[llvm-dev] trouble hoisting GlobalValues

Hal Finkel via llvm-dev
Fri, 11 Dec 2015 22:33:00 -0000

----- Original Message -----
> From: "Rafael Espíndola via llvm-dev" <llvm-dev at
lists.llvm.org>
> To: "Steve King" <steve at metrokings.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Friday, December 11, 2015 4:28:33 PM
> Subject: Re: [llvm-dev] trouble hoisting GlobalValues
>
> On 11 December 2015 at 16:53, Steve King via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hello LLVM,
> > To reduce the code-size cost of relocations, I'm trying to hoist
> > GlobalValues that are used many times.  A new pass hides each
> > hoisted
> > GV behind a BITCAST in the dominating BB.  The pass then updates
> > users
> > with the output of the BITCAST.  This pass works properly AFAICT.
> >
> > The problems come in instruction selection.
> > SelectionDAGBuilder::visitBitCast() treats the BITCAST as a no-op
> > and
> > eventually users of the GV fold the relocation as if hoisting never
> > happened.  Experiments turning the BITCAST into a copytoreg failed
> > and
> > anyway I feel like I'm just shooting in the dark.
> >
> > Can anyone can suggest a strategy for lowering a hoisted GV?  The
> > end
> > result should be that the GV materializes as a simple
> > move-immediate
> > to register.

Also, something similar-sounding to this is done in
lib/Transforms/Scalar/ConstantHoisting.cpp

 -Hal

>
> Nothing simple. One possibility would be to make constants explicitly
> materialized. Something like:
>
> https://github.com/apple/swift/blob/master/docs/SIL.rst#global-addr
>
> Which would also be useful for architectures where some constants are
> expensive to place in a register.
>
> Cheers,
> Rafael
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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory