[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 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
>
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