[llvm-dev] Built in progmem variables in AVR

[Dylan McKay via llvm-dev]

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?



On Thu, Oct 10, 2019 at 3:16 AM Łukasz Kostka via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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.
>
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