[Bug tree-optimization/31460] New: vectorizer failed to work on simple loop

[Bug tree-optimization/31460] New: vectorizer failed to work on simple loop

[hjl at lucon dot org]

bash-3.1$ cat pmaxsw.c
typedef short vec_t;

extern __attribute__((aligned(16))) vec_t x [64];
extern __attribute__((aligned(16))) vec_t y [64];
extern __attribute__((aligned(16))) vec_t m [64];

void
foo ()
{
  int i;

  for (i = 0; i < 20; i++)
#if 0
    m [i] = (x [i] < y [i]) ? y [i] : x[i];
#else
    if (x [i] < y [i])
      m [i] = y [i];
    else
      m [i] = x[i];
#endif
}
bash-3.1$ make
/usr/gcc-4.3/bin/gcc -O2 -mssse3 -ftree-vectorize -ftree-vectorizer-verbose=1
-S pmaxsw.c

pmaxsw.c:9: note: for a in pmaxsw.s; do \
          insn=`basename $a .s`; \
          echo Check vectorizer on $insn.c:; \
          grep $insn $a | grep xmm; \
        done
Check vectorizer on pmaxsw.c:
make: *** [all] Error 1
bash-3.1$ 

bash-3.1$ cat pmaxsw.c
typedef short vec_t;

extern __attribute__((aligned(16))) vec_t x [64];
extern __attribute__((aligned(16))) vec_t y [64];
extern __attribute__((aligned(16))) vec_t m [64];

void
foo ()
{
  int i;

  for (i = 0; i < 20; i++)
#if 1
    m [i] = (x [i] < y [i]) ? y [i] : x[i];
#else
    if (x [i] < y [i])
      m [i] = y [i];
    else
      m [i] = x[i];
#endif
}
bash-3.1$ make
/usr/gcc-4.3/bin/gcc -O2 -mssse3 -ftree-vectorize -ftree-vectorizer-verbose=1
-S pmaxsw.c

pmaxsw.c:12: note: LOOP VECTORIZED.
pmaxsw.c:9: note: vectorized 1 loops in function.
for a in pmaxsw.s; do \
          insn=`basename $a .s`; \
          echo Check vectorizer on $insn.c:; \
          grep $insn $a | grep xmm; \
        done
Check vectorizer on pmaxsw.c:
        pmaxsw  x(%rip), %xmm0
        pmaxsw  x+16(%rip), %xmm0
bash-3.1$ 

Why can't vectorizer recognize

    if (x [i] < y [i])
      m [i] = y [i];
    else
      m [i] = x[i];

is the same as

    m [i] = (x [i] < y [i]) ? y [i] : x[i];


-- 
           Summary: vectorizer failed to work on simple loop
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hjl at lucon dot org
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460

[Bug tree-optimization/31460] vectorizer failed to work on simple loop

[steven at gcc dot gnu dot org]

------- Comment #1 from steven at gcc dot gnu dot org  2007-04-03 16:36 -------
Maybe you can show the assembly output you're expecting?

tree level if-conversion for the vectorizer _should_ be able to recognize this
case. It may be that it's just not set up to deal with x86* insns of this kind.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460

[Bug tree-optimization/31460] vectorizer failed to work on simple loop

[hjl at lucon dot org]

------- Comment #2 from hjl at lucon dot org  2007-04-03 17:37 -------
For code:

typedef short vec_t;

extern __attribute__((aligned(16))) vec_t x [64];
extern __attribute__((aligned(16))) vec_t y [64];
extern __attribute__((aligned(16))) vec_t m [64];

void
foo ()
{
  int i;

  for (i = 0; i < 64; i++)
    if (x [i] < y [i])
      m [i] = y [i];
    else
      m [i] = x [i];
}

I am expecting:

.globl foo
        .type   foo, @function
foo:
.LFB2:
        movdqa  y(%rip), %xmm0
        movl    $16, %eax
        pmaxsw  x(%rip), %xmm0
        movdqa  %xmm0, m(%rip)
        .p2align 4,,7
.L2:
        movdqa  y(%rax), %xmm0
        pmaxsw  x(%rax), %xmm0
        movdqa  %xmm0, m(%rax)
        addq    $16, %rax
        cmpq    $128, %rax
        jne     .L2
        rep ; ret
.LFE2:
        .size   foo, .-foo

But I got

.globl foo
        .type   foo, @function
foo:
.LFB2:
        xorl    %ecx, %ecx
        .p2align 4,,7
.L2:
        movzwl  x(%rcx,%rcx), %edx
        movzwl  y(%rcx,%rcx), %eax
        cmpw    %ax, %dx
        cmovge  %edx, %eax
        movw    %ax, m(%rcx,%rcx)
        addq    $1, %rcx
        cmpq    $64, %rcx
        jne     .L2
        rep ; ret
.LFE2:
        .size   foo, .-foo


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460

[Bug tree-optimization/31460] if(a) a[i] = xxx; else a[i] = yyy; is not converted to if (a) ddd= xxx; else ddd = yyy; a[i] = ddd;

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2007-04-03 17:49 -------
This is not the vectorizer's fault.
>tree level if-conversion for the vectorizer _should_ be able to recognize this
> case. It may be that it's just not set up to deal with x86* insns of this kind.
It cannot because there is a store there and no other pass sinks the store.

So basically 
if(a) a[i] = xxx; else a[i] = yyy;
is not converted to
 if (a) ddd= xxx; else ddd = yyy; a[i] = ddd;

Which means this is really a dup of another bug.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
            Summary|vectorizer failed to work on|if(a) a[i] = xxx; else a[i]
                   |simple loop                 |= yyy; is not converted to
                   |                            |if (a) ddd= xxx; else ddd =
                   |                            |yyy; a[i] = ddd;


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460

[Bug tree-optimization/31460] if(a) a[i] = xxx; else a[i] = yyy; is not converted to if (a) ddd= xxx; else ddd = yyy; a[i] = ddd;

[dorit at il dot ibm dot com]

------- Comment #4 from dorit at il dot ibm dot com  2007-04-03 19:56 -------
yes, this is indeed a known problem (I don't know if there's a PR open for it).
It is one of the tree-ifcvt enhancements that Victor was going to tackle for
4.3 (item (2.3) in http://gcc.gnu.org/wiki/AutovectBranchOptimizations?). 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460

[Bug tree-optimization/31460] if(a) a[i] = xxx; else a[i] = yyy; is not converted to if (a) ddd= xxx; else ddd = yyy; a[i] = ddd;

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2007-04-03 20:00 -------
(In reply to comment #4)
> yes, this is indeed a known problem (I don't know if there's a PR open for it).
> It is one of the tree-ifcvt enhancements that Victor was going to tackle for
> 4.3 (item (2.3) in http://gcc.gnu.org/wiki/AutovectBranchOptimizations?). 

Plus it was mentioned before this does not belong in tree-ifcvt either.
See http://gcc.gnu.org/ml/gcc-patches/2006-05/msg00494.html

And there is already a bug about this, see PR25553

*** This bug has been marked as a duplicate of 25553 ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31460