[LLVMdev] Preferred alignment of globals > 16bytes

[Hal Finkel]

On Thu, 20 Sep 2012 10:18:46 -0700
Chris Lattner <clattner at apple.com> wrote:

> On Sep 18, 2012, at 3:11 AM, Richard Osborne <richard at xmos.com> wrote:
> >>> I want it to be configurable. On my target there is no advantage
> >>> to aligning anything by more than a 4 bytes - it just wastes
> >>> space. I'll try to put together a proposal for making it possible
> >>> to set this per target.
> >> Ok, after considering this a bit more, how about we take a
> >> different approach: despite its name, TargetData is really mostly
> >> for consumption by the mid-level optimizers.
> >> getPreferredAlignment is really something that only
> >> TargetLowering[ObjectFile] should be using.  What do you think
> >> about sinking getPreferredAlignment down to it? This will make it
> >> much easier to target-hookize this.
> >> 
> >> -Chris
> > Hi Chris,
> > 
> > Sorry for not replying sooner but I've haven't had a chance to look
> > at this until now. I think your suggestion makes a lot of sense.
> > The preferred alignment will be a lot easier to change if it is not
> > embedded in the IR in the form of the target data string. I started
> > trying to moving the functions to get preferred alignment to
> > TargetLowering but found there are a few mid-level optimizers that
> > make use of the preferred alignment. In particular:
> > 
> > * lib/Analysis/ValueTracking.cpp - uses getPreferredAlignment() to
> > determine number of trailing zeros of the address of a global
> > defined in the current module.
> 
> This "works", but isn't safe.  It is relying on an assumption that
> the backend will provide at least that alignment for a global, and
> assuming that nothing later than it will change the alignment to 1.
> This needs to be fixed.
> 
> > * lib/Transforms/Utils/InlineFunction.cpp - Uses
> > getPrefTypeAlignment() to set the alignment of the alloca it
> > introduces when a byval argument is inlined
> > * lib/Transforms/IPO/ConstantMerge.cpp - Uses
> > getPreferredAlignment() to determine the alignment of the merged
> > constant.
> 
> Can't these just leave the alignment unset and let the code generator
> do its thing?
> 
> > * lib/Transforms/Vectorize/BBVectorize.cpp - Depending on what
> > options are set this pass avoids vectorization if it would
> > introduce loads and store of less than the alignment returned by
> > getPreferredAlignment().
> 
> I can't comment on this specifically, but this also sounds wrong.
> Hal?

This is correct, but I don't think that it has anything to do with
globals.

Some SIMD ISAs allow efficient loads and stores of vectors with
the scalar-element alignment and some don't. For those that don't, we
don't want the vectorizer to form these unaligned loads because this
might cause prevent vector-aligned loads from being generated. In
theory we could always form the unaligned loads and let the legalizer
break them up as necessary, but this often leads to suboptimal code
generation for various reasons.

> 
> > * lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp -
> > getPrefTypeAlignment() is used to explictly set the alignment of
> > allocas with unspecified alignment. Also instcombine tries to
> > enforce preferred alignment for loads and stores if possible.
> 
> 
> This case is more interesting to me - the idea here is that
> instcombine is trying to expose alignment information beyond the ABI
> alignment for optimization passes, allowing other passes to benefit
> from it - and allowing alignment inference of loads and stores to the
> alloca, which may help codegen.
> 
> I tend to think that this is the wrong approach - optimizations that
> would benefit from higher alignment should themselves force the
> alloca to be more aligned, not rely on instcombine to do it.  The
> code generator should wait until the alloca gets a specific alignment
> (which presumably will be the preferred alignment) and then bump up
> the alignment of loads and stores based on that alignment, instead of
> just taking what IR has and blindly going with it.

Interesting, I did not know about this; it seems like the vectorizer
should hook into this as well.

 -Hal

> 
> -Chris
> 



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory