[Bug target/82618] Inefficient double-word subtration on x86_64

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82618

--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
Maybe a peephole2 pattern for sub<mode>_3 with:

"find_regno_note (peep2_next_insn (0), REG_UNUSED, operand[0])"

constraint that converts to equivalent compare would do the trick?

Hopefully, later passes (cprop_hardreg) would eliminate the move.