[Bug c/19703] New: Poor optimisation of loop test

[Bug c/19703] New: Poor optimisation of loop test

[andrewhutchinson at cox dot net]

Missed optimization

gcc version 4.0.0 20041205 (experimental)
4.0.0/cc1.exe -quiet -v looprv.c -quiet -dumpbase
looprv.c -mmcu=atmega169 -auxbase looprv -Os -Wall -version -funsigned-char
-funsigned-bitfields -fpack-struct -fshort-enums -o looprv.s

Down counting loop, uses expensive compare EQ (-n) instead of compare >=0.
Testcase as follows:

volatile char  value6;
extern void foo6(char);
void testloop6(void)
{
int i;
for (i=100;i>= 0;i-=10)
{
   if (!value6)
   {
       foo6(i);
   } 
}

Loop test in RTL is compare NE -10. It should be compare GE 0 - which
is(generally) free. First dump of Expanded RTL show the compare.

-- 
           Summary: Poor optimisation of loop test
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: andrewhutchinson at cox dot net
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] Poor optimisation of loop test

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-30 03:17 -------
(In reply to comment #1)
> Hmm, on most targets it is true that != is the same case as >=.
s/case/cost/.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] Poor optimisation of loop test

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-30 03:17 -------
Hmm, on most targets it is true that != is the same case as >=.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |tree-optimization
 GCC target triplet|                            |avr
           Keywords|                            |missed-optimization
            Summary|Poor optimisation of loop   |Poor optimisation of loop
                   |test                        |test


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] Poor optimisation of loop test

[andrewhutchinson at cox dot net]

------- Additional Comments From andrewhutchinson at cox dot net  2005-01-30 04:58 -------
Subject: Re:  Poor optimisation of loop test

I am not sure what makes you think that. Compare with ZERO is 
invariabley cheaper than compare with "n".
The former is "free" sign status following any conditioning setting 
instruction - like subtract!
Its even the sign bit of the result!

subi r28,10
cpi   r28, -10
brpl  looptop

subi r28,10
brpl looptop

or did I miss something?

pinskia at gcc dot gnu dot org wrote:

>------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-30 03:17 -------
>Hmm, on most targets it is true that != is the same case as >=.
>
>  
>





-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-30 05:25 -------
Confirmed (sorry about that I was thinking of something different at the time).
Here is a better example for normal targets (PPC and x86 too):
extern void foo6(int);
void testloop6 (void)
{
  int i;
  i = 100;
  do {
      foo6 ( i);
    i-=10;
  } while (i != 0);
}

The tree dump for the loop on x86:
<L0>:;
  foo6 (i);
  D.1592 = (unsigned int) i + 0fffffff6;
  i = (int) D.1592;
  if (D.1592 != 0) goto <L0>; else goto <L1>;

Note this is not IV-OPTS messing up but DOM changing the if statement.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
 GCC target triplet|avr                         |
   Last reconfirmed|0000-00-00 00:00:00         |2005-01-30 05:25:46
               date|                            |
            Summary|Poor optimisation of loop   |[4.0 Regression] Poor
                   |test                        |optimisation of loop test,
                   |                            |DOM causing unsigned to int
                   |                            |and missing combine compares
   Target Milestone|---                         |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-30 19:20 -------
Even though DOM is the problem, IV-OPTS can be improved so we don't create the casts in the first 
place, patch here to do that: <http://gcc.gnu.org/ml/gcc-patches/2005-01/msg02209.html>.

I will try get a testcase where DOM messes up even without IV-OPTs doing anything.
And here is that testcase, yes it is werid for someone to do this but ...:
extern void foo6(int);
void testloop6 (void)
{
  unsigned int D1133;
  int i;
  i = 100;
  do {
    foo6 (i);
    i = ((unsigned int) i) + 4294967286U;
  }while (i != 0);
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-02-10 21:04 -------
Lowering the priority as IV-OPTs has worked around DOM's problems.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P2                          |P3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0/4.1 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[mmitchel at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.0                       |4.0.1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0/4.1 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu dot org
   Last reconfirmed|2005-01-30 05:25:46         |2005-06-26 18:03:23
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0/4.1 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[mmitchel at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.1                       |4.0.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-17 12:37 -------
This has been fixed on the mainline now.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |4.0.0
      Known to work|                            |3.4.0 4.1.0
            Summary|[4.0/4.1 Regression] Poor   |[4.0 Regression] Poor
                   |optimisation of loop test,  |optimisation of loop test,
                   |DOM causing unsigned to int |DOM causing unsigned to int
                   |and missing combine compares|and missing combine compares


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[mmitchel at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.0.2                       |4.0.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703

[Bug tree-optimization/19703] [4.0 Regression] Poor optimisation of loop test, DOM causing unsigned to int and missing combine compares

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2005-11-04 15:36 -------
Since this is only a missed optimization, closing as fixed for 4.1.0.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|4.0.3                       |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19703