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

[SHEN Hao]

On Wed, May 12, 2010 at 3:53 PM, SHEN Hao <hao.shen at imag.fr> wrote:
> On Mon, May 10, 2010 at 7:19 PM, John Criswell <criswell at uiuc.edu> wrote:
>> 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.
>
> Thanks for your suggestion. I am using the LLVM for a simulator related
> research project. I need to modify all memory address related instructions
> such as load/store, br and call. As I should modify all the call related
> instructions, I can only support general functional calls. That's why I do not
> like all these intrinsic instructions.
>
> I like your suggestion of adding a LLVM pass. I think it's a good way to solve
> this problem with some works. Now I am now taking a look of existing
> LLVM code and I wish I can solve it ASAP.
>

Fortunately, in the lib/Target/CBackend/CBackend.cpp, I found the
CWriter::lowerIntrinsics function which is a good example for my
work. The LLVM pass can be realized based on this example.

> Best regards,
> Hao
>
>>
>> -- 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.
>>>> _______________________________________________
>>>> 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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>
>
>
> --
> Hao Shen
>



-- 
Hao Shen