[Bug inline-asm/15228] New: useless copies of floating point operands

["sylvain dot pion at sophia dot inria dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

Consider the following function :

// Forces a double to memory
double force2memdouble(double x) {
  asm("" : "=m"(x) : "m"(x));
  return x;
}

This is used to force a double value in memory for platforms like x86
where the FP registers have too much precision.

It gets compiled by GCC (3.4 and 3.3.3, with -O2 -fomit-frame-pointer) to :

force2memdouble:
        subl    $12, %esp
        fldl    16(%esp)
        fstpl   (%esp)
        fldl    (%esp)
        addl    $12, %esp
        ret

I believe that the store/load is useless, and that the function could be
compiled as simply :

force2memdouble:
        fldl    4(%esp)
        ret


It is interesting to note that when using "int", "float" or "long double"
instead of "double", then there is no such useless store/load.


So, is this something that could be improved in GCC ?

Maybe the code responsible for asm() adds too many requirements
on the operands for doubles ?

-- 
           Summary: useless copies of floating point operands
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sylvain dot pion at sophia dot inria dot fr
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15228