Hi, when investigating a regression, I realized that we create a superfluous load on S390. The snippet looks something like
LA %r10, 0(%r8,%r9) LLH %r4, 0(%r10) meaning the address in r10 is computed by an LA even though LLH supports the addressing already. The same address is used multiple times so combine cannot do something about it. Looking into fwprop, I realized it actually tries to propagate the address but exits because we specify higher costs for an address with index than for one without. This was meant to account for the fact that, in general and all other things being equal, not every instruction can handle indexed addressing mode. Now, in this case, fwprop actually knows the instructions it propagates into and could decide based on the full costs, seeing that it would not be more expensive. Currently, it recursively descends to the parts that are going to be propagated or replaced and compares the costs of both without regarding the full instruction. Would it make sense to enhance fwprop with a more detailed cost evaluation or are there other passes that should do the same - i.e. what's the preferred way to solve this? Is changing the address costs in the backend depending on addressing mode sensible at all? As far as I can see, the x86 backend also changes costs depending on global properties (i.e. to prefer fewer registers when addressing). Regards Robin
