https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84114
Richard Biener <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |missed-optimization Target| |aarch64, x86_64-*-* Status|UNCONFIRMED |NEW Last reconfirmed| |2018-01-30 Version|unknown |8.0 Ever confirmed|0 |1 --- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- This is probably related to targetm.sched.reassociation_width where reassoc will widen a PLUS chain so several instructions will be executable in parallel without dependences. Thus, (x + (y + (z + w))) -> (x + y) + (z + w). When all of them are fed by multiplications this goes from four fmas to two. It's basically a target request we honor so it works as designed. At some point I thought about integrating FMA detection with reassociation.