Back end question.

[Ian Lance Taylor]

"Dmitry" <mittie@ukr.net> 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