[Bug tree-optimization/56717] New: Enhance Dot-product pattern recognition to avoid mult widening.

[ysrumyan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56717

             Bug #: 56717
           Summary: Enhance Dot-product pattern recognition to avoid mult
                    widening.
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ysrumyan@gmail.com


Comparing performance of icc and gcc compilers we found out that for one
important benchmark from eembc 1.1 suite gcc produces very poor code in
comparison with icc. This deficiency can be illustrated by the following simple
example:

typedef signed short s16;
typedef signed long  s32;
void bar (s16 *in1, s16 *in2, s16 *out, int n, s16 scale)
{
  int i;
  s32 acc = 0;
  for (i=0; i<n; i++)
    acc += ((s32) in1[i] * (s32) in2[i]) >> scale;
  *out = (s16) acc;
}
gcc performes mult widening conversion for it which does not look reasonable
and leads to suboptiml code for x86 at least.

I assume that Dot-prodeuct pattern recognition can be simply enhanced to accept
such case by allowing the following stmts:

     type x_t, y_t;
     TYPE1 prod;
     TYPE2 sum = init;
   loop:
     sum_0 = phi <init, sum_1>
     S1  x_t = ...
     S2  y_t = ...
     S3  x_T = (TYPE1) x_t;
     S4  y_T = (TYPE1) y_t;
     S5  prod = x_T * y_T;
     [S6  prod = (TYPE2) prod;  #optional]
     S6' prod1 = prod1 <bin-op> <opnd>
     S7  sum_1 = prod1 + sum_0;

where S6' is vectorizable.