[LLVMdev] Proposal: Adding aligned instruction bundle support to MC

[Jim Grosbach]

On Dec 6, 2012, at 2:45 PM, Eli Bendersky <eliben at google.com> wrote:

>> How will these bundles interact with ARM codegen? The constant island pass in particular.
>> 
> 
> Hi Jim,
> 
> This is a great question.
> From the compiler's point of view, these bundles indeed pose a problem
> for the constant island pass. At this point for the NaCl platform we
> disable this pass, generating instructions like movt instead (NaCl
> requires at least v7 for ARM). However, as a future enhancement we'd
> like to follow gcc's path and adjust the pass to coexist with
> potential bundling by computing worst-case estimates for the sizes of
> bundle-locked instruction sequences.

That's going to be challenging. Not impossible, and you can probably leverage the existing basic-block alignment code, but very tricky.

I'm curious. Have you done any code size comparisons using movw/movt instead of constant pools? What was the impact?

-Jim

> 
> From the assembler's point of view, assembly code that refers to data
> using labels should generate errors in the event that these labels
> become too too far to reach, similarly to the current situation in
> gas.
> 
> Eli