[PATCH] Generate better location ranges for some register-described variables.

[David Blaikie]

On Fri, May 30, 2014 at 3:37 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>> On May 30, 2014, at 3:31 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Fri, May 30, 2014 at 3:12 PM, Eric Christopher <echristo at gmail.com> wrote:
>>> On Fri, May 30, 2014 at 3:03 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>> On Fri, May 30, 2014 at 3:00 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
>>>>>
>>>>> On Fri, May 30, 2014 at 12:45 PM, Eric Christopher <echristo at gmail.com>
>>>>> wrote:
>>>>>>
>>>>>> On Fri, May 30, 2014 at 10:26 AM, Adrian Prantl <aprantl at apple.com> wrote:
>>>>>>>
>>>>>>>> On May 29, 2014, at 10:03 PM, Alexey Samsonov <vonosmas at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>> Yeah, well, we can add "FrameTeardown" machine instruction flag in
>>>>>>>> addition to "FrameSetup", and annotate machine instructions added in frame
>>>>>>>> lowering code. But do we expect many users of this flag (assuming that this
>>>>>>>> usage is also somewhat questionable)?
>>>>>>>
>>>>>>> The FrameSetup flag is also only used by the debug info, so at least it
>>>>>>> wouldn’t be any more questionable than that :-)
>>>>>
>>>>>
>>>>> OK. Why don't we land the patch as-is now (don't terminate the register at
>>>>> the end of MBB if this register is only modified in frame setup and the last
>>>>> MBB - it should be safe), and then replace "the last MBB" condition with
>>>>> checking a "FrameTeardown" flag, attached to certain machine instructions?
>>>>> In this way we won't have to special-case certain registers and make
>>>>> assumptions about their constant-ness.
>>>>
>>>> Given the limited nature of this fix (both in the fix's possible scope
>>>> and intent/purpose) I'm personally sort of inclined to do this by
>>>> whitelisting certain registers instead, if that were/is possible...
>>>> seems simpler and still sufficient for the required purpose here.
>>>>
>>>> Can we easily just test whether a register is the stack/frame pointer?
>>>> (I actually know so little about this that the difference between
>>>> those two things isn't straight in my head and I'm not sure which, or
>>>> both, we need for the common/intended case here)
>>>>
>>>
>>> If we need to stack realign we might not be able to do this as easily.
>>> I'm not a huge fan of either method, but I think that the assumption
>>> that Alexey has might make more sense than whitelisting.
>>
>> Fair enough.
>>
>>> My logic:
>>> Whitelisting involves assuming that various registers won't be used in
>>> ways that surprise us and I can think of a few ways where it could
>>> happen: stack realignment, inline asm, a smarter stack coloring
>>> algorithm.
>>
>> Can inline asm clobber the frame pointer & we actually tolerate that,
>> find the right variables, etc, when that happens? (how?)
>>
>> Can't quite picture the stack coloring you have in mind & Alexey
>> commented on the stack realignment issue.
>>
>> The only other idea I'd float would be another simpler solution,
>> though a bit more expressive: just a check to see that the particular
>> register(s) (stack/frame pointers, whichever things it is we actually
>> use to describe the location of stack variables) don't change beyond
>> the prologue, and if not, describe any variables that are relative to
>> that register as having that location for the whole program.
>
> That won’t work for values that are spilled to the stack, because they only live on the stack for part of the function.

Do we get those right currently?

> [And for (-O0) allocas that are described in the MMI table, we already do what you described].

Right - but the problem Alexey is trying to address is what ASan does,
where it makes one big alloca and describes variables as being within
that alloca - so the MMI sidetable handling doesn't fire...

>
> -- adrian
>
>>
>> But, yeah, that's not /lots/ easier than finding any registers that
>> happen to meet that property.
>>
>> - David
>>
>>>
>>> -eric
>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>
>>>>>> :)
>>>>>>
>>>>>>>> I can also make this patch even less general and just special-case the
>>>>>>>> frame pointer and the stack pointer registers, so that the code wouldn't
>>>>>>>> pretend to solve a general problem we're facing.
>>>>>>>
>>>>>>> The frame pointer should be stable over the entire function, special
>>>>>>> casing it seems to be appropriate. I don’t know whether, e.g, stack coloring
>>>>>>> would cause the stack pointer to be modified in the middle of a function.
>>>>>>>
>>>>>>
>>>>>> FWIW it may not _now_ but there's nothing from stopping it either. :\
>>>>>>
>>>>>> -eric
>>>>>
>>>>>
>>>>> --
>>>>> Alexey Samsonov
>>>>> vonosmas at gmail.com
>