[LLVMdev] datalayout optimization question

[Sebastien DELDON-GNB]

Hi Jim,

First of all, thanks for the quick answer. I'm just asking this question because in presence of target specific 'datalayout' information I see significant speed-up on benchmarks, in certain rare cases I also see slowdowns. So before digging into the sources to understand what's going on, I wanted to understand if there is an identified set of optimizations that are known to benefit of 'datalayout' information.

Best Regards
Seb

> -----Original Message-----
> From: Jim Grosbach [mailto:grosbach at apple.com]
> Sent: Tuesday, April 17, 2012 12:30 AM
> To: Sebastien DELDON-GNB
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] datalayout optimization question
> 
> Are you expecting to use a single set of IR files to generate code for
> multiple targets? Otherwise I'm not sure why your .ll files would
> necessarily lack the data layout information. The reason I ask is that
> LLVM IR is fundamentally not target independent in ways far beyond the
> presence or absence of the data layout string.
> 
> For gory details: http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-
> October/043719.html.
> 
> Beyond that, rather large, caveat, I'm not sure of the answer to your
> specific question. It's not so much which optimizations are turned off,
> per se, but rather that some won't have the information they need
> (alignment data, for example) to do a good job, so they'll miss
> opportunities.
> 
> Regards,
>   Jim
> 
> On Apr 16, 2012, at 9:06 AM, Sebastien DELDON-GNB
> <sebastien.deldon at st.com> wrote:
> 
> > Hi all,
> >
> > We are developing our own compiler front-end and use LLVM as back-
> end. We generate .ll files and use opt/llc to generate target specific
> code. When I generate datalayout information in my .ll file it seems
> that it enables more optimization from LLVM opt. How can I determine
> which optimization are driven by datalayout information generation ?
> > In other words, when I don't generate target specific datalayout
> information, what are the optimizations that are turned off ?
> >
> > Thanks for answers
> > Best Regards
> > Seb
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