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From: Jeff Law <jeffreyalaw@gmail.com>
To: Jan Hubicka <hubicka@ucw.cz>
Cc: Richard Biener <rguenther@suse.de>, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Improve DSE to handle stores before __builtin_unreachable ()
Date: Sat, 24 Jun 2023 08:24:10 -0600	[thread overview]
Message-ID: <ad374587-251f-2db2-34a7-92a2dbe7712b@gmail.com> (raw)
In-Reply-To: <ZJRPyeuKxXHfI/2e@kam.mff.cuni.cz>



On 6/22/23 07:42, Jan Hubicka wrote:
>>
>>
>> On 6/22/23 00:31, Richard Biener wrote:
>>> I think there's a difference in that __builtin_trap () is observable
>>> while __builtin_unreachable () is not and reaching __builtin_unreachable
>>> () invokes undefined behavior while reaching __builtin_trap () does not.
>>>
>>> So the isolation code marking the trapping code volatile should be
>>> enough and the trap () is just there to end the basic block
>>> (and maybe be on the safe side to really trap).
>> Agreed WRT observability -- but that's not really the point of the trap and
>> if we wanted we could change that behavior.
>>
>> The trap is there to halt execution immediately rather than letting it keep
>> running.  That was a design decision from a security standpoint. If we've
>> detected that we're executing undefined behavior, stop rather than
>> potentially letting a malicious actor turn a bug into an exploit.
> 
> Also as discussed some time ago, the volatile loads between traps has
> effect of turning previously pure/const functions into non-const which
> is somewhat sad, so it is still on my todo list to change it this stage1
> to something more careful.   We discussed internal functions trap_store
> and trap_load which will expand to load/store + trap but will make it
> clear that side effect does not count to modref.
It's been a long time since I looked at this code -- isn't it the case 
that we already must have had a load/store and that all we've done is 
change its form (to enable more DCE) and added the volatile marker?

Meaning that it couldn't have been pure/cost before, could it?  Or is it 
the case that you want to not have the erroneous path be the sole reason 
to spoil pure/const detection -- does that happen often in practice?

jeff

  reply	other threads:[~2023-06-24 14:24 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20230620070009.C11983858D1E@sourceware.org>
2023-06-20 13:27 ` Jeff Law
2023-06-21  6:41   ` Richard Biener
2023-06-21  9:55     ` Jan Hubicka
2023-06-21 14:04     ` Jeff Law
2023-06-22  6:31       ` Richard Biener
2023-06-22 13:36         ` Jeff Law
2023-06-22 13:42           ` Jan Hubicka
2023-06-24 14:24             ` Jeff Law [this message]
2023-06-25 16:33               ` Jan Hubicka
2023-06-26 17:21   ` Jan Hubicka
2023-06-26 22:37     ` Jeff Law
2023-06-20  6:59 Richard Biener

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