Paul, On Sunday 04 June 2006 17:57, Paul Brook wrote: > Because then you have several different patterns for the same > operation. The different variants of movsi should be part of the same > pattern so that the compiler can change its mind which variant it > wants to use.
Together with the comments of Rask Ingemann (Thanks, Rask!), I understand now what you mean. But regarding the fact that swpb() needs a temporary register - or alternative - clobber the input register - how can I model this behaviour in a single insn? > You're confusing constraints and predicates. general_operand is the > predicate. The predicate says under which conditions the insn will > match. The constraints tell regalooc/reload how to make sure the > operands of the instruction are valid. Yes, my wording was incorrect. But I know already the difference from the manual. > Tightening the predicates isn't sufficient (and may not even be > neccessary). You need to set the constraints so that the compiler > knows *how* to fix invalid instructions. And if I have 4 different constraints in a single insn, and only one of them is needing a temporary register, how do I model this? This may be the biggest problem. And because byte writes are so common, it deserves a good implementation. I can't waste a temporary register for each load/store. > The compilcation is that while constraints give sufficient > information for the compiler to generate correct code they don't help > generating good code. There are often non-obvious target specific > ways of reloading invalid addresses. So reload has additional hooks > (eg. GO_IF_LEGITIMATE_ADDRESS) to provice clever ways of fixing > invalid operands. I will look into this region of code to understand what's going on there. Thanks, Paul. regards Wolfgang -- We're back to the times when men were men and wrote their own device drivers. (Linus Torvalds)
