[LLVMdev] [RFC] AArch64: Should we disable GlobalMerge?

Renato Golin
Thu, 26 Feb 2015 12:09:10 -0000

On 26 February 2015 at 00:57, Ahmed Bougacha <ahmed.bougacha at gmail.com>
wrote:
> -- A way forward
> One obvious way to improve it is: look at uses of globals, and try to
> form sets of globals commonly used together.  The tricky part is to
> define heuristics for "commonly".  Also, the pass then becomes
much
> more expensive.  I'm currently looking into improving it, and will
> report if I come up with a good solution.  But this shouldn't stop us
> from disabling it, for now.

Hi Ahmed,

Before "moving forward", it would be good to understand what in
GlobalMerge is impacting what in LTO.

With LTO becoming more important nowadays, I agree we have to balance
the compiler optimisations to work well with it, but by turning things
off we might be impacting unknown code in an unknown way.

We'll never know how unknown code behaves, but if at least we
understand what of GM affects what of LTO, then people using unknown
code will have a more informed view on what to disable, when.

cheers,
--renato