https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101697
--- Comment #6 from Jeffrey A. Law <law at gcc dot gnu.org> --- So this issue has come up again in the context of LRA conversion which happens to trip over the same bug, but with a different testcase. At the core of this problem is reload and LRA will both generate invalid RTL when performing register eliminations. Specifically, they will create an autoinc addressing mode where the incremented/decremented register is used elsewhere in the same insn as a source operand. Such RTL has been considered invalid as long as I can remember. The H8 backend does try to prevent this behavior by checking for this scenario and rejecting such insns in the insn condition. But in both the reload and LRA cases, they make substitutions in the original insn without validating the resulting insn (which would have failed). Even if they did try to validate the resulting insn, neither has a code generation strategy to deal with a failed substitution during register eliminations. Paul K. indicated how the pdp11 port handles these cases with constraints. Using constraints alone was insufficient to fix this problem, but using constraints in conjunction with the existing insn condition checks does seem to fix this problem. I'm currently upstreaming the various bits to make that happen.
