https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98350
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Version|unknown |11.0
Ever confirmed|0 |1
Last reconfirmed| |2021-01-05
Status|UNCONFIRMED |NEW
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
There is no built-in way, and yes, reassoc-width is known to have this effect.
What could be done is move/duplicate FMA discovery from
pass_optimize_widening_mul to reassoc(*). The simplistic idea would be to
perform a separate FMA detection on the OPS array.
The question is how to handle imperfect chains where reassoc would order
after rank, like
a[i] += b[i]* c[i] + d[i] + f[i] * g[i] + h[i] + k[i] * l[i] + m[i] + p[i];
and also how to not "break" the special heuristics the current FMA formation
pass has. Alternatively altering rewrite_expr_tree_parallel only to avoid
splitting FMA chains in unwanted ways would be possible.
(*) note since reassoc doesn't handle signed integer arithmetic it cannot
fully replace late FMA detect
