[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 3:53 PM, Adrian Prantl <aprantl at apple.com> wrote:

>
> On Mar 10, 2015, at 3:35 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> 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
>
>
> Fine. I’ll do it this way then.
>
> .
>
>
>> 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?
>
>
> As for DW_AT_frame_base, according to the PR, gdb just crashes if it
> encounters a DW_OP_piece in there. More generally, since the expression
> inside DW_AT_frame_base is just going to be evaluated when a DW_OP_fbreg is
> encountered, it makes the result of an expression like "DW_OP_fbreg 4
> DW_OP_piece 4 DW_OP_fbreg 8 DW_OP_piece 4” highly ambiguous.
>

I'm still really not following how piece, frame_base, and fbreg are all
connected to this issue.

When/where do we use a piece to describe the frame_base?
Is GDB's (which version?) implementation of fbreg not respecting the size
of the piece specified in a _piece in frame_base?


>
> For the larger expression, I suppose it actually works with the “defined
> by the ABI”-strategy. Consider a 2-byte struct on x86_64 where the lower
> byte is in al and the higher byte is in bl: DW_OP_reg0 DW_OP_piece 1
> DW_OP_reg4 DW_OP_piece 1.
> And the same struct in al:bh needs some shifting either way.
>
>
>
>
>> b) is doable, if ugly, but I’d need some help with a).
>>
>
> Ugly (but compact) it is, then! :-)
>
> thanks!
> adrian
>
>
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