On Sun, 22 Nov 2020, Jeff Law via Gcc-patches wrote: > This is the primary patch for cc0 removal on the H8 port.? It doesn't > have any of the optimization work and many patterns are simply disabled > at this time.? It's working well enough to not regress the testsuite. > > The H8 is similar to the m68k and other ports in that the vast majority > of instructions clobber the condition codes, including most of the > arithmetic insns that reload needs to use. While there's a special > adds/subs that does not modify the condition codes, they only accept > constant addends of 1, 2 and 4. With that in mind, this port does not > expose cc0 until after reload. So most patterns are defined using > define_insn_and_split. The splitter adds the appropriate clobbers.
JFTR (as I'm repeating a previous note for another port): if you'd went for exposing cc0 *before* reload (adding a clobber to each pattern that clobbers, then specifying exceptions removing them), the amount of (required) define_insn_and_splits would have been zero; typically much less churn in the port. That approach also has the benefit of insns not "silently" changing behavior at reload-time. You might think a parallel with a clobber for each insn hampers some rtl optimizations, but I found that to be not an issue at all, doing that for CRIS (at least when comparing to the cc0 version). brgds, H-P
