[PATCH] Generate fused widening multiply-and-accumulate operations only when the widening multiply has single use

[Yufeng Zhang]

Hi,

This patch changes the widening_mul pass to fuse the widening multiply 
with accumulate only when the multiply has single use.  The widening_mul 
pass currently does the conversion regardless of the number of the uses, 
which can cause poor code-gen in cases like the following:

typedef int ArrT [10][10];

void
foo (ArrT Arr, int Idx)
{
  Arr[Idx][Idx] = 1;
  Arr[Idx + 10][Idx] = 2;
}

On AArch64, after widening_mul, the IR is like

  _2 = (long unsigned int) Idx_1(D);
  _3 = Idx_1(D) w* 40;                           <----
  _5 = Arr_4(D) + _3;
  *_5[Idx_1(D)] = 1;
  _8 = WIDEN_MULT_PLUS_EXPR <Idx_1(D), 40, 400>; <----
  _9 = Arr_4(D) + _8;
  *_9[Idx_1(D)] = 2;

Where the arrows point, there are redundant widening multiplies.

Bootstrap successfully on x86_64.

The patch passes the regtest on aarch64, arm and x86_64.

OK for the trunk?

Thanks,
Yufeng

p.s. Note that x86_64 doesn't suffer from this issue as the 
corresponding widening multiply accumulate op is not available on the 
target.

gcc/

        * tree-ssa-math-opts.c (convert_plusminus_to_widen): Call
        has_single_use () and not do the conversion if has_single_use ()
        returns false for the multiplication result.diff --git a/gcc/tree-ssa-math-opts.c b/gcc/tree-ssa-math-opts.c
index f7f8ec9..d316990 100644
--- a/gcc/tree-ssa-math-opts.c
+++ b/gcc/tree-ssa-math-opts.c
@@ -2425,12 +2425,16 @@ convert_plusminus_to_widen (gimple_stmt_iterator *gsi, 
gimple stmt,
 
      It might also appear that it would be sufficient to use the existing
      operands of the widening multiply, but that would limit the choice of
-     multiply-and-accumulate instructions.  */
+     multiply-and-accumulate instructions.
+
+     If the widened-multiplication result has more than one uses, it is
+     probably wiser not to do the conversion.  */
   if (code == PLUS_EXPR
       && (rhs1_code == MULT_EXPR || rhs1_code == WIDEN_MULT_EXPR))
     {
       if (!is_widening_mult_p (rhs1_stmt, &type1, &mult_rhs1,
-                              &type2, &mult_rhs2))
+                              &type2, &mult_rhs2)
+         || !has_single_use (rhs1))
        return false;
       add_rhs = rhs2;
       conv_stmt = conv1_stmt;
@@ -2438,7 +2442,8 @@ convert_plusminus_to_widen (gimple_stmt_iterator *gsi, 
gimple stmt,
   else if (rhs2_code == MULT_EXPR || rhs2_code == WIDEN_MULT_EXPR)
     {
       if (!is_widening_mult_p (rhs2_stmt, &type1, &mult_rhs1,
-                              &type2, &mult_rhs2))
+                              &type2, &mult_rhs2)
+         || !has_single_use (rhs2))
        return false;
       add_rhs = rhs1;
       conv_stmt = conv2_stmt;