Support for Low Power Modes

[RossPorter506]

Hi,

Is there any way to interact with the MSP430's various low power modes?
In C this is done through compiler intrinsics like __bis_SR_register() and __bic_SR_register_on_exit().

[cr1901]

This was discussed here. Short version:

• LLVM doesn't implement the intrinsics.
• An optimized inline asm implementation needs to know the surrounding codegen before codegen actually occurs.
• A Rust implementation of the intrinsics requires an LLVM feature that is likely to never be stabilized.
• A slightly less optimal version using naked functions should be possible, where the outer naked interrupt handler- the one that the vector points to- immediately calls an inner interrupt function. I've never personally implemented this, but @pftbest gave an example code snippet¹. I also believe @vadixidav got it working, and I never followed up.

1. Note that the code snippet implements a hypothetical __bis_SR_register_on_entry() :P. I think on_exit semantics would require pushing a fake return address and flags onto the stack, and then jmping to the inner function, and then bic, since the inner function will end with reti.

[vadixidav]

It has been a while since I touched this. We ended up not using MSP430 ultimately, so I didn't feel the need to continue working on this. Everything I did should be available in the fork I have. If something is missing (downstream), let me know, and I might be able to dig it up.