[LLVMdev] Prevent clang from replacing code with library calls

[David Sela]

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
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>
>
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