[LLVMdev] Contants generation - proposal

[Demikhovsky, Elena]

> I think that the improved behavior for consts should be acceptable in the large model. But that's just me.
By default, all constants should be in a special read-only section, and this section may be far from the text section.

I'm working with JIT model or with only one object file. The code model, when all constants are near, we can call "LargeJIT". (Is it sounds better then LargeNearConst?)
For X86 target JITDefault will be translated CodeModel::LargeJIT for x64 and CodeModel::Small for 32-bit.

And the TargetLoweringObjectFile already has CodeModel inside. So the code will look like :

const MCSection *
TargetLoweringObjectFile::getSectionForConstant(SectionKind Kind) const {
  if (getCodeModel() == CodeModel::LargeJIT)
    return TextSection;
  if (Kind.isReadOnly() && ReadOnlySection != 0)
    return ReadOnlySection;

  return DataSection;
}

-  Elena


-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com] 
Sent: Tuesday, June 25, 2013 23:31
To: Demikhovsky, Elena
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Contants generation - proposal

Hi Elena,

> (2) Proposal
> Define one more Code Model, let's say "LargeNearConst", which will allow to put constants in .text.

Isn't that a little heavy-handed? The large model only requires the less efficient access for symbols we can't control, and in fact x86 still uses pc-relative conditional branches within a function so it can't pretend to support a single function >2GB in code size even now.

I think that the improved behaviour for consts should be acceptable in the large model. But that's just me.

> I'd like to hear people's opinion. Will it help other targets? I'm ready to send a patch.

An analogous improvement would be useful for AArch64, which means that if the new model *is* justified it would probably be useful there too.

Cheers.

Tim.
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