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

Clang optimizes code by replacing some parts with efficient library
functions.

For example the following code:

for (i=0;i<size;++i)
       dest[i]=src[i];

will be compiled to (target=ARM assembly):

bl      __aeabi_memcpy(PLT)

The compile cmd:

/usr/share/android-arm-l14-toolchain/bin/clang31 -cc1 -triple
arm-none-linux-androideabi -S -target-abi aapcs-linux -target-cpu arm1022e
-backend-option -arm-enable-ehabi -backend-option
-arm-enable-ehabi-descriptors -backend-option -arm-ignore-has-ras
-internal-isystem
/usr/share/android-arm-l14-toolchain/lib/clang/3.1/include
-internal-externc-isystem
/usr/share/android-arm-l14-toolchain/bin/../sysroot/usr/include -o
myMemcpy.s -x c myMemcpy.c


Is there a flag or some other way to prevent the compiler from replacing
the code with library calls?


Thanks in advance,
David
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Hi David,

"-fno-builtin” is probably what you want.

-Jim
> On Aug 4, 2014, at 2:19 AM, David Sela <sela.david at gmail.com>
wrote:
> 
> Clang optimizes code by replacing some parts with efficient library
functions.
> 
> For example the following code:
> 
> 
> for (i=0;i<size;++i)
>        dest[i]=src[i];
> will be compiled to (target=ARM assembly):
> 
> 
> bl      __aeabi_memcpy(PLT)
> The compile cmd:
> 
> /usr/share/android-arm-l14-toolchain/bin/clang31 -cc1 -triple
arm-none-linux-androideabi -S -target-abi aapcs-linux -target-cpu arm1022e
-backend-option -arm-enable-ehabi -backend-option -arm-enable-ehabi-descriptors
-backend-option -arm-ignore-has-ras -internal-isystem
/usr/share/android-arm-l14-toolchain/lib/clang/3.1/include
-internal-externc-isystem
/usr/share/android-arm-l14-toolchain/bin/../sysroot/usr/include -o myMemcpy.s -x
c myMemcpy.c
> 
> 
> 
> Is there a flag or some other way to prevent the compiler from replacing
the code with library calls?
> 
> 
> 
> Thanks in advance,
> David
> 
> 
> 
> _______________________________________________
> 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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Hi Jim,
I have tried "-fno-builtin" but it didn't help.
Thanks,
David


On Tue, Aug 5, 2014 at 12:12 AM, Jim Grosbach <grosbach at apple.com>
wrote:
> Hi David,
>
> "-fno-builtin” is probably what you want.
>
> -Jim
>
> On Aug 4, 2014, at 2:19 AM, David Sela <sela.david at gmail.com>
wrote:
>
> Clang optimizes code by replacing some parts with efficient library
> functions.
>
> For example the following code:
>
> for (i=0;i<size;++i)
>        dest[i]=src[i];
>
> will be compiled to (target=ARM assembly):
>
> bl      __aeabi_memcpy(PLT)
>
> The compile cmd:
>
> /usr/share/android-arm-l14-toolchain/bin/clang31 -cc1 -triple
> arm-none-linux-androideabi -S -target-abi aapcs-linux -target-cpu arm1022e
> -backend-option -arm-enable-ehabi -backend-option
> -arm-enable-ehabi-descriptors -backend-option -arm-ignore-has-ras
> -internal-isystem
> /usr/share/android-arm-l14-toolchain/lib/clang/3.1/include
> -internal-externc-isystem
> /usr/share/android-arm-l14-toolchain/bin/../sysroot/usr/include -o
> myMemcpy.s -x c myMemcpy.c
>
>
> Is there a flag or some other way to prevent the compiler from replacing
> the code with library calls?
>
>
> Thanks in advance,
> David
>
>
> _______________________________________________
> 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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