Re: single precision complex bug in g77 - was Testing g77 with LAPACK 3.0

[Jim Wilson <wilson@cygnus.com>]

Using current egcs development sources, it works if I compile with -O, and
fails without.

Without optimization, it is a calling convention problem.  We try to pass
a single precision complex to the function cabs1.0, which requires loading
two SFmode values.  We generate RTL to load them both into the same register
and this code obviously doesn't work.

(insn 26 24 28 (set (reg:SF 4 a0)
        (mem/f:SF (reg:SI 77) 0)) -1 (nil)
    (nil))

(insn 28 26 30 (set (reg:SF 4 a0)
        (mem/f:SF (plus:SI (reg:SI 77)
                (const_int 4 [0x4])) 0)) -1 (nil)
    (nil))

This has always been a problem with the complex support.  There was some
discussion last year about trying to fix this in the alpha port, but I don't
know what came of that.

The bad code comes from emit_move_insn_1 in the CONCAT case.  It does
=>        emit_insn (GEN_FCN (mov_optab->handlers[(int) submode].insn_code)
                     (gen_realpart (submode, x), gen_realpart (submode, y)));
          emit_insn (GEN_FCN (mov_optab->handlers[(int) submode].insn_code)
                     (gen_imagpart (submode, x), gen_imagpart (submode, y)));
which emits two loads into the same register when x is a register big
enough to hold the entire complex value.  This code can work for registers
only if the register is equal to or smaller than one half the size of the
complex type.

The single register was chosen by the mips FUNCTION_ARG macro.

The problem can probably be fixed in either area.  It isn't clear which is
best.  We need to make sure that we emit correct ABI compliant code for this
case which may constrain the possible solutions.

Jim