[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 3:14 PM, David Blaikie <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?
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.

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/20150310/960af6e2/attachment.html>