ASAN_MARK poison (but even ASAN_MARK with "..W.." will
> prevent those) and uneliminable when stored after ASAN_MARK poison.
> 
> For the "fn spec" for now, I'd just go with "..R.." for ASAN_CHECK and
> NULL for ASAN_MARK for now.

I'm a bit scared of that even, :) especially in stage4.  If you think
it's safe enough, I can give it a shot, but honestly I wouldn't have
much time to deal with the fallout now (hence the quick revert).

Paolo