[Bug middle-end/14060] New: An unused register is saved to the stack.

[Bug middle-end/14060] New: An unused register is saved to the stack.

[kazu at cs dot umass dot edu]

/* Use ./cc1 -mh -O2 -fomit-frame-pointer.  */

extern long bar (long, long *);

void
foo (long z)
{
  long a[2];

  a[0] = z;

  while (1)
    {
      if (bar (z, a))
	return;
    }
}

Note that er5 is saved but not used anywhere in the function.

_foo:
	mov.l	er4,@-er7
	mov.l	er5,@-er7
	subs	#4,er7
	subs	#4,er7
	mov.l	er0,er4
	mov.l	er0,@er7
.L2:
	mov.l	er7,er1
	mov.l	er4,er0
	jsr	@_bar
	mov.l	er0,er0
	beq	.L2
	adds	#4,er7
	adds	#4,er7
	mov.l	@er7+,er5
	mov.l	@er7+,er4
	rts

-- 
           Summary: An unused register is saved to the stack.
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: normal
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: h8300-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14060

[Bug middle-end/14060] An unused register is saved to the stack.

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-02-07 06:38 -------
My guess is as follows:

Before the register allocation, we have this.

(insn 46 10 12 0 (set (reg/f:SI 20)
        (plus:SI (reg/f:SI 11 fp)
            (const_int -8 [0xfffffff8]))) 36 {*addsi_h8300hs} (nil)
    (nil))

There is no matching constraint for this insn
(because H8 is a two-address machine).
So the source is spilled.
But it turns out that (plus (reg:SI 11) (const_int -8)) is equal to (reg:SI sp).
So an addition instruction becomes a move instruction:

(set (reg:SI 20) (reg:SI sp))

But the reload or the register allocator doesn't go back and say that
the spill made above is not necessary anymore.

Before this patch

http://gcc.gnu.org/ml/gcc-patches/2004-01/msg03252.html

I used to see a use of er5 (although a stupid one).

eliminate_regs_in_insn() is called too late to notice that
the spill is not needed.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14060

[Bug middle-end/14060] An unused register is saved to the stack.

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14060

[Bug middle-end/14060] An unused register is saved to the stack.

[kazu at cs dot umass dot edu]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-02-09 17:07:37
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14060

[Bug middle-end/14060] An unused register is saved to the stack.

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2005-02-08 10:19 -------
Kazu, is this still a problem?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14060