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

Eli Bendersky eliben at google.com
Fri Dec 7 11:12:26 PST 2012

>> 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?

Even if we did, it was probably a while ago and they have to be done
again. We'll look into it when upstreaming the relevant parts.


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