Wrong RTL instruction deleted

[Uros Bizjak]

Hello!

The testcase below produces different results when compiled with 
different optimization levels:

--cut here--
#include <stdio.h>

float foo ()
{
  unsigned int MASK = 0x80000000;
  return (float &) MASK;
}

int main() {
  printf ("%f\n", foo());
  return 0;
}
--cut here--

g++ -O1 yyy.cpp produces correct value:
./a.out
-0.000000
g++ -O1 yyy.cpp
./a.out
0.000000
g++ -O3 yyy.cpp
./a.out
36.658997

The asm code of foo() function is correct at -O1:
_Z3foov:
.LFB13:
        pushl   %ebp    #
.LCFI0:
        movl    %esp, %ebp      #,
.LCFI1:
        subl    $16, %esp       #,
.LCFI2:
        movl    $-2147483648, -4(%ebp)  #, MASK
        flds    -4(%ebp)        #
        leave
        ret

But for -O2, the move of constant to stack is missing:
_Z3foov:
.LFB13:
        pushl   %ebp    #
.LCFI0:
        movl    %esp, %ebp      #,
.LCFI1:
        subl    $16, %esp       #,
.LCFI2:
        flds    -4(%ebp)        #
        leave
        ret

Following is the RTL dump of .15.cse2, when foo() is compiled with -O2:
(note 6 2 9 NOTE_INSN_FUNCTION_BEG)

;; Start of basic block 0, registers live: (nil)
(note 9 6 11 0 [bb 0] NOTE_INSN_BASIC_BLOCK)

(insn 11 9 14 0 (set (mem/i:SI (plus:SI (reg/f:SI 20 frame)
                (const_int -4 [0xfffffffc])) [2 MASK+0 S4 A32])
        (const_int -2147483648 [0x80000000])) 35 {*movsi_1} (nil)
    (nil))

(insn 14 11 18 0 (set (reg:SF 61)
        (mem:SF (plus:SI (reg/f:SI 20 frame)
                (const_int -4 [0xfffffffc])) [3 S4 A32])) 60 {*movsf_1} 
(nil)
    (nil))

(note 18 14 21 0 NOTE_INSN_FUNCTION_END)


And in .16.life, (insn 11) is simply missing (there is no 
NOTE_INSN_DELETED):
(note 6 2 9 NOTE_INSN_FUNCTION_BEG)

;; Start of basic block 0, registers live: 6 [bp] 7 [sp] 16 [argp] 20 
[frame]
(note 9 6 14 0 [bb 0] NOTE_INSN_BASIC_BLOCK)

(insn 14 9 18 0 (set (reg:SF 61)
        (mem:SF (plus:SI (reg/f:SI 20 frame)
                (const_int -4 [0xfffffffc])) [3 S4 A32])) 60 {*movsf_1} 
(nil)
    (nil))

(note 18 14 21 0 NOTE_INSN_FUNCTION_END)

In -O2, -O3, there is no initialization of temporary value on stack, so 
the result is undefined.
This behaviour was found when PR16111 was analyzed, but can be also 
related to PR13366.

Uros.