[Bug tree-optimization/49795] New: vectorization of conditional code happens only on local variables

[Bug tree-optimization/49795] New: vectorization of conditional code happens only on local variables

[vincenzo.innocente at cern dot ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

           Summary: vectorization of conditional code happens only on
                    local variables
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincenzo.innocente@cern.ch


in this example loop1 does not vectorize, loop2 does 
const int N=64;
float c[N];
float d[N];

void loop1() {
  for (int i=0; i!=N; ++i) {
    if (c[i]<0) d[i] = -d[i];
  }
}

void loop2() {
  for (int i=0; i!=N; ++i) {
    float tmp = d[i];
    if (c[i]<0) tmp = -tmp;
    d[i]=tmp;
  }
}

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-07-20 11:57:56 UTC ---
At least from C++0x memory model or OpenMP POV that's highly desirable.
In loop1 d[i] isn't written unconditionally, in loop2 it is, so transforming
loop1 code into loop2 might introduce data races.

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-07-20 12:00:32 UTC ---
Interesting. Then I would be curious to know what other respected compilers vs
OpenMP do in this area, eg, Intel..

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[vincenzo.innocente at cern dot ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

--- Comment #3 from vincenzo Innocente <vincenzo.innocente at cern dot ch> 2011-07-20 12:32:21 UTC ---
my actual code looks more like this
void loop() {
  for (int i=0; i!=N; ++i) {
    d[i]=a[i]+b[i];
    if (c[i]<0) d[i] = -d[i];
  }
}
where d[i] IS written unconditionally (and does not vectorize either)

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-07-20 12:41:07 UTC ---
That is something different, yeah, in that case the transformation doesn't
introduce new data races and is desirable as well, not just for vectorization.

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.07.20 14:53:41
                 CC|                            |rguenth at gcc dot gnu.org,
                   |                            |spop at gcc dot gnu.org
     Ever Confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-20 14:53:41 UTC ---
I think you at least need -ftree-loop-if-convert-stores to vectorize
conditional stores, but it doens't seem to work in this case.

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[vincenzo.innocente at cern dot ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

--- Comment #6 from vincenzo Innocente <vincenzo.innocente at cern dot ch> 2011-07-20 16:59:20 UTC ---
actually  -ftree-loop-if-convert-stores does the "trick" with -Ofast

things are not fully consistent though
of these four loop I get the following
notice how the combination -ftree-loop-if-convert-stores -03 vectorize the
first BUT not the second!


const int N=1024;
float __attribute__ ((aligned(16))) a[N];
float __attribute__ ((aligned(16))) b[N];
float __attribute__ ((aligned(16))) c[N];
float __attribute__ ((aligned(16))) d[N];

void loop1() {
  for (int i=0; i!=N; ++i) {
    d[i]=a[i]+b[i];
    if (c[i]<0) d[i] = -d[i];
  }
}

void loop2() {
  for (int i=0; i!=N; ++i) {
    float tmp = a[i]+b[i];
    if (c[i]<0) tmp = -tmp;
    d[i]=tmp;
  }
}

void loop3() {
  for (int i=0; i!=N; ++i) {
    d[i] = (c[i]>0) ? a[i]+b[i] : -a[i]-b[i];
  }
}

void loop4() {
  for (int i=0; i!=N; ++i) {
    float tmp = a[i]+b[i];
    tmp = (c[i]>0) ? tmp : -tmp;
    d[i] = tmp;
  }
}




c++ -Wall -O3  -ftree-vectorizer-verbose=2  -c test/testBug.cpp -o bha.o

test/testBug.cpp:9: note: vectorized 0 loops in function.

test/testBug.cpp:17: note: LOOP VECTORIZED.
test/testBug.cpp:16: note: vectorized 1 loops in function.

test/testBug.cpp:24: note: vectorized 0 loops in function.

test/testBug.cpp:31: note: not vectorized: unsupported data-type bool
test/testBug.cpp:30: note: vectorized 0 loops in function.
pb-d-128-141-131-10:Octave innocent$ c++ -Wall -O3  -ftree-vectorizer-verbose=2
 -c test/testBug.cpp -o bha.o -ftree-loop-if-convert-stores

test/testBug.cpp:10: note: LOOP VECTORIZED.
test/testBug.cpp:9: note: vectorized 1 loops in function.

test/testBug.cpp:16: note: vectorized 0 loops in function.

test/testBug.cpp:24: note: vectorized 0 loops in function.

test/testBug.cpp:30: note: vectorized 0 loops in function.
pb-d-128-141-131-10:Octave innocent$ c++ -Wall -Ofast 
-ftree-vectorizer-verbose=2  -c test/testBug.cpp -o bha.o
-ftree-loop-if-convert-stores

test/testBug.cpp:10: note: LOOP VECTORIZED.
test/testBug.cpp:9: note: vectorized 1 loops in function.

test/testBug.cpp:17: note: LOOP VECTORIZED.
test/testBug.cpp:16: note: vectorized 1 loops in function.

test/testBug.cpp:25: note: LOOP VECTORIZED.
test/testBug.cpp:24: note: vectorized 1 loops in function.

test/testBug.cpp:31: note: LOOP VECTORIZED.
test/testBug.cpp:30: note: vectorized 1 loops in function.

[Bug tree-optimization/49795] vectorization of conditional code happens only on local variables

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to vincenzo Innocente from comment #6)

loop1, loop2, and loop4 all vectorize now at -O3.
loop3 can vectorize with -O3 -fno-trapping-math (it can also be vectorize at
-O3 on x86_64 with -march=skylake-avx512 or on aarch64 with SVE enabled).


I almost want to say there is not much to do here.
also about comment #0, the vectorizer does happen for -mavx2. So maybe this is
enough these days ...