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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/95060] vfnmsub132ps is not generated with
 -ffast-math
Date: Tue, 12 May 2020 06:56:14 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95060

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |11.0
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-05-12
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1
             Target|                            |x86_64-*-* i?86-*-*

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
FMA generation already folds the FMA stmt:

      if (cond)
        fma_stmt =3D gimple_build_call_internal (IFN_COND_FMA, 5, cond, mul=
op1,
                                               op2, addop, else_value);
      else
        fma_stmt =3D gimple_build_call_internal (IFN_FMA, 3, mulop1, op2, a=
ddop);
      gimple_set_lhs (fma_stmt, gimple_get_lhs (use_stmt));
      gimple_call_set_nothrow (fma_stmt, !stmt_can_throw_internal (cfun,
                                                                   use_stmt=
));
      gsi_replace (&gsi, fma_stmt, true);
      /* Follow all SSA edges so that we generate FMS, FNMA and FNMS
         regardless of where the negation occurs.  */
      gimple *orig_stmt =3D gsi_stmt (gsi);
      if (fold_stmt (&gsi, follow_all_ssa_edges))
        {
          if (maybe_clean_or_replace_eh_stmt (orig_stmt, gsi_stmt (gsi)))
            gcc_unreachable ();
          update_stmt (gsi_stmt (gsi));

but not the negate it feeds since with -ffast-math we have
-((a[i] * b[i]) + c[i]) as canonical form it seems (reassoc does this).

float r[8], a[8], b[8], c[8];

void
test_fnms (void)
{
  for (int i =3D 0; i < 8; i++)
    r[i] =3D -((a[i] * b[i]) + c[i]);
}

would be an alternative testcase, not handled without -ffast-math either.

I'd suggest to fold the single-use stmt of the fma_stmts lhs if any
[and if it is a negate].=