I see. Ok let's say it this way. If I have an move instruction that if a destination operand is an accumulator can set some status bits, like zero or sign. How can I use this feature with the compiler? For example, after "mov" to the accumulator the compiler could use the status bits from that "mov" with the branch insn (without a seperate compare). Dmitry. --- Исходное сообщение --- От кого: "Ian Lance Taylor" <iant@xxxxxxxxxx> Кому: "Dmitry" <mittie@xxxxxxx> Дата: 4 мая 2011, 17:29:03 Тема: Re: Back end question. > "Dmitry" <mittie@xxxxxxx> writes: > > > Ok. But what if the compiler uses that unnecessary clobber for a > > branch or another CC dependent insn before reload, and if I remove > > that clobber in a define_split after reload, could I end up with a > > corrupted code? > > I'm sorry, I don't understand what you are trying to say. The compiler > can't use a clobber. Simply removing a clobber can't cause corrupted > code, because the compiler was already assuming that the value in the > register was unpredictable. > > Ian > > > --- Исходное сообщение --- > > От кого: "Ian Lance Taylor" <iant@xxxxxxxxxx> > > Кому: "Dmitry" <mittie@xxxxxxx> > > Дата: 29 апреля 2011, 19:52:44 > > Тема: Re: Back end question. > > > > > > > >> "Dmitry" <mittie@xxxxxxx> writes: > >> > >> > I have one more question. If I have, for example, a define_insn rtx which matches a pattern generated by previous define_expand rtx. define_expand and define_split combination produces either a simple "set" rtx or "set" plus clobber of CC reg. And if in define_insn I specify a constraint that requires such an operand type which should generate a CC clobber, but initially there could be a combination of operands that does not clobber a CC and also does not satisfy constraints, so compiler must reload one or two operands in appropriate registers which satisfies constraints. In the case those appropriate registers should clobber a CC does a compiler add a suitable clobber rtx as written in define_split, or it just changes the registers names without any knowledge of a necessary clobber? > >> > >> If I understand your question correctly, then the compiler is not going > >> to add a clobber. Reload does not change the shape of the instruction. > >> It just shuffles registers to match the constraints. If you have > >> constraints which require a clobber, then the insn pattern should have a > >> clobber. You can use define_split to remove an unnecessary clobber > >> after reload is complete. > >> > >> Ian > >> > >> > >
