[Lldb-commits] [PATCH] D82863: [LLDB] Add support to resize SVE registers at run-time

Pavel Labath via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Thu Sep 3 05:45:39 PDT 2020

labath added a reviewer: jasonmolenda.
labath added a comment.

I'd like to also pull in Jason for this, since this is really the trickiest part of the whole patchset.

What this patch essentially does is bake in the knowledge of the arm sve registers and their various configurations into the lldb client (and the `DynamicRegisterInfo` class in particular). Before we go into the implementation details, we should answer the question whether we are ok with that.

I am personally fine with that (if it gets encapsulated a bit better), because we can't avoid knowing about the registers of some architecture for various other reasons (core file debugging, eh_frame parsing, stubs which don't support qRegisterInfo, etc.). In fact, I'd be very happy if this got us closer towards a state where the stub does not need to send eh_frame and other register numbers over. Also, hardcoding may be the only way to get reasonable performance out of this, since a fully generic solution of asking about the register information after every stop would likely be prohibitively slow (well.. I suppose we could come up with some way of letting the stub to notify us when the register configuration has changed).

I believe Jason is also ok some hardcoding, in principle. However,
a) I don't want to speak for him,
b) there's still a lot of details to be worked out.

For example, one of the ideas floated around previously was based on the client and server communicating 4 things about each register:

- name
- the register number (for `p` packets, etc.)
- size
- offset (for e.g. `g` packet)

The register name and number are fine, but the other two things are sort of variable with SVE. The register size was mainly there as a cross-check, and so we could conceivably just drop it from the list (or use the max value or some other placeholder for sve registers in particular). But that still leaves the question of the `g` offsets...

It might be interesting data point to look at how the sve registers are described in gdb's target.xml descriptions.



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