https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98981
--- Comment #3 from Jim Wilson <wilson at gcc dot gnu.org> --- I suppose cost model problems could explain why combine didn't do the optimization. I didn't have a chance to look at that. I still think there is a fundmental problem with how we represent SImode operations, but again cost model problems could explain why my experiments to fix that didn't work as expected. I probably didn't look at that when I was experimenting with riscv.md changes. Your patch does look useful, but setting cost to 1 for MULT is wrong, and would be just as wrong for DIV. That is OK for PLUS, MINUS, and NEG though. I think a better option is to set *total = 0 and return false. That gives no extra cost to the sign extend, and recurs to get the proper cost for the operation underneath. That would work for MUL and DIV. I found code in the rs6000 port that does this.
