llvm dev - Aug 2014 - [LLVMdev] Prevent clang from replacing code with library calls

> On Aug 7, 2014, at 4:12 AM, Anton Korobeynikov <anton at
korobeynikov.info> wrote:
> 
>> I downloaded the latest NDK (r10) and compiled with the following cmd:
>> arm-linux-androideabi-clang myMemcpy.c -S -fno-builtin -o0
>> It still produces assembly with a call to "memcpy". Maybe the
-fno-builtin
>> is broken also in the latest release.
> It's not broken. Even freestanding implementation is supposed to
> provide mem* functions.
> 
It’s also supposed to be possible to write a memcpy() implementation in C
without the compiler’s idiom recognizer just transforming it into a call to the
system memcpy(). -fno-builtin is supposed to be how to do that, right?

-jim

On Thu, Aug 7, 2014 at 2:49 PM, Jim Grosbach <grosbach at apple.com>
wrote:
>
> > On Aug 7, 2014, at 4:12 AM, Anton Korobeynikov <anton at
korobeynikov.info>
> wrote:
> >
> >> I downloaded the latest NDK (r10) and compiled with the following
cmd:
> >> arm-linux-androideabi-clang myMemcpy.c -S -fno-builtin -o0
> >> It still produces assembly with a call to "memcpy".
Maybe the
> -fno-builtin
> >> is broken also in the latest release.
> > It's not broken. Even freestanding implementation is supposed to
> > provide mem* functions.
> >
>
> It’s also supposed to be possible to write a memcpy() implementation in C
> without the compiler’s idiom recognizer just transforming it into a call to
> the system memcpy(). -fno-builtin is supposed to be how to do that, right?
>
IIRC -ffreestanding might be the solution.

-- Sean Silva

>
> -jim
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Its seems like a syntax bug in clang. You must use optimization flag
together with the *no-builtin* flag.

The following command produces assembly *with* a call to memcopy:

arm-linux-androideabi-clang -S myMemcpy.c -fno-builtin

The following command produces assembly *without* a call to memcopy:

arm-linux-androideabi-clang -S myMemcpy.c -fno-builtin -O1

The *no-builtin* flag is coupled with the optimization flag.

Thanks,
David




On Fri, Aug 8, 2014 at 1:49 AM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
>
> On Thu, Aug 7, 2014 at 2:49 PM, Jim Grosbach <grosbach at apple.com>
wrote:
>
>>
>> > On Aug 7, 2014, at 4:12 AM, Anton Korobeynikov <anton at
korobeynikov.info>
>> wrote:
>> >
>> >> I downloaded the latest NDK (r10) and compiled with the
following cmd:
>> >> arm-linux-androideabi-clang myMemcpy.c -S -fno-builtin -o0
>> >> It still produces assembly with a call to "memcpy".
Maybe the
>> -fno-builtin
>> >> is broken also in the latest release.
>> > It's not broken. Even freestanding implementation is supposed
to
>> > provide mem* functions.
>> >
>>
>> It’s also supposed to be possible to write a memcpy() implementation in
C
>> without the compiler’s idiom recognizer just transforming it into a
call to
>> the system memcpy(). -fno-builtin is supposed to be how to do that,
right?
>>
>
> IIRC -ffreestanding might be the solution.
>
> -- Sean Silva
>
>
>>
>> -jim
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
>
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