[LLVMdev] Call to non-constant memset() being generated where libc is not available

[Nick Lewycky]

David Meyer wrote:
> I am compiling something without standard libraries, including no libc.
>
> Somehow, this bitcode is being generated by plain array manipulation code:
>
>    %12 = add i8 %11, -19
>    %tmp35 = icmp sgt i32 %9, 1
>    %smax36 = select i1 %tmp35, i32 %9, i32 1
>    call void @llvm.memset.p0i8.i32(i8* getelementptr inbounds ([100 x
> i8]* @global_array_char, i32 0, i32 0), i8 %12, i32 %smax36, i32 1, i1
> false)
>
> (global_array_char is an array of length 100, which only partially gets
> used.)

That's not quite enough to go on. What's the original code, and what is 
this? Are we looking at an array initialization here?

> This gets lowered by LLC to a library call to memset(), which cannot be
> satisfied.
>
> Any advice?

As Duncan pointed out, memset may be generated as part of the ABI even 
in a freestanding (ie., no libc) environment. I think memcpy is in the 
same boat.

If you have a different target with a different ABI that doesn't provide 
memset/memcpy, you could try writing your own generic lowering in 
Target/X86. Or you could always link against something native that 
provides memset/memcpy, which is what I strongly recommend.

> Thanks,
> David Meyer
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev