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* [Bug middle-end/43182]  New: gcc could not vectorize this simple loop (un-handled data-ref)
@ 2010-02-25 23:37 changpeng dot fang at amd dot com
  2010-02-25 23:45 ` [Bug middle-end/43182] " pinskia at gcc dot gnu dot org
                   ` (5 more replies)
  0 siblings, 6 replies; 8+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-02-25 23:37 UTC (permalink / raw)
  To: gcc-bugs

gcc 4.5 can not vectorize this simple loop:

void foo(int a[], int n) {
 int i;
 for(i=1; i< n; i++)
  a[i] = a[0];
}

"gcc -O3 -fdump-tree-vect-all -c foo.c" shows:
foo.c:3: note: not vectorized: unhandled data-ref 
foo.c:3: note: bad data references.
foo.c:1: note: vectorized 0 loops in function.

It seems gcc gets confused at a[0] and gives up vectorization. There
is no dependence in this loop, and we should teach gcc to handle a[0]
to vectorize it.


-- 
           Summary: gcc could not vectorize this simple loop (un-handled
                    data-ref)
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: changpeng dot fang at amd dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] gcc could not vectorize this simple loop (un-handled data-ref)
  2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
@ 2010-02-25 23:45 ` pinskia at gcc dot gnu dot org
  2010-02-25 23:50 ` [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n] pinskia at gcc dot gnu dot org
                   ` (4 subsequent siblings)
  5 siblings, 0 replies; 8+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-02-25 23:45 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #1 from pinskia at gcc dot gnu dot org  2010-02-25 23:45 -------
Actually a[0] should be load hoisted from the loop as it not changed from
inside the loop at all.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |alias, missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]
  2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
  2010-02-25 23:45 ` [Bug middle-end/43182] " pinskia at gcc dot gnu dot org
@ 2010-02-25 23:50 ` pinskia at gcc dot gnu dot org
  2010-02-25 23:54 ` pinskia at gcc dot gnu dot org
                   ` (3 subsequent siblings)
  5 siblings, 0 replies; 8+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-02-25 23:50 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #2 from pinskia at gcc dot gnu dot org  2010-02-25 23:50 -------
So currently inside LIM (which does load motion in general):
  D.2724_7 = a_6(D) + D.2723_5;
  D.2725_8 = *a_6(D);
  *D.2724_7 = D.2725_8;

But LIM/alias oracle does not know that D.2723_5 has a range of [4, n_3*4]
which means D.2724_7 can never equal a_6 so we don't pull out the load from
a_6.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-02-25 23:50:10
               date|                            |
            Summary|gcc could not vectorize this|GCC does not pull out a[0]
                   |simple loop (un-handled     |from loop that changes a[i]
                   |data-ref)                   |for i:[1,n]


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]
  2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
  2010-02-25 23:45 ` [Bug middle-end/43182] " pinskia at gcc dot gnu dot org
  2010-02-25 23:50 ` [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n] pinskia at gcc dot gnu dot org
@ 2010-02-25 23:54 ` pinskia at gcc dot gnu dot org
  2010-02-26 18:53 ` changpeng dot fang at amd dot com
                   ` (2 subsequent siblings)
  5 siblings, 0 replies; 8+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-02-25 23:54 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #3 from pinskia at gcc dot gnu dot org  2010-02-25 23:54 -------
Related to PR 29751 but that only does a simple method and does not handle this
case as we need range info.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]
  2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
                   ` (2 preceding siblings ...)
  2010-02-25 23:54 ` pinskia at gcc dot gnu dot org
@ 2010-02-26 18:53 ` changpeng dot fang at amd dot com
  2010-02-26 18:55 ` pinskia at gcc dot gnu dot org
  2010-02-26 19:06 ` changpeng dot fang at amd dot com
  5 siblings, 0 replies; 8+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-02-26 18:53 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #4 from changpeng dot fang at amd dot com  2010-02-26 18:53 -------
Here is another similar case but more general. We know that a(j) and a(i)
never access the same memory location. intel ifort can vectorize this
triangular
loop:

      do 10 j = 1,n
         do 20 i = j+1, n
            a(i) = a(i) - aa(i,j) * a(j)
  20     continue
  10  continue


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]
  2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
                   ` (3 preceding siblings ...)
  2010-02-26 18:53 ` changpeng dot fang at amd dot com
@ 2010-02-26 18:55 ` pinskia at gcc dot gnu dot org
  2010-02-26 19:06 ` changpeng dot fang at amd dot com
  5 siblings, 0 replies; 8+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-02-26 18:55 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #5 from pinskia at gcc dot gnu dot org  2010-02-26 18:55 -------
(In reply to comment #4)
> Here is another similar case but more general.

Actually it is a totally different case.  Please file a new bug with that case;
though there might already be a bug about that one.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]
  2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
                   ` (4 preceding siblings ...)
  2010-02-26 18:55 ` pinskia at gcc dot gnu dot org
@ 2010-02-26 19:06 ` changpeng dot fang at amd dot com
  5 siblings, 0 replies; 8+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-02-26 19:06 UTC (permalink / raw)
  To: gcc-bugs



------- Comment #6 from changpeng dot fang at amd dot com  2010-02-26 19:06 -------

> 
> Actually it is a totally different case.  Please file a new bug with that case;
> though there might already be a bug about that one.
> 

I could not see the difference even though j is not a compile-time constant.
(it
is an invariant to the innermost loop). I can say:

GCC does not pull out a[j] from loop that changes a[i] for i:[j+1,n]


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182


^ permalink raw reply	[flat|nested] 8+ messages in thread

* [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]
       [not found] <bug-43182-4@http.gcc.gnu.org/bugzilla/>
@ 2021-07-26  7:45 ` pinskia at gcc dot gnu.org
  0 siblings, 0 replies; 8+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-07-26  7:45 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43182

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

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

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So even though we can vectorize this loop these days, the non-vectorized loop
still has the load each iteration.
at -O2:
.L3:
        movl    (%ecx), %edx
        addl    $4, %eax
        movl    %edx, -4(%eax)
        cmpl    %ebx, %eax
        jne     .L3

^ permalink raw reply	[flat|nested] 8+ messages in thread

end of thread, other threads:[~2021-07-26  7:45 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2010-02-25 23:37 [Bug middle-end/43182] New: gcc could not vectorize this simple loop (un-handled data-ref) changpeng dot fang at amd dot com
2010-02-25 23:45 ` [Bug middle-end/43182] " pinskia at gcc dot gnu dot org
2010-02-25 23:50 ` [Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n] pinskia at gcc dot gnu dot org
2010-02-25 23:54 ` pinskia at gcc dot gnu dot org
2010-02-26 18:53 ` changpeng dot fang at amd dot com
2010-02-26 18:55 ` pinskia at gcc dot gnu dot org
2010-02-26 19:06 ` changpeng dot fang at amd dot com
     [not found] <bug-43182-4@http.gcc.gnu.org/bugzilla/>
2021-07-26  7:45 ` pinskia at gcc dot gnu.org

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