[PATCH] Fix the remainder of PR22762 (GDB is crashing on DW_OP_piece being used inside of DW_AT_frame_base)

[David Blaikie]

On Tue, Mar 10, 2015 at 4:28 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

>  Yes this is exactly the edge case that we were relying on up to now.
> There are two problems I have with that:
>
> a) how do we implement the “defined by the ABI” predicate correctly?
> Assume that it’s always the subregister at offset 0 and wait until someone
> complains?
>
>
> I'd probably be OK with this, open to other opinions, but it seems pretty
> simply like "we stuffed this number in a big register, but we don't need
> all the bits in the big register, so take the lowest bits that we specify"
> - any platform that did anything stranger than that... well, we should
> probably have a talk with/about them anyway, so I wouldn't mind if them
> running into this feature was what caused us to have that discussion.
>
>  You mean like X86 vector registers?  They're implicitly arrays/structs
> of values of a smaller size. I don't know that the value of interest always
> ends up in the "low" part, but maybe it does. I really don't have a handle
> on X86 vector instructions.
>

Can't say I know a great deal about it - but sure, if we can concoct a test
case where part of a variable resides in part of a vector register, it'd be
worth checking that it behaves appropriately.


>  --paulr
>
>
>
> *From:* llvm-commits-bounces at cs.uiuc.edu [mailto:
> llvm-commits-bounces at cs.uiuc.edu] *On Behalf Of *David Blaikie
> *Sent:* Tuesday, March 10, 2015 3:36 PM
> *To:* Adrian Prantl
> *Cc:* reviews+D8233+public+f5c9e8745840f37b at reviews.llvm.org;
> llvm-commits at cs.uiuc.edu
> *Subject:* Re: [PATCH] Fix the remainder of PR22762 (GDB is crashing on
> DW_OP_piece being used inside of DW_AT_frame_base)
>
>
>
>
>
>
>
> On Tue, Mar 10, 2015 at 3:26 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>
>
>  On Mar 10, 2015, at 3:14 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
>
>
>
>
> On Tue, Mar 10, 2015 at 2:50 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
> Hi echristo, dblaikie,
>
> http://llvm.org/bugs/show_bug.cgi?id=22762
> The symptom was that DW_AT_frame_base should never use a
> DW_OP_(bit)_piece, the bug was that AddMachineRegPiece incorrectly created
> pieces to describe values that occupy only a subregister. Change this to
> emit a bit mask instead.
>
> I'm posting this for review because emitting the bit mask increases the
> size occupied for DWARF expressions for sub-registers (~5 bytes for a
> 32-bit subregister). Previously we would (incorrectly!) use DW_OP_piece to
> describe a value occupying part of a register. However, "DW_OP_piece
> provides a way of describing how large a part of a variable a particular
> DWARF location description refers to.",
>
>
> The spec also says "If the piece is located in a register, but does not
> occupy the entire register, the placement of the piece within that register
> is defined by the ABI. " - so we can use this in some cases at least.
> Should we? I assume it just means if we say _piece of size 1 in a register
> of size 4 we get the low byte (whatever definition of 'low' there is)?
>
>
> not the size and offset of an entire variable inside a super-register. The
> way that most debuggers implement DW_OP_piece this sort of works out for
> subregisters that are at offset 0, but it causes confusion if the
> expression needs to be composed (such as in DW_AT_frame_base, or if the
> subregister contains only a part of the variable).
>
>
>
> Yes this is exactly the edge case that we were relying on up to now. There
> are two problems I have with that:
>
> a) how do we implement the “defined by the ABI” predicate correctly?
> Assume that it’s always the subregister at offset 0 and wait until someone
> complains?
>
>
> I'd probably be OK with this, open to other opinions, but it seems pretty
> simply like "we stuffed this number in a big register, but we don't need
> all the bits in the big register, so take the lowest bits that we specify"
> - any platform that did anything stranger than that... well, we should
> probably have a talk with/about them anyway, so I wouldn't mind if them
> running into this feature was what caused us to have that discussion.
>
>
>   b) it doesn’t compose well, so we’d need to explicitly forbid it inside
> of DW_AT_frame_base and inside of a larger piece expression.
>
>
> This bit I don't really understand - perhaps you could provide some
> expression examples?
>
>
>  b) is doable, if ugly, but I’d need some help with a).
>
>
>
> -- adrian
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150310/e041895e/attachment.html>