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.
paul
