[PATCH] D15641: [Driver] Pass -O* to the gold plugin via -plugin-opt

[Mehdi Amini via cfe-commits]

> On Dec 19, 2015, at 8:11 AM, James Molloy <james at jamesmolloy.co.uk> wrote:
> 
> It's interesting to think about, but not something I'm intending to attack right now. 
> 
> I mainly just wanted to know the answer to Joerg's question, and now I do :)


I missed Joerg’s question earlier, I didn’t know what you were referring to. So I’ll answer here: AFAIK per-function attributes do not record the optimization level but only some optimization attributes: minsize, optsize, option. 
A fundamental issue is that the pipeline can change between O0/O1/O2/O3, and this is intrinsically problematic since the pipeline cannot be “per-function” overall.
What we were thinking to implement would be some information at the module level to instruct the ThinLTO optimizer of the flags to enable/disable, probably in global metadata.

The CodeGenOpt::Level has the same issue that it could be used to change the pipeline theoretically. It is not a problem at the time since (AFAIK) the CodeGen pipeline is intrinsically per function, but this may change with the new instruction selector.

— 
Mehdi


> 
> James
> On Sat, 19 Dec 2015 at 17:17, Mehdi Amini via cfe-commits <cfe-commits at lists.llvm.org <mailto:cfe-commits at lists.llvm.org>> wrote:
> 
> 
> Sent from my iPhone
> 
> > On Dec 18, 2015, at 11:11 PM, James Molloy <James.Molloy at arm.com <mailto:James.Molloy at arm.com>> wrote:
> >
> > Hi Mehdi,
> >
> >> On 18 Dec 2015, at 23:17, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
> >>
> >> The alternative to the command line flag is to encode the optimization level in the bitcode itself.
> >
> > You may have answered Joerg's question here - I take it this is not yet implemented?
> 
> Not implemented. We have settled on the various tradeoff. Are you interested to think about this?
> --
> Mehdi
> >
> > James
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