llvm dev - Jul 2010 - [LLVMdev] about llvm2.7's inline assembly

Hello,

Whenever I compile a C program containing "FD_SET" function using
llvm-gcc
2.7 (with -emit-llvm option), I got the following inline assembly in my
llvm byte code:

  %asmtmp = call %struct.__mbstate_t asm sideeffect "cld; rep; stosl",
"={cx},={di},{ax},0,1,~{dirflag},~{fpsr},~{flags},~{memory}"(i32 0,
i32
32, i32* getelementptr inbounds (%struct.__sigset_t* @socks, i32 0, i32
0, i32 0)) nounwind ; <%struct.__mbstat  %asmresult = extractvalue
%struct.__mbstate_t %asmtmp, 0 ; <i32> [#uses=1]  store i32 %asmresult,
i32* %__d0
  %asmresult1 = extractvalue %struct.__mbstate_t %asmtmp, 1 ; <i32>
[#uses=1]
  store i32 %asmresult1, i32* %__d1
  %0 = load i32* @sock, align 4

However, if I use llvm-gcc 2.6, this inline assembly function is not
generated in the byte code.

I was wondering if there is any way to "not generating" this inline
assembly function call in 2.7. Any comments will be appreciated.

Thank you!

Hi Sang Kil Cha,
> Whenever I compile a C program containing "FD_SET" function using
llvm-gcc
> 2.7 (with -emit-llvm option), I got the following inline assembly in my
> llvm byte code:
this comes directly from your system headers (/usr/include/bits/select.h)
and as such doesn't really have anything to do with llvm-gcc:

#if defined __GNUC__ && __GNUC__ >= 2

# if __WORDSIZE == 64
#  define __FD_ZERO_STOS "stosq"
# else
#  define __FD_ZERO_STOS "stosl"
# endif

# define __FD_ZERO(fdsp) \
   do {                                                                        \
     int __d0, __d1;                                                           \
     __asm__ __volatile__ ("cld; rep; " __FD_ZERO_STOS                
\
                           : "=c" (__d0), "=D" (__d1)       
\
                           : "a" (0), "0" (sizeof (fd_set)  
\
                                           / sizeof (__fd_mask)),              \
                             "1" (&__FDS_BITS (fdsp)[0])          
\
                           : "memory");                               
\
   } while (0)

#else   /* ! GNU CC */

Maybe there's a good reason not to just use memset, but I don't know
what it is.

Ciao,

Duncan.

> this comes directly from your system headers (/usr/include/bits/select.h)
> and as such doesn't really have anything to do with llvm-gcc:
iirc there were some fixincludes hacks for FD_ZERO thing. Maybe are
presented presented in one version and does not - in another, thus the
difference...

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University