[avr-llvm-devel] Important stuff

[Borja F. <bor...@gm...>]

We need to figure out some important things that need to be introduced in
the compiler before we advance with bigger things. The things i'm talking
about are:

1) aligning data wider than 1 byte in even registers to favour movws and
argument passing into functions.
 int16 a = b; // a = r21:r20, b = r13:r12
 movw r20, r12 <--- good reg allocation

 int16 a = b; // a = r20:r21, b = r12:r13
 mov r21, r13
 mov r20, r12 <--- bad reg allocation

2) THE BIG ONE: working with register pairs. there are some instructions
like sbiw/adiw, movw, ld/st that only work with register pairs. we need to
figure out how to introduce them.
For example, I wrote a pass that converts 2 moves in a row into a movw. This
is very basic, but i find it bad, because if the compiler places an
instruction between both moves this pass wont be able to replace it with the
movw. Also i find it very hacky, because the compiler doesnt know that there
is really a move instruction that work with 16bit data so it doesnt have a
global view of the costs and probably worsening reg allocation.

If we worked with pseudo regs of 16 bits this should be straight forward,
but we would face the problem i mentioned some time ago, all data operations
would need custom lowering, because LLVM isnt able to split a wider reg into
subregs (say R1312 into R13:R12). An initial guess would be manually
splitting the reg into its subregs, perform the operation and finally
recombining the subregs again into the bigger one for next instruction, but
im not sure yet if this is a good solution.

We really need to think about it because it's a very important design
decision. This was my first email into llvm's dev mailing list, but i really
didnt got any real solution.
If you can think of any other points related to this, add them.