[LLVMdev] [ARM] [PIC] optimizing the loading of hidden global variable

[Weiming Zhao]

I just gave a try to MegeGlobal with alias because I thought it's easy to do. However, another issue with it is:
Although I got aliases like:
@h0 = alias getelementptr inbounds (... at _MergedGlobals, 0, 0)
@h1 = alias getelementptr inbounds (... at _MergedGlobals, 0, 1)
@h2 = alias getelementptr inbounds (... at _MergedGlobals, 0, 2)

They cannot be lowered to correct asm. The all be aliases of _MergedGlobals:
	.globl	h0
.set h0, _MergedGlobals
	.globl	h1
.set h1, _MergedGlobals
	.globl	h2
.set h2, _MergedGlobals

I guess there is no support in asm to alias to a member of struct, right?

Thanks,
Weiming

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation


-----Original Message-----
From: Rafael Espíndola [mailto:rafael.espindola at gmail.com] 
Sent: Friday, March 14, 2014 1:49 PM
To: Weiming Zhao
Cc: Tim Northover; LLVM Developers Mailing List; Jim Grosbach; Nick Kledzik
Subject: Re: [LLVMdev] [ARM] [PIC] optimizing the loading of hidden global variable

On 14 March 2014 14:34, Weiming Zhao <weimingz at codeaurora.org> wrote:
> Hi Rafael,
>
> Yes, merging gv prevents linker to do garbage collection. Should it be implemented as a peephole pass? If we do it too early, the distance between GVs are not fixed yet.

Correct. It would be somewhere in CodeGen, I am not exactly sure where.

> PS:
> Below is the GCC output with "extern" hidden:
>         ldr     r2, .L2
>         stmfd   sp!, {r3, lr}
>         .save {r3, lr}
> .LPIC0:
>         add     r0, pc, r2
>         bl      _Z4initPv(PLT)
>         ldr     r1, .L2+4
> .LPIC1:
>         add     r0, pc, r1
>         bl      _Z4initPv(PLT)
>         ldr     r0, .L2+8
> .LPIC2:
>         add     r0, pc, r0
>         ldmfd   sp!, {r3, lr}
>         b       _Z4initPv(PLT)
> .L3:
>         .align  2
> .L2:
>         .word   g0-(.LPIC0+8)
>         .word   g1-(.LPIC1+8)
>         .word   g2-(.LPIC2+8)

That is pretty neat too.

Cheers,
Rafael