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

[John Criswell]

SHEN Hao wrote:
> 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.
>   

It's possible to write an LLVM pass that converts calls to llvm.memset() 
into calls to an external memset() function.  You could then modify your 
version of llvm-gcc to run this pass.

However, is there really a problem with letting llvm-gcc use the 
intrinsic, even if you're compiling a C library?  If the code generator 
replaces llvm.memset() with inline assembly code that does the same 
thing, then you get the correct behavior.  If it modifies the 
llvm.memset() call to call an external memset() function, then the 
program ends up using the implementation of memset() from the C library 
that you're compiling.  Either way, I'd think you'd get correct behavior.

-- John T.

> 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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>>     
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