[Bug c++/43529] New: G++ doesn't optimize away empty loop when index is a double

[Bug c++/43529] New: G++ doesn't optimize away empty loop when index is a double

[evouga at gmail dot com]

Empty loops where the loop index is an integer are optimized away, but the
following loop is not (presumably because it is more difficult to prove that it
terminates in finite time?)

int main()
{
        for(double i=0; i<1e9; i+=1);
}

Command line: g++ -O3


-- 
           Summary: G++ doesn't optimize away empty loop when index is a
                    double
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: evouga at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

[Bug tree-optimization/43529] G++ doesn't optimize away empty loop when index is a double

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2010-03-26 11:45 -------
Confirmed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|c++                         |tree-optimization
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-03-26 11:45:52
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

[Bug tree-optimization/43529] G++ doesn't optimize away empty loop when index is a double

[changpeng dot fang at amd dot com]

------- Comment #2 from changpeng dot fang at amd dot com  2010-06-04 23:15 -------
Interesting! What's the difference between 17 and 18?
------------
int main()
{
  double i;
  for(i=0; i<18; i+=1); /* gcc -O3, empty loop not removed */
}
----------------
int main()
{
  double i;
  for(i=0; i<17; i+=1); /* gcc -O3, empty loop removed */
}


-- 

changpeng dot fang at amd dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |changpeng dot fang at amd
                   |                            |dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

[Bug tree-optimization/43529] G++ doesn't optimize away empty loop when index is a double

[changpeng dot fang at amd dot com]

------- Comment #3 from changpeng dot fang at amd dot com  2010-06-04 23:29 -------
(In reply to comment #2)
> Interesting! What's the difference between 17 and 18?
> ------------
> int main()
> {
>   double i;
>   for(i=0; i<18; i+=1); /* gcc -O3, empty loop not removed */
> }


The funny thing occurs in gcc 4, not gcc 6:

        .file   "empty.c"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        xorl    %eax, %eax
        .p2align 4,,10
        .p2align 3
.L2:
        addl    $1, %eax
        cmpl    $18, %eax
        jne     .L2
        rep
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (Ubuntu 4.4.1-4ubuntu9) 4.4.1"
        .section        .note.GNU-stack,"",@progbits


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

[Bug tree-optimization/43529] G++ doesn't optimize away empty loop when index is a double

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2010-06-05 00:05 -------
>Interesting! What's the difference between 17 and 18?

Exact representation.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

[Bug tree-optimization/43529] G++ doesn't optimize away empty loop when index is a double

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2010-06-05 10:40 -------
(In reply to comment #4)
> >Interesting! What's the difference between 17 and 18?
> 
> Exact representation.

Complete unrolling.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

[Bug tree-optimization/43529] G++ doesn't optimize away empty loop when index is a double

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43529

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
   Target Milestone|---                         |10.0
             Status|NEW                         |RESOLVED

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
the 19 case was fixed in GCC 4.5.0 uptill somewhere between 200 and 2000.
2000 and 1e9 is fixed in GCC 10 for C++ by ... and r10-7522-g75efe9cb1f8938 .