Re: rfc: constant pool and floats

[Aldy Hernandez <aldyh at redhat dot com>]

Thanks for taking the time on this.

> >>That's the problem.  Clearly, (reg:SF 120) is not equal to the
> >>CONST_INT; it's equal to a SFmode CONST_DOUBLE.
> >
> >I played around kludging the REG_EQUAL to be equal to a CONST_DOUBLE.
> >That didn't fix the problem because when we fill in the constant pool
> >(force_const_mem) with the value, we fill it in with a const_int/SFmode
> >pair, which still causes the abort while dumping the constant pool.
> 
> Where did the CONST_INT come from, if not the REG_EQUAL note?

Ah, see below.

> >  /* If VALUE is a floating-point mode, access it as an integer of the
> >     corresponding size.  This can occur on a machine with 64 bit 
> >registers
> >     that uses SFmode for float.  This can also occur for unaligned 
> >float
> >     structure fields.  */
> >  if (GET_MODE_CLASS (GET_MODE (value)) != MODE_INT
> >      && GET_MODE_CLASS (GET_MODE (value)) != MODE_PARTIAL_INT)
> >    value = gen_lowpart ((GET_MODE (value) == VOIDmode
> >			  ? word_mode : int_mode_for_mode (GET_MODE 
> >			  (value))),
> >			 value);
> 
> This code is not wrong, as such, but I'm a bit confused about what then 
> happens to 'value'.  The routine appears to return it, but there's no 
> documentation about what the routine is *supposed* to return, and every 
> use of it in the compiler appears to ignore the return value.
> 
> Anyway, what you want to look for is the first point where SFmode and 
> CONST_INT get associated with each other.

The code above sets 'value' to the const_int.  Further down we recurse into
store_bit_field:

          /* Fetch that unit, store the bitfield in it, then store
             the unit.  */
          tempreg = copy_to_reg (op0);
=>        store_bit_field (tempreg, bitsize, bitpos, fieldmode, value);

Where:
	tempreg = (reg:SI 119)
	fieldmode = SFmode
	value = (const_int 1202908642)

Store_bit_field eventually calls emit_move_insn with:
	dst = (subreg:SF (reg:SI 119) 0)
	src = (const_int xx)

*This* is the first time we associate an SF with a const_int.

I'm a bit confused here, so I'm not sure if this is related, but 
emit_move_insn calls rs6000_emit_move with the above 'dst' and 'src',
which calls force_reg(SFmode, (const_int xx)), which is where we get
the REG_EQUAL note:

(insn 11 10 0 (set (reg:SF 120)
        (mem/u/i:SF (lo_sum:SI (reg:SI 121)
                (symbol_ref/u:SI ("*.LC0") [flags 0x2])) [0 S4 A32])) -1 (nil)
    (expr_list:REG_EQUAL (const_int 1202908642 [0x47b2ede2])
        (nil)))

It looks like store_bit_field is at fault by calling the emit_move_insn
with an incompatible source/dest.  Is (set (reg:SF) (const_int)) valid
rtl?  David had mentioned it was, but now I'm not sure.

Aldy