Converting inline ASM to intrinsic for x64 igraph

Question

I'm compiling from source the python extension IGRAPH for x64 instead of x86 which is available in the distro. I have gotten it all sorted out in VS 2012 and it compiles when I comment out as follows in src/math.c

#ifndef HAVE_LOGBL
long double igraph_logbl(long double x) {
long double res;
/**#if defined(_MSC_VER)
  __asm { fld [x] }
  __asm { fxtract }
  __asm { fstp st }
  __asm { fistp [res] }
 #else
 __asm__ ("fxtract\n\t"
 "fstp  %%st" : "=t" (res) : "0" (x));
 #endif*/
    return res;
 }
#endif

The problem is I don't know asm well and I don't know it well enough to know if there are issues going from x86 to x64. This is a short snippet of 4 assembly intsructions that have to be converted to x64 intrinsics, from what I can see.

Any pointers? Is going intrinsic the right way? Or should it be subroutine or pure C?

Edit: Link for igraph extension if anyone wanted to see http://igraph.sourceforge.net/download.html

There is no intrinsic for fxtract, so that plan fails immediately. If you're willing to switch you non-long doubles, you can extract the exponent easily with SSE2 intrinsics (that'll be much quicker than fxtract, by the way). — harold

Goz Goz · Accepted Answer · 2012-10-30T20:57:43

In x64 floating point will generally be performed using the SSE2 instructions as these are generally a lot faster. Your only problem here is that there is no equivalent to the fxtract op in SSE (Which generally means the FPU version will be implemented as a compound instruction and hence very slow). So implementing as a C function will likely be just as fast on x64.

I'm finding the function a bit hard to read however as from what I can tell it is calling fxtract and then storing an integer value to the address pointed to by a long double. This means the long double is going to have a 'partially' undefined value in it. As best I can tell the above code assembly shouldn't work ... but its been a VERY long time since I wrote any x87 code so I'm probably just rusty.

Anyway the function, appears to be an implementation of logb which you won't find implemented in MSVC. It can, however, be implemented as follows using the frexp function:

long double igraph_logbl(long double x) 
{
    int exp = 0;
    frexpl( x, &exp );
    return (long double)exp;
 }

Converting inline ASM to intrinsic for x64 igraph

1 Answers