[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.org via Gcc-bugs]

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

Andrew Pinski  changed:

   What|Removed |Added

   Severity|normal  |enhancement

--- Comment #7 from Andrew Pinski  ---
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

[Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]

[changpeng dot fang at amd dot com]

--- 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


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

[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]

--- 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.


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

[Bug middle-end/43182] GCC does not pull out a[0] from loop that changes a[i] for i:[1,n]

[changpeng dot fang at amd dot com]

--- 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]


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

[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]

--- 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.


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pinskia at gcc dot gnu dot org changed:

   What|Removed |Added

 Status|UNCONFIRMED |NEW
 Ever Confirmed|0   |1
   Last reconfirmed|-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

[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]

--- 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.


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