Re: Fix handling of call clobbering readonly-result

[Olivier Hainque <hainque at act-europe dot fr>]

Richard Earnshaw wrote:
> The fundamental failure is coming from an incorrect cse'ing of the calls 
> to test3.

 Thanks for the feedback. Your analysis about cse doing something wrong 
 appears confirmed in the RTL dumps for my reduced case with
 positive/negative functions (see below).

> Unfortunately, I don't know enough about CSE to begin to fix this :-(

 Neither do I. I gave it a try and will keep searching, but Richard Kenner's
 help would be welcome at this point.

 Richard (Kenner), could you please give a hand here ? A description of the
 problem follows. Thanks in advance.


 For this testcase:

     int negative (int x)
     {
       return (x < 0);
     }

     int positive (int x)
     {
       return (x > 0);
     }

     int main()
     {
       int x;

       x = 80;
       if (negative (x))
	 abort ();
       if (! positive (x))
	 abort ();

       x = 100;
       if (negative (x))
	 abort ();
       if (! positive (x))
	 abort ();

       return 0;
     }

 An arm-elf cross compiler from the current sources generates wrong O2 code,
 missing the second call to "positive".

 The .cse2 dump excerpts below show the anomaly: the first call's return value
 (pseudo 53) is reused as the result of the second call.
 
 (call_insn/u 36 35 37 2 0x401d4738 (parallel [
            (set (reg:SI 0 r0)
                (call (mem:SI (symbol_ref:SI ("^positive")...
                    (const_int 0 [0x0])))
            (use (const_int 0 [0x0]))
            (clobber (reg:SI 14 lr))
        ]) 190 {*call_value_symbol} (nil)
    (expr_list:REG_EH_REGION (const_int -1 [0xffffffff])
        (nil))
    (expr_list (use (reg:SI 0 r0 [ x ]))
        (nil)))

 (insn 37 36 40 2 0x401d4738 (set (reg:SI 53)                      <===
        (reg:SI 0 r0)) 124 {*arm_movsi_insn} (nil)                 <===
    (insn_list:REG_RETVAL 35
      (expr_list:REG_EQUAL (expr_list (symbol_ref:SI ("^positive"))
                (expr_list (reg/v:SI 49 [ x ])
                    (nil)))
            (nil))))
 [...]
 (call_insn/u 74 73 75 6 0x401d4738 (parallel [
            (set (reg:SI 0 r0)
                (call (mem:SI (symbol_ref:SI ("^positive")...
                    (const_int 0 [0x0])))
            (use (const_int 0 [0x0]))
            (clobber (reg:SI 14 lr))
        ]) 190 {*call_value_symbol} (nil)
    (expr_list:REG_EH_REGION (const_int -1 [0xffffffff])
        (nil))
    (expr_list (use (reg:SI 0 r0 [ x ]))
        (nil)))

 (insn 75 74 78 6 0x401d4738 (set (reg:SI 57)
        (reg:SI 53)) 124 {*arm_movsi_insn} (nil)                   <===
    (insn_list:REG_RETVAL 73
      (expr_list:REG_EQUAL (expr_list (symbol_ref:SI ("^positive"))
                (expr_list (reg/v:SI 49 [ x ])
                    (nil)))
            (nil))))

 (insn 78 75 91 6 0x401d4738 (set (reg:CC 24 cc)                   
        (compare:CC (reg:SI 53)                                    <===
            (const_int 0 [0x0]))) 162 {*arm_cmpsi_insn} (nil)
    (nil))