[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 11, 2015, at 11:22 AM, Adrian Prantl <aprantl at apple.com> wrote:
> 
> 
>> On Mar 11, 2015, at 11:16 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> 
>> On Tue, Mar 10, 2015 at 3:53 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> 
>>> On Mar 10, 2015, at 3:35 PM, David Blaikie <dblaikie at gmail.com <mailto: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.
>> 
>> 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?
> 
> On x86_64-pc-linux-gnux32 the frame base is ebp, but in 64-bit mode ebp doesn’t have DWARF register number, so we need a way saying “the lower 32 bits or rbp”. Before this patch we would emit "DW_OP_reg(rbp) DW_OP_piece 4", after this patch it is "DW_OP_reg(rbp) DW_OP_constu 0xffffff DW_OP_and".
> 
>> Is GDB's (which version?) implementation of fbreg not respecting the size of the piece specified in a _piece in frame_base?
> 
> According to the PR (I didn’t verify that) gdb crashes when it encounter a DW_OP_piece in DW_AT_frame_base.
> -> http://llvm.org/bugs/show_bug.cgi?id=22762 <http://llvm.org/bugs/show_bug.cgi?id=22762>
( Even if gdb wouldn’t be crashing we cannot use a DW_OP_piece in DW_AT_frame_base because of the ambiguity outlined in http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150309/265207.html <http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150309/265207.html>. )

> 
> -- adrian
> 
>>  
>> 
>> 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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