default_handler no longer includes _e

[LunNova]

In cortex-m, default_handler had a local variable for the exception for convenience when debugging:

rust-embedded/cortex-m@8394581#diff-86a9d5942521ed33df4dccb8dbf23016R164

It's missing now that default_handler is in cortex-m-rt:

https://github.com/japaric/cortex-m-rt/blob/8842d0efd7863c2dc23edd75548a658af59afe6b/src/lib.rs#L458

Is this intentional?

[japaric]

Is this intentional?

Yes, I don't recall the exact reason but the load operation was not elidable in release mode (because it's a volatile load) yet the variable was not inspectable (in release mode) because of inlining / optimization. This might have been related to the bloating we used to have due to repeated exception / interrupt handlers. (cf. rust-embedded/cortex-m#19)

I think we could provide GDB (Python script) helpers for this debugging stuff since the involved registers are available on all devices and always have the same addresses / semantics. I'm not familiar with GDB scripting though.

If you do want to perform some logic based on the current exception / interrupt in the default handler then you can always override the default handler using the default_handler! macro.

[japaric]

DefaultHandler now includes an irqn argument which is the IRQ number of the exception being serviced (negative = core exception, zero or positive = device specific interrupt).