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

[Adrian Prantl]

> On May 30, 2014, at 4:55 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
> 
> 
> On Fri, May 30, 2014 at 4:17 PM, Adrian Prantl <aprantl at apple.com> wrote:
> 
> > On May 30, 2014, at 4:13 PM, Adrian Prantl <aprantl at apple.com> wrote:
> >
> >
> >> On 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.
> >
> > Instead of checking for it being the last basic block we could check for something like (BB.getLastNonDebugInstr()->isReturn()) then we would handle cases with multiple return blocks. The only problem that I see is that the last basic block may also contain non-epilogue code. In fact, the entire function may only be a single basic block. A safer condition might be “register is modified in the prologue && has precisely one non-prologue-def in each returning basic block”?
> > Then again that condition would break on a backend that pops each callee-saved register from the stack one-by-one.
> >
> > What about this definition: The epilogue is everything after the last instruction that has a DebugLoc attached to it?
> 
> Sorry, I realized didn’t actually answer you question :-)
> I agree that this is a big improvement, but I’m not convinced that the condition for detecting the epilogue you mention is safe, for the same reasons that Eric mentioned earlier. When we find a safe condition for the epilogue this is good to go. My current favorite is looking at the line table.
> 
> Could you elaborate on line table solution?

Epilogue is everything that is dominated by the last instruction that has a DebugLoc attached to it. This incidentally also happens to be the debugger’s interpretation.

>  
> 
> -- adrian
> 
> >
> > -- adrian
> >>
> >>
> >>>
> >>> 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
> >
> 
> 
> 
> 
> -- 
> Alexey Samsonov
> vonosmas at gmail.com