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

[Adrian Prantl]

> 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 <mailto:aprantl at apple.com>> wrote:
> 
>> On Mar 10, 2015, at 3:14 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> 
>> On Tue, Mar 10, 2015 at 2:50 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> Hi echristo, dblaikie,
>> 
>> http://llvm.org/bugs/show_bug.cgi?id=22762 <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.

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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