llvm dev - May 2010 - [LLVMdev] How can I remove Intrinsic Functions during llvm-gcc compilation?

Hi, all,

I am using llvm-gcc --emit-llvm to generate byte code. With llvm
readable ll format, I found some standard C library function such as
llvm.memset.
In fact, I'm trying to compile newlibc with llvm, I do not need this
kind of llvm functions. How can I remove them during the compilation?

Best regards,
-- 
Hao Shen

Hi Hao Shen,
> I am using llvm-gcc --emit-llvm to generate byte code. With llvm
> readable ll format, I found some standard C library function such as
> llvm.memset.
this is not a C library function, it is an LLVM intrinsic.  An intrinsic is
analogous to a builtin in gcc.  An intrinsic may be expanded out into a code
sequence by the compiler, or may get turned into a library call (which sounds
like is what you are seeing).
> In fact, I'm trying to compile newlibc with llvm, I do not need this
> kind of llvm functions. How can I remove them during the compilation?
You can't avoid them.  The same problem exists with gcc: you can't
always
avoid having gcc use the gcc memset builtin.  However it has to be said
that gcc generates its memset builtin less often than llvm-gcc generates
llvm.memset, so this is not as visible.  Also (I'm not sure about this)
it may be that on some platforms gcc always expands builtin_memset into a
code sequence rather than generating a call to the library memset function.
However, gcc is not obliged to use a code sequence even in a freestanding
environment.  The environment is always required to provide memset.  Here's
what the gcc docs say:

   GCC requires the freestanding environment provide `memcpy',
`memmove',
  `memset' and `memcmp'.

Ciao,

Duncan.

Thanks a lot for your answer.
As what you said, I can not have any options to avoid generating this kind
of intrinsic for byte code. Is it possible to modify gcc and ask it take
all memset liked functions as a general function call? I know this solution
is less performance efficient, but I would like to have it for my llvm
assembly level modification works.

But anyway, thanks for you help.
Hao

On Mon, May 10, 2010 at 6:30 AM, Duncan Sands <baldrick at free.fr>
wrote:
> Hi Hao Shen,
>
>> I am using llvm-gcc --emit-llvm to generate byte code. With llvm
>> readable ll format, I found some standard C library function such as
>> llvm.memset.
>
> this is not a C library function, it is an LLVM intrinsic.  An intrinsic is
> analogous to a builtin in gcc.  An intrinsic may be expanded out into a
code
> sequence by the compiler, or may get turned into a library call (which
sounds
> like is what you are seeing).
>
>> In fact, I'm trying to compile newlibc with llvm, I do not need
this
>> kind of llvm functions. How can I remove them during the compilation?
>
> You can't avoid them.  The same problem exists with gcc: you can't
always
> avoid having gcc use the gcc memset builtin.  However it has to be said
> that gcc generates its memset builtin less often than llvm-gcc generates
> llvm.memset, so this is not as visible.  Also (I'm not sure about this)
> it may be that on some platforms gcc always expands builtin_memset into a
> code sequence rather than generating a call to the library memset function.
> However, gcc is not obliged to use a code sequence even in a freestanding
> environment.  The environment is always required to provide memset.
 Here's
> what the gcc docs say:
>
>   GCC requires the freestanding environment provide `memcpy',
`memmove',
>  `memset' and `memcmp'.
>
> Ciao,
>
> Duncan.
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>


-- 
Hao Shen