[LLVMdev] PIC16 removal details

[Jim Grosbach]

On Sep 21, 2011, at 2:53 PM, Dan Gohman wrote:

> On Sep 20, 2011, at 10:59 PM, Matthew Hilt wrote:
> 
>> I've been looking closely at LLVM as a means to developing a new toolchain for an MCU core of very similar architecture.  To that end, the once included PIC16 backend might be a valuable reference.  I found a message in April of this year that indicated it had been dropped from new releases however, and that were it to be resumed "it will be largely a rewrite".
>> 
>> I'm wondering if I can get some details as to why it was dropped firstly and also, why would it be 'largely a rewrite' if it were resumed?  Thanks in advance.
> 
> LLVM is designed for "normal" architectures. The further from
> approximately the intersection of x86 and ARM, the more it takes to
> use LLVM effectively. PIC16, we learned, is well out there, from
> LLVM's perspective.

An eight-bit, accumulator based, Harvard ISA is the Trifecta of Doom as far as LLVM is concerned, basically.

> As one example, LLVM is built to optimize for machines with numerous
> general-purpose registers. It's a stretch to even say that PIC16 has
> a single general-purpose register. The PIC16 backend developers
> asked for ways to prevent LLVM from promoting variables from memory
> into registers, because it was pessimizing code for PIC16. However,
> LLVM can't do that without also shutting down large parts of itself,
> because of the pervasive assumption that registers are where the
> action is.


To be fair, I don't think that was a good approach to that problem. Neither here nor there at this point, though.

-Jim