[new-regalloc] Problem on x86 with initialization of ra_reg_info

[Geert Bosch]

Hi Daniel,

The new register allocator crashes badly on x86 at line 703 of 
new-regalloc.c, trying to access the degree of ra_reg_info[6] which
is 0:
      /* If it's of significant degree, place it on the spill_worklist.  */
      if (ra_reg_info[reg_num]->degree >= reg_freedom (reg_num))
        {
          if (rtl_dump_file != NULL && debug_new_regalloc > 1)
            fprintf (rtl_dump_file, "Inserting %d onto spill_worklist\n", reg_nu
m);
          SET_REGNO_REG_SET (spill_worklist, reg_num);
        }

This seems to be a result of ra_reg_info not being initialized properly.
Only registers 0, 3 and 7 were initialized (%eax, %edx and %esp).
This seems to be because the test for is_reg_candidate returns false
for all other registers.

/* Determine if regno is a candidate for this pass of register
   allocation.  */
static int
is_reg_candidate (regno)
     unsigned int regno;
{
  return DF_REGNO_FIRST_DEF (df_analyzer, regno) != 0;
}

There is not really a comment that tells what the condition is
for a register to be a candidate for register allocation, and
I can't figure out what DF_REGNO_FIRST_DEF means as this macro
is undocumented. One of the problems with this extensive use of
macros is that it is not clear what types are returned by various
expressions.

I would guess that it would somehow return the instruction where
the given register is first defined in the dataflow information
contained in df_analyzer. Then however it doesn't make sense to
only consider a register to be a candidate for allocation if it
actually is defined somewhere.

I'm trying to understand this code as I'd like to help getting it
in good shape, so please bear with my comments, I hope they'll
improve the code in the end.

  -Geert