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
