[LLVMdev] module level assembly optimization

[Rafael Espíndola]

On 14 October 2013 21:56, reed kotler <rkotler at mips.com> wrote:
> I would like to do constant pools over an entire module instead of just on a
> per function basis as constant islands does it now.
>
> It seems there are two options for this:
>
> 1) collect the machine functions with their machine instructions for the
> whole module, edit them as needed and then
> call asmprinter for them one at a time.
>
> 2) collect all the instruction streams and store them in lists, one per
> section and put them all out at the end.
>
> There are possibly other kinds of module level optimizations that could
> benefit from this.
>
> Some of this might be better done by moving this functionality into target
> independent code but I think I could do it all
> without such changes.
>
> Any thoughts?

My experience is here consists of only once debugging an issue on the
ARM constant island pass, so take my opinion with a grain of salt

When looking at the pass, it is easy to notice how much it looks like
an assembler doing relaxations. It needs to know exact offsets, adding
data to an island changes offsets, etc. It is also easy to notice how
hard it is to test a pass like that that is nested deep inside
codegen.

My thought at the time was that it would be nice to implement the pass
in the assembler itself by having codegen use pseudo instructions.
Something like

.const_island_set_reg r4, 0x12345678

The assembler would then be responsible for creating the islands and
producing a load (or using something like movw + movt if it was better
in that case.

The advantages would be
* The assembler already naturally handles exact addresses.
* It would be much easier to test.
* It would work across functions

Then again, it is entirely possible I missed something that mandates
this pass being in codegen.

Cheers,
Rafael