[Bug rtl-optimization/23361] New: Can't eliminate empty loops with power of two step and variable bounds

[Bug rtl-optimization/23361] New: Can't eliminate empty loops with power of two step and variable bounds

[chris at bubblescope dot net]

This may be related to bug 19001 (loops with power of two step and variable bounds not unrolled)

Neither of the following empty loops is eliminated:

void foo(int a, int b)
{ for(;a!=b;a+=4); }

void foo(int a, int b)
{ for(;a<b;a+=4); }

Similarily to 19001, these loops come about because of iterating over an range of pointers doing 
something for each element (for example deleting them) which for some types turns out to be a null 
operation. Fixing this will allow a large number of workarounds in libstdc++ to be removed.

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           Summary: Can't eliminate empty loops with power of two step and
                    variable bounds
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: chris at bubblescope dot net
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-12 18:57 -------
With -funsafe-loop-optimizations we remove them.

Using -Wunsafe-loop-optimizations, we get:
t.c:2: warning: cannot optimize loop, the loop counter may overflow


I mentioned this before to transform the loops such that we know if they overflow or not and Zdenek 
said it was not useful which I feel is still wrong.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |tree-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[chris at bubblescope dot net]

------- Additional Comments From chris at bubblescope dot net  2005-08-12 18:58 -------
*** Bug 23362 has been marked as a duplicate of this bug. ***

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-12 19:10 -------
See http://gcc.gnu.org/ml/gcc-patches/2005-06/msg02275.html which is the email I was talking 
about.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-08-12 19:10:31
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[dberlin at dberlin dot org]

------- Additional Comments From dberlin at gcc dot gnu dot org  2005-08-12 23:43 -------
Subject: Re:  Can't eliminate empty loops with
	power of two step and variable bounds

On Fri, 2005-08-12 at 19:10 +0000, pinskia at gcc dot gnu dot org wrote:
> ------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-12 19:10 -------

Personally, i think -funsafe-loop-optimizations should be on by default
in -O3, with a warning for when we rely on it.

It's *incredibly* rare that a user actually intends for a loop counter
to be able to overflow.




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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

Re: [Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[Daniel Berlin]

On Fri, 2005-08-12 at 19:10 +0000, pinskia at gcc dot gnu dot org wrote:
> ------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-12 19:10 -------

Personally, i think -funsafe-loop-optimizations should be on by default
in -O3, with a warning for when we rely on it.

It's *incredibly* rare that a user actually intends for a loop counter
to be able to overflow.

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2005-08-13 10:01 -------
One thing is that if 'a' and 'b' are originally pointers of the same type, it 
should be clear the the loop can be removed. When they are lowered to integers, 
instead, we lose the precious alignment information. Can't the empty loop be 
removed before the pointers are lowered?
Or am I missing something?

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[bonzini at gcc dot gnu dot org]

------- Additional Comments From bonzini at gcc dot gnu dot org  2005-09-15 13:46 -------
if these ints are signed, you should be able to remove these loops.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361

[Bug tree-optimization/23361] Can't eliminate empty loops with power of two step and variable bounds

[chris at bubblescope dot net]

------- Additional Comments From chris at bubblescope dot net  2005-09-15 14:02 -------
Expanding slightly, I tried the following 4 functions. All were removed by -funsafe-loop-optimisations, 
but only foo3 was removed by -O3 without -funsafe-loop-optimisations. I can't see a good reason to 
remove foo3 but not remove foo4?

void foo(int a, int b)
{ for(;a!=b;a+=4); }


void foo2(int a, int b)
{ for(;a<b;a+=4); }

void foo3(int*a, int* b)
{ for(;a<b;a++); }

void foo4(int*a, int*b)
{ for(;a!=b;a++); }

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23361