Re: [PATCH] Improve dead code elimination with -fsanitize=address (PR84307)

[Paolo Bonzini <bonzini@gnu.org>]

On 13/02/2018 00:49, Jakub Jelinek wrote:
> On Tue, Feb 13, 2018 at 12:41:28AM +0100, Paolo Bonzini wrote:
>> On 09/02/2018 19:07, Jakub Jelinek wrote:
>>> On Fri, Feb 09, 2018 at 07:01:08PM +0100, Richard Biener wrote:
>>>>> which indeed fixes the testcase and seems not to break asan.exp.
>>>>
>>>> Huh. Need to double check why that makes sense ;) 
>>>
>>> I think it does, for both ASAN_CHECK and ASAN_MARK the pointer argument
>>> is the second one, the first one is an integer argument with flags.
>>> And ASAN_MARK, both poison and unpoison, works kind like a clobber on the
>>> referenced variable, before unpoison it is generally inaccessible and after
>>> poison too.
>>
>> This was too optimistic. :(
>>
>> In use-after-scope-types-1.C, after the patch FRE+DSE are able to
>> optimize away the problematic read.  In general it seems to me that the
>> sanitizer passes should be before DSE if we want ASAN builtins to have
>> precise info, otherwise some reads or stores might not be
>> instrumented---GCC was being lucky here.
>>
>> The obvious change here is:
> 
> That is way too early, I don't think this is a good idea.
> Certainly not in stage4.

Certainly, for now I'll revert.

But can you expand on why it's too early?  Indeed I suppose it may
affect inlining decisions, on the other hand it seems dangerous to apply
instrumentation after pretty much any optimization pass.

Thanks,

Paolo