[llvm-dev] trouble hoisting GlobalValues

[Rafael Espíndola via llvm-dev]

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.

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