Rust Roadmap 2021: Adding a useful dev mode and making it the default
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- 3 min
In this blog entry following the Call for Rust 2021 Roadmap items, I shall layout my vision for giving Rust the ability to create debuggable binaries which will also fit in the flash of a microcontroller.
This request has multiple motivations but following the Call for Rust 2021 Roadmap items rules I'm notating them in the agile user story way:
- As an embedded developer I would like to be able to create debuggable applications
- As an embedded developer I would also like to be able to create applications which fit into the very limited flash available to microcontrollers
So let me give you a bit more background about the status quo and what I mean by those two stories before giving you my ideas how to address them.
Debuggable applications which fit into flash
The "problem" with Rust applications in general is that once even just the bare-minimum optimisations are enabled, the compiler will completely restructure the whole application in a way that makes debugging hard or even impossible.
On the other hand, if optimisations are completely disabled,
happily churn out code which is so highly inefficient and large that it either
won't fit in the available flash space or be so slow that crucial deadlines can
not be met (like USB handling) and the application will simply fail; quite
often those two issues also rear their ugly heads at the same time. It also
panicing branches into code which is not even supposed to be
anything more than a compiler hint, cf. Issue 68208.
Other languages do a lot better by creating more efficient code without any optimisations but they also offer optimisation levels which don't or only lightly effect the debugability of an application built with them.
In addition to the code generation problems it's also a big stumbling block and
source of surprise and frustration that
rustc uses dev mode by default,
often creating unusable binaries or even just errors.
Imagine exploring Rust coming from C and seeing that the most basic program suddenly takes up many kB of precious flash instead of a few hundred Bytes when compiled with default flags (and including a fixed overhead of a few hundred bytes):
text data bss dec hex filename 532 0 0 532 214 miniblink.elf
text data bss dec hex filename 9976 0 4 9980 26fc blinky
I think it should be possible to come up with a set of debugging friendly basic
optimisations applied at
opt-level=g which operate similar to clang's or
-Og. Those are similar to
opt-level=1 but disable debugging harming
optimisations. Furthermore this level should be the default for
dev mode to
make Rust in embedded scenarios useful by default!
Thanks for your attention and hope to blog to y'all soon!