https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95060
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 = gimple_build_call_internal (IFN_COND_FMA, 5, cond, mulop1,
op2, addop, else_value);
else
fma_stmt = gimple_build_call_internal (IFN_FMA, 3, mulop1, op2, addop);
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 = 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 = 0; i < 8; i++)
r[i] = -((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].
