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

[Robinson, Paul]

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.

You mean like X86 vector registers?  They're implicitly arrays/structs of values of a smaller size. I don't know that the value of interest always ends up in the "low" part, but maybe it does. I really don't have a handle on X86 vector instructions.
--paulr

From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of David Blaikie
Sent: Tuesday, March 10, 2015 3:36 PM
To: Adrian Prantl
Cc: reviews+D8233+public+f5c9e8745840f37b at reviews.llvm.org; llvm-commits at cs.uiuc.edu
Subject: Re: [PATCH] Fix the remainder of PR22762 (GDB is crashing on DW_OP_piece being used inside of DW_AT_frame_base)



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
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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