[LLVMdev] Contants generation - proposal

[Demikhovsky, Elena]

Hi all,

I analyzed more and ready (1) to describe the problem and (2) I have a proposal.

(1) Problem description:
In X86_64 target the Code Model is "Large". It means that address is 64-bit and IP-relative memory operand can't be used in this case.
(Because in IP-relative memory operand the displacement is 32-bit).

In order to load constant, we use 2 instructions.
movabsq $.LCPI0_0, %rcx
vmulpd  (%rcx), %ymm0, %ymm0

It happens because .LCPI0_0 is in .rodata section and instruction itself is in .text.

If I put the constant in .text, the code will look much better:
vmulpd  .LCPI0_0(%rip), %ymm0, %ymm0

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

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

Thank you.

-  Elena


-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com] 
Sent: Tuesday, June 25, 2013 15:08
To: Demikhovsky, Elena
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Contants generation

Hi again,

Actually, I've just been looking at the existing code and the ARM solution may be over-complicated for this situation.

You should be able to override EmitConstantPool directly, or possibly even just override getSectionForConstantKind in X86LinuxTargetObjectFile (and perhaps others) to return .text.

Tim.
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