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

[Корчагин Василий]

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.
>>
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