Built in progmem variables in AVR

Hello.

I was wondering if it is possible to implement placing RO variables within progmem sections. For now it is possible to just use [[gnu::progmem]] and read variables using assembly. My proposal is to:

  • Use those variables as any other, without creating assembly.
  • While reading them, compiler has all necessary knowledge where the variable is and can generate assembly

Possible future improvements:

  • Automatic placement of variables in __flash sections.
  • Take care of reading across 64KB boundary
  • If a variable is const, it can be places automatically in some __flash section

AVR devices are old, but still widely used. This feature could improve code quality even further.

Hello Łukasz,

I was wondering if it is possible to implement placing RO variables within progmem sections. For now it is possible to just use [[gnu::progmem]] and read variables using assembly

This is already implemented, via address spaces. At the moment, the IR frontend can control which memory to place global variables in by declaring it with addrspace(1), which will place the variable in read-only program memory. When omitted, LLVM assumes the default address space 0, which corresponds to read/write SRAM data memory.

The address space is tracked through all uses of the global variable, and the AVR backend will correctly select an ‘LPM’ instruction to load from program memory when the address space is one, otherwise it will emit an ‘LD’ instruction to load from data memory. You shouldn’t need to implement this boilerplate yourself.

There are some good points here

  • Automatic placement of variables in __flash sections.

    • avr-rust issue: https://github.com/avr-rust/rust/issues/74
    • I’m a big fan of automatic placement of const/RO variables in flash memory, but there hasn’t been very much critical debate of it, which makes me hesitant to build it into LLVM proper. As far as my reasoning has gone, I think we should be able to add a pass to do this safely, but whether it should be on by default needs more debate IMO
  • Take care of reading across 64KB boundary

    • Good point, we don’t currently recognize the RAMP{X,Y,Z} IO registers, which are required for >64kb addressing.
  • If a variable is const, it can be places automatically in some __flash section

  • Related to above - should this be on by default? Is there any possible codegen breakage it could cause?

As I recall, reading a byte from program memory requires an extra cycle. But maybe if you're cycle counting, you should just be programming in assembly.

Steve

Hi.

IMO explicit placing a addrspace(1) is fine. If I decide to port my code to other platform which does not have those quirks (like 32 bit arm), I could always use:

For AVR
#define PROGMEM addrspace(1)
For ARM
#define PROGMEM

There may be a use case when someone doesn’t want to automatically place const variable in program space for performance reasons. IMO simple #define solution is good enough.

What I care most about, is automatic instruction selection of RAMPZ, LPM, managing 64KB boundry and letting compiler do it.
That would already greatly simplify code, without a need for hand rolled assembly for reading and special types.

Sorry if I’m chiming in with an irrelevant comment :slight_smile: … but placing read only variables (in C, const variables) in program memory/flash is also one of those things that I think has been discussed in fairly great detail by the AVR GCC guys when they were building their original compiler. I’m pretty sure I remember coming across old threads on this.

They discuss it here: https://www.nongnu.org/avr-libc/user-manual/pgmspace.html. They are talking about C but it would probably be relevant more generally?

They seemed to have lots of good reasons why it was not optimal compiler design and the person writing the program should explicitly define when a constant should be defined in program memory.

Certainly on the most basic level, for anything except strings, these constants will only amount to a very small proportion of the code size, wouldn’t they?

It makes sense for the frontend to reason about storage if enough is known and I would think the frontend is the right place to add any such logic.

(As an example, in a future version of the swift compiler for targeting AVR, I want to make it automatically intern strings as C strings and add addrspace(1) to them.)

Just my 2 cents worth… I know far less than you guys :)…

Carl