[Bug c/37488] New: register allocation spills floats needlessly

[jstrother9109 at gmail dot com]

In the provided testcase, gcc spills an xmm register onto the stack even though
there is only one register being used.  This does not occur with similar code
using general purpose registers.

<BEGIN TESTCASE>

const void* test(int action, void* ptr)
{
   static void * const addrs[] = {&&L1, &&L2};

   if (action == 0) {
      return addrs;
   } else {
      char* ip = ptr;

      register double reg_f_a;
      double reg_f[1];

      reg_f_a = 0.0;
      reg_f[0] = 0.0;

      goto *ip;

L1: {
      int t1 = *(int*)(++ip);
      reg_f_a = reg_f_a + reg_f[t1];
      goto *(++ip);
   }

L2:
      *(double*)ptr = reg_f_a;
   }

   return 0;
}

<END TESTCASE>

The above code compiled with -O3 -march=i686 -msse2 -mfpmath=sse produces the
following bit of assembly

<BEGIN OUTPUT>
        movl    1(%ebx), %eax
        addl    $2, %ebx
        movsd   -32(%ebp), %xmm0
        addsd   -16(%ebp,%eax,8), %xmm0
        movl    %ebx, %eax
        movsd   %xmm0, -32(%ebp)
        jmp     *%eax
<END OUTPUT>

The xmm0 register should remain the home register for reg_f_a, so there should
be no need for the store/load.  Other usages of xmm0 should be placed in xmm1. 
So the output should read:

<BEGIN MODIFIED 1>
        movl    1(%ebx), %eax
        addl    $2, %ebx
        addsd   -16(%ebp,%eax,8), %xmm0
        movl    %ebx, %eax
        jmp     *%eax
<END MODIFIED 1>

As a possibly related issue, there is also no reason why a copy of %ebx is made
prior to performing the jump.  This could just as easily be

<BEGIN MODIFIED 2>
        movl    1(%ebx), %eax
        addl    $2, %ebx
        addsd   -16(%ebp,%eax,8), %xmm0
        jmp     *%ebx
<END MODIFIED 2>

So, as you can see, three out of the seven instructions can be removed, as well
as two of four memory references.

The version of gcc is 4.2.3 (Ubuntu 4.2.3-2ubuntu7)


-- 
           Summary: register allocation spills floats needlessly
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jstrother9109 at gmail dot com
 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=37488