Re: autovectorization in gcc

[David Malcolm <dmalcolm@redhat.com>]

On Wed, 2019-01-09 at 09:56 +0000, Jonathan Wakely wrote:
> On Wed, 9 Jan 2019 at 09:50, Andrew Haley wrote:
> > I don't agree. Sometimes vectorization is critical. It would be
> > nice
> > to have a warning which would fire if vectorization failed. That
> > would
> > surely help the OP.
> 
> Dave Malcolm has been working on something like that:
> https://gcc.gnu.org/ml/gcc-patches/2018-09/msg01749.html

Yes: this code is in trunk for gcc 9, but it doesn't help much for the
case given elsewhere in this thread:

#include <cmath>

extern float data [ 32768 ] ;

extern void vf1()
{
   #pragma vectorize enable
   for ( int i = 0 ; i < 32768 ; i++ )
     data [ i ] = std::sqrt ( data [ i ] ) ;
}

Compiling on this x86_64 box with -fopt-info-vec-missed shows the
rather cryptic:

g++ -c /tmp/sqrt-test.cc -O3 -mavx2 -fopt-info-vec-missed
/tmp/sqrt-test.cc:8:24: missed: couldn't vectorize loop
/tmp/sqrt-test.cc:8:24: missed: not vectorized: control flow in loop.
/home/david/coding/gcc-python/gcc-svn-trunk/install-dogfood/include/c++/9.0.0/cmath:464:27: missed: statement clobbers memory: __builtin_sqrtf (_1);

and with -fopt-info-vec-all-internals shows:

g++ -c /tmp/sqrt-test.cc -O3 -mavx2 -fopt-info-vec-all-internals

Analyzing loop at /tmp/sqrt-test.cc:8
/tmp/sqrt-test.cc:8:24: note:  === analyze_loop_nest ===
/tmp/sqrt-test.cc:8:24: note:   === vect_analyze_loop_form ===
/tmp/sqrt-test.cc:8:24: missed:   not vectorized: control flow in loop.
/tmp/sqrt-test.cc:8:24: missed:  bad loop form.
/tmp/sqrt-test.cc:8:24: missed: couldn't vectorize loop
/tmp/sqrt-test.cc:8:24: missed: not vectorized: control flow in loop.
/tmp/sqrt-test.cc:5:13: note: vectorized 0 loops in function.
/home/david/coding/gcc-python/gcc-svn-trunk/install-dogfood/include/c++/9.0.0/cmath:464:27: note:  === vect_slp_analyze_bb ===
/home/david/coding/gcc-python/gcc-svn-trunk/install-dogfood/include/c++/9.0.0/cmath:464:27: note:   === vect_analyze_data_refs ===
/home/david/coding/gcc-python/gcc-svn-trunk/install-dogfood/include/c++/9.0.0/cmath:464:27: note:   got vectype for stmt: _1 = data[i_12];
vector(8) float
/home/david/coding/gcc-python/gcc-svn-trunk/install-dogfood/include/c++/9.0.0/cmath:464:27: missed:  not vectorized: not enough data-refs in basic block.
/home/david/coding/gcc-python/gcc-svn-trunk/install-dogfood/include/c++/9.0.0/cmath:464:27: missed: statement clobbers memory: __builtin_sqrtf (_1);
/tmp/sqrt-test.cc:8:24: note:  === vect_slp_analyze_bb ===
/tmp/sqrt-test.cc:8:24: note:   === vect_analyze_data_refs ===
/tmp/sqrt-test.cc:8:24: note:   got vectype for stmt: data[i_12] = _7;
vector(8) float
/tmp/sqrt-test.cc:8:24: missed:  not vectorized: not enough data-refs in basic block.
/tmp/sqrt-test.cc:10:1: note:  === vect_slp_analyze_bb ===
/tmp/sqrt-test.cc:10:1: note:   === vect_analyze_data_refs ===
/tmp/sqrt-test.cc:10:1: missed:  not vectorized: not enough data-refs in basic block.

I had to turn on -fdump-tree-all to try to figure out what that
"control flow in loop" was; it seems to be a guard against the input to
value being negative:

  <bb 3> [local count: 1063004407]:
  # i_12 = PHI <0(2), i_6(7)>
  # ivtmp_10 = PHI <32768(2), ivtmp_2(7)>
  # DEBUG i => i_12
  # DEBUG BEGIN_STMT
  _1 = data[i_12];
  # DEBUG __x => _1
  # DEBUG BEGIN_STMT
  _7 = .SQRT (_1);
  if (_1 u>= 0.0)
    goto <bb 8>; [99.95%]
  else
    goto <bb 4>; [0.05%]

  <bb 8> [local count: 1062472912]:
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 531495]:
  __builtin_sqrtf (_1);

I'm not sure where that control flow came from: it isn't in
  sqrt-test.cc.104t.stdarg
but is in
  sqrt-test.cc.105t.cdce
so I think it's coming from the argument-range code in cdce.

Arguably the location on the statement is wrong: it's on the loop
header, when it presumably should be on the std::sqrt call.

Shall I file a bugzilla about this?

Dave