https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115395
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> --- It needs epilogue vectorization to trigger and it's the path re-using the vector accumulator from the earlier loop that goes wrong when the main vector loop is skipped. We apply the initial value adjustment to the scalar result but the continuation fails to do this and the epilogue vector epilogue expects the earlier code to have done it. IIRC we force "optimization" of this to be disabled but obviously somehow fail to do this for SLP.
