[LLVMdev] Lowering "memcpy" intrinsic function on ARM using LDMIA/STMIA

[Sandeep Patel]

-fno-builtin is the flag you want.

deep

On Wed, Feb 9, 2011 at 10:18 PM, Корчагин Василий
<vasiliy.korchagin at gmail.com> wrote:
> 09.02.2011 18:57, Jason Kim пишет:
>> On Wed, Feb 9, 2011 at 5:02 AM, Vasiliy Korchagin
>> <vasiliy.korchagin at gmail.com>  wrote:
>>> Hi,
>>>
>>> llvm emits code for "memcpy" on ARM as consecutive ldr/str commands, and
>>
>> Hmm, this happens elsewhere as well (x86?). Perhaps what we need is a
>> switch to disable memset/memcpy lowering?
>>
>
> Do you offer to call libc memset/memcpy functions always instead of
> intrinsic lowering? It seems not a good idea, because often (especially
> in cases of small chunks of memory) consecutive ldm/stm instructions are
> more efficient than memcpy call.
>
>>> further combines them into ldm/stm with special pass after register
>>> allocation. But ldm/stm commands require registers to go in ascending order,
>>> what is often not so after regalloc, therefore some str/ldr commands. For
>>> example such code:
>>>
>>> struct Foo {int a, b, c, d; }
>>> void CopyStruct(struct Foo *a, struct Foo *b) { *a = *b; }
>>>
>>> compiled to:
>>>
>>> ldmia    r1, {r2, r3, r12}
>>> ldr    r1, [r1, #12]
>>> stmia    r0, {r2, r3, r12}
>>> str    r1, [r0, #12]
>>> bx    lr
>>>
>>> I ran different tests and always regalloc allocates at least one register
>>> not in ascending order.
>>>
>>> What is your ideas to overcome this issue? Maybe llvm should emit code for
>>> "memcpy" straight into ldm/stm or exchange registers before combining
>>> ldr/str to make them go in ascending order or fix somehow register
>>> allocator?
>>>
>>> Best regards, Vasiliy.
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>