llvm dev - Mar 2014 - [LLVMdev] [ARM] [PIC] optimizing the loading of hidden global variable

Hi,

 

When I’m compiling a code with –fvisibility=hidden –fPIC for ARM, I find
that LLVM generates less optimized code than GCC.

 

For example:

test.cpp:

 

void init(void *);

 

int g0[100];

int g1[100];

int g2[100];

 

void foo() {

  init(&g0);

  init(&g1);

  init(&g2);

}

 

Clang will emit 1 GOT entry for each GV and 2 instructions to get the
address: 

        ldr     r0, .LCPI0_2

        add     r0, r0, r4

         bl      _Z4initPv(PLT)

 

GCC  does this only for the first GV. The rest GV address are computed
directly:

        ldr     r4, .L2

.LPIC0:

        add     r4, pc, r4   è get &g0 via GOT_PC Relative

        mov     r0, r4

        bl      _Z4initPv(PLT)

        add     r0, r4, #400    è get &g1

        bl      _Z4initPv(PLT)

        add     r0, r4, #800  è get &g2

        ldmfd   sp!, {r4, lr}

        b       _Z4initPv(PLT)

.L3:

        .align  2

.L2:

        .word   .LANCHOR0-(.LPIC0+8)  è 1 GOT offset entry

 

It seems it’s a missing optimizing opportunity for LLVM both in code size
and performance, any ideas? If so, I can open a bug and try to fix it.

 

Thanks,

Weiming

 

 

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

 

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Hi Weiming,

On 12 March 2014 17:43, Weiming Zhao <weimingz at codeaurora.org>
wrote:
> Clang will emit 1 GOT entry for each GV and 2 instructions to get the
> address:
>
> GCC  does this only for the first GV. The rest GV address are computed
> directly:
This looks like it would be the job of lib/Transforms/GlobalMerge.cpp.
It looks like ARM runs it in all cases, perhaps it doesn't understand
some ELF linkage subtleties?

Cheers.

Tim.

Hi Tim,

Thanks for the pointer. It seems GlobalMerge only considers static/local GVs:
  if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
      continue;
Let me try some experiments in GlobalMerge.

Another place might be in ARMISelLowering.cpp :: LowerGlobalAddressELF(), but I
think changing GlobalMerge makes more sense.

Thanks,
Weiming

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


-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com] 
Sent: Wednesday, March 12, 2014 12:50 PM
To: Weiming Zhao
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] [ARM] [PIC] optimizing the loading of hidden global
variable

Hi Weiming,

On 12 March 2014 17:43, Weiming Zhao <weimingz at codeaurora.org>
wrote:
> Clang will emit 1 GOT entry for each GV and 2 instructions to get the
> address:
>
> GCC  does this only for the first GV. The rest GV address are computed
> directly:
This looks like it would be the job of lib/Transforms/GlobalMerge.cpp.
It looks like ARM runs it in all cases, perhaps it doesn't understand some
ELF linkage subtleties?

Cheers.

Tim.

Hi Tim,

The global merge pass puts the GVs into a sturcture to guarantee their
address are contiguous.
It works for static GVs but for global hidden GVs, this will cause name
resoltion fail during linking .o into .so

Any thoughs?

Thanks,
Weiming

> Hi Weiming,
>
> On 12 March 2014 17:43, Weiming Zhao <weimingz at codeaurora.org>
wrote:
>> Clang will emit 1 GOT entry for each GV and 2 instructions to get the
>> address:
>>
>> GCC  does this only for the first GV. The rest GV address are computed
>> directly:
>
> This looks like it would be the job of lib/Transforms/GlobalMerge.cpp.
> It looks like ARM runs it in all cases, perhaps it doesn't understand
> some ELF linkage subtleties?
>
> Cheers.
>
> Tim.
>