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

[David Blaikie]

On Fri, May 30, 2014 at 3:57 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
>
> On Fri, May 30, 2014 at 3:54 PM, Adrian Prantl <aprantl at apple.com> wrote:
>>
>>
>> > On May 30, 2014, at 3:51 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
>> >
>> >
>> > On Fri, May 30, 2014 at 3:44 PM, Adrian Prantl <aprantl at apple.com>
>> > wrote:
>> >
>> > > On May 30, 2014, at 3:40 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> > >
>> > > 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?
>> >
>> > Partially. We get them right until the end of the current basic block.
>> >
>> > The patch should improve this situation by the way:
>> > <bbX>:
>> >   DBG_VALUE $rsp, 32 !"var"  // "var is spilled on stack.
>> > // ... more code ..
>> > <bbY>:
>> >   DBG_VALUE $r9, 0, !"var"  // "var" is now loaded to "r9".
>> >
>> > Earlier we used to terminate the location of "var" at the end of bbX,
>> > but now we won't do
>> > this, as we know "rsp" doesn't change in the function.
>>
>> Yeah, but only if we can detect the Epilogue and ignore the def of $rsp
>> that restores the register at the end of the function.
>
>
> Yes. So, how do you feel about the plan suggested above:
>
> 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.

For my money, I'd hesitate to add more invasive changes like
FrameTeardown for this change unless we knew they were useful for
something beyond this short-term fix.

But I don't know where the right tipping point there is (from "this is
an isolated workaround that can be removed when we get general
variable tracking" to "this change is going to be an ongoing cost to
maintain and/or a pain to remove when we have something better").

- David

>
>>
>>
>> >
>>
>> > But there is no code that would insert DBG_VALUES after they are
>> > reloaded again.
>> >
>> >
>> > >
>> > >> [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…
>> >
>> > Awright.
>> >
>> > Yeah, MMI side table doesn't really work for indirect descriptions, when
>> > we need to deref the memory stored at reg+offset.
>> >
>> >
>> > >
>> > >>
>> > >> -- 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
>> >
>> >
>> >
>> >
>> > --
>> > Alexey Samsonov
>> > vonosmas at gmail.com
>>
>
>
>
> --
> Alexey Samsonov
> vonosmas at gmail.com