[LLVMdev] Clang question

[Ryan Taylor]

Owen,

  Nevermind. bb-vectorize causes this optimization I see, I have disabled
it.

  I am still curious though, what is the syntactically correct way to just
remove the -memcpy using -fno-builtin, I have tried both
-fno-builtin[-memcpy] and the "gcc" version -fno-builtin-memcpy?

On Mon, Mar 5, 2012 at 3:00 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:

>
> Owen,
>
>   Clang doesn't accept this as an option; however, it did accept
> -fno-builtin (the more general for all usage) and this has seemed to work.
> Thank you.
>
>   My other question would then be how to lower vector instructions, such
> as extractelement, insertelement and shufflevector. These should be solved
> by ld/st/address calculation, correct? This is somewhat of the same problem
> it seems to me, or not?
>
>
> On Mon, Mar 5, 2012 at 1:44 PM, Owen Anderson <resistor at mac.com> wrote:
>
>> Does -fno-builtin[-memcpy] handle this?
>>
>> --Owen
>>
>> On Mar 5, 2012, at 12:35 PM, Eric Christopher wrote:
>>
>> > You'll need to do the work then. I'd also question why? On most
>> platforms a decent memcpy exists.
>> >
>> > -eric
>> >
>> > On Mar 5, 2012, at 12:28 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
>> >
>> >> I would like it to always be lowered, I don't want it.
>> >>
>> >> On Mon, Mar 5, 2012 at 12:27 PM, Eric Christopher <echristo at apple.com>
>> wrote:
>> >> You don't have memcpy or want it to always lower it?
>> >>
>> >> -eric
>> >>
>> >> On Mar 5, 2012, at 11:56 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:
>> >>
>> >>> Christoph,
>> >>>
>> >>> Yes, you are correct on the lifetime calls, they are just markers for
>> liveness.
>> >>>
>> >>> However, the backend is not optimizing these calls away. I could try
>> to deal with them outside of llvm but I was hoping for a cleaner solution
>> using llvm?
>> >>>
>> >>> On Mon, Mar 5, 2012 at 11:51 AM, Christoph Erhardt <
>> christoph at sicherha.de> wrote:
>> >>> Hi Ryan,
>> >>>
>> >>> the compiler is free to insert implicit calls to memcpy(), for
>> instance
>> >>> for assignments from one struct/class variable to another. The same
>> goes
>> >>> for memset(), which may be inserted implicitly for the initialization
>> of
>> >>> local structs or arrays.
>> >>>
>> >>> The good news is that the backend normally optimizes these calls away
>> >>> where possible, replacing them with simple moves - at least as long as
>> >>> the number of bytes to copy does not exceed a certain threshold.
>> >>>
>> >>> As for the llvm.lifetime intrinsics, take a look at the documentation:
>> >>> http://llvm.org/docs/LangRef.html#int_memorymarkers
>> >>> If I'm not mistaken, these calls seem to be used to mark the lifespan
>> of
>> >>> a stack-allocated object.
>> >>>
>> >>> Regards,
>> >>> Christoph
>> >>>
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>> >>
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>
>
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