On 06/04/2018 07:31 AM, Paul Koning wrote: > The internals manual in its description of the "matching constraint" says > that it works for cases where the in and out operands are somewhat different, > such as *p++ vs. *p. Obviously that is meant to cover post_inc side effects. > > The curious thing is that auto-inc-dec.c specifically avoids doing this: if > it finds what looks like a suitable candidate for auto-inc or auto-dec > optimization but that operand occurs more than once in the insn, it doesn't > make the change. The result is code that's both larger and slower for > machines that have post_inc etc. addressing modes. The gccint documentation > suggests that it was the intent to optimize this case, so I wonder why it is > avoided. I wouldn't be terribly surprised if the old flow.c based auto-inc discovery handled this, but the newer auto-inc-dec.c doesn't. The docs were probably written prior to the conversion.
jeff
