commit 69511ae80480815385abef7dfa095a8bd2961144
author Jason Molenda <jmolenda@apple.com> Tue Sep 09 14:11:39 2025 -0700
committer GitHub <noreply@github.com> Tue Sep 09 14:11:39 2025 -0700
tree 03239f13c702d98514ab33f9c3ccb0afba13c0eb
parent c4b17bf9ed46acbd8b28987f2a6262964e4e2072

[lldb] Unwind through ARM Cortex-M exceptions automatically (#153922)

When a processor faults/is interrupted/gets an exception, it will stop
running code and jump to an exception catcher routine. Most processors
will store the pc that was executing in a system register, and the
catcher functions have special instructions to retrieve that & possibly
other registers. It may then save those values to stack, and the author
can add .cfi directives to tell lldb's unwinder where to find those
saved values.

ARM Cortex-M (microcontroller) processors have a simpler mechanism where
a fixed set of registers are saved to the stack on an exception, and a
unique value is put in the link register to indicate to the caller that
this has taken place. No special handling needs to be written into the
exception catcher, unless it wants to inspect these preserved values.
And it is possible for a general stack walker to walk the stack with no
special knowledge about what the catch function does.

This patch adds an Architecture plugin method to allow an Architecture
to override/augment the UnwindPlan that lldb would use for a stack
frame, given the contents of the return address register. It resembles a
feature where the LanguageRuntime can replace/augment the unwind plan
for a function, but it is doing it at offset by one level. The
LanguageRuntime is looking at the local register context and/or symbol
name to decide if it will override the unwind rules. For the Cortex-M
exception unwinds, we need to modify THIS frame's unwind plan if the
CALLER's LR had a specific value. RegisterContextUnwind has to retrieve
the caller's LR value before it has completely decided on the UnwindPlan
it will use for THIS stack frame.

This does mean that we will need one additional read of stack memory
than we currently do when unwinding, on Armv7 Cortex-M targets. The
unwinder walks the stack lazily, as stack frames are requested, and so
now if you ask for 2 stack frames, we will read enough stack to walk 2
frames, plus we will read one extra word of memory, the spilled LR value
from the stack. In practice, with 512-byte memory cache reads, this is
unlikely to be a real performance hit.

This PR includes a test with a yaml corefile description and a JSON
ObjectFile, incorporating all of the necessary stack memory and symbol
names from a real debug session I worked on. The architectural default
unwind plans are used for all stack frames except the 0th because
there's no instructions for the functions, and no unwind info. I may
need to add an encoding of unwind fules to ObjectFileJSON in the future
as we create more test cases like this.

This PR depends on the yaml2macho-core utility from
https://github.com/llvm/llvm-project/pull/153911 to run its API test.

rdar://110663219