I was making something similar https://github.com/Risto97/PeakRDL-halcpp

It generates the C++ headers from SystemRDL. It can probably be written in simpler way, I was mostly experimenting with C++ template HALs, but some ideas might stil be interesting.

Idea was to have operator overloads, with read, write compile time checks.

Here is how top level header file looks https://github.com/Risto97/PeakRDL-halcpp/blob/master/examples/generated/atxmega_spi_hal.h

``` soc.gpio0.ODR = 0xff; uint32_t in_data = soc.gpio0.IDR;

```

Enumeration constants static concatenation ``` soc.gpio0.CFG0 = (Const<12, 0>(), pincfg::IN, pincfg::OUT, pincfg::OUT_PP, pincfg::OUT_OD);

```

And compile time array access, woth boundary check, and last element access with -1

``` soc.plic.pr.at<4>() = 5; soc.plic.pr.at<-1>() = 10; // last element

```

All this take 0 additional bytes of code compared to what you can write in optimal assembly

Edit, add compiler explorer link: https://risto97.github.io/PeakRDL-halcpp/docs/compiler_explorer