Re: [PATCH] PR69195, Reload confused by invalid reg equivs

[Jeff Law <law@redhat.com>]

On 03/14/2016 03:56 AM, Richard Biener wrote:

>> Undefined?  Most likely.  But we still have to do something sensible. As
>> Jakub noted, a user could create the problematic code just as easily as
>> DCE/DSE, so IRA probably needs to be tolerant of this situation.
>>
>> So it seems like you're suggesting we leave DCE/DSE alone (declaring this
>> usage undefined) and fix IRA to be tolerant, right?
>
> Tolerant as in not crash?  Yes.
Right.  Tolerant as in not crash.

>
>>> Using alloca to create/jump to code on the stack should work (we might
>>> transform that into a decl though).
>>
>> Given that executable stacks are a huge security hole, I'd be willing to go
>> out on a limb and declare that undefined as well.  It's not as clear cut,
>> but that's the argument I'd make.
>
> Well, I thought about somebody trying to build trampolines in a way exposed
> to GCC.
Right or other dynamic, short-lived code fragments.

>
>> And yes, I realize that goes in opposition to what GCC has allowed for 20+
>> years.  I still think it'd be the right thing to do.
>
> Did we allow this?  Not by design but rather by accident I suppose.
I don't think it was ever specifically allowed or disallowed; like many 
of the old extensions, it was never crisply defined.

I can distinctly remember having to declare that taking the address of a 
blob of code on the stack, then calling/jumping to it after the 
containing function went out of scope as undefined.  I think it was the 
address of a trampoline, but I'm not entirely sure -- there's a small 
chance it was user-created code.  I only remember it because I was 
surprised at how controversial it was to declare that as undefined :(

That most likely predates egcs, so the discussion is not likely in the 
public archives.  It may have been a private discussion between Kenner, 
Jim, Doug and myself or some subset thereof.


jeff