[Bug other/22067] New: Inconsistent multiply by immediate

[Bug other/22067] New: Inconsistent multiply by immediate

[pthaugen at us dot ibm dot com]

PR17103 states that mulli is better than decomposing into shift/add sequence.
Following is example where we are being inconsistent about that decision.

Compiled with gcc -O2 -mcpu=power4 -m32


struct S {
  int i1,i2,i3,i4,i5,i6;
}s[10];

int y;

int test1(int j, int x)
{

 y = y * 24;	// shift/sub
 s[j].i1 = 1;	// mulli

 return (x * 24);	// mulli
}

-- 
           Summary: Inconsistent multiply by immediate
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pthaugen at us dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067

[Bug middle-end/22067] Inconsistent multiply by immediate

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-06-14 21:23 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|other                       |middle-end
     Ever Confirmed|                            |1
  GCC build triplet|powerpc64-linux             |
   GCC host triplet|powerpc64-linux             |
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2005-06-14 21:23:27
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067

[Bug middle-end/22067] Inconsistent multiply by immediate

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2005-09-17 03:49 -------
The mult_cost argument to choose_mult_variant() and the cost of the best
shift/add sequence are not always representing the same operation, so the
comparison is not always correct.  The synth_mult cost is just the cost of the
multiplication by a constant, but the mult_cost argument includes the full
rtx_cost of the insn, including address cost.  For example

(mult:SI (mem/c/i:SI (reg/f:SI 123) [5 y+0 S4 A32])
    (const_int 24 [0x18]))

rtx_cost will include the cost of the MEM.  This artificially inflates the cost
of the MULT, making the shift/add sequence seem more efficient.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067

[Bug middle-end/22067] Inconsistent multiply by immediate

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2005-09-17 04:28 -------
I think the patch in attachment 9747 that subtracts the cost of operand0 is the
correct solution.  One generally does want the cost of the operands included, so
rtx_costs is correct.  choose_mult_variant would consider too long a sequence if
it *added* the cost of operand0, so the correct solution appears to be
subtracting the cost.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dje at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067

[Bug middle-end/22067] Inconsistent multiply by immediate

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-09-17 20:18 -------
Subject: Bug 22067

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	dje@gcc.gnu.org	2005-09-17 20:17:55

Modified files:
	gcc            : ChangeLog expmed.c 

Log message:
	PR middle-end/22067
	* expmed.c (expand_mult): Substitute simple register for op0 when
	computing max_cost.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.9968&r2=2.9969
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/expmed.c.diff?cvsroot=gcc&r1=1.232&r2=1.233



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067

[Bug middle-end/22067] Inconsistent multiply by immediate

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2005-09-17 20:20 -------
Patch committed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067

[Bug middle-end/22067] Inconsistent multiply by immediate

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22067