Re: pb expanding insn with "use"
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> Aurelien Buhrig <aurelien.buhrig.gcc@xxxxxxxxx> writes: > >> I defined this insn which uses a "temporary" register whose value is >> used for next rotation by 1: >> >> (define_insn "rotrhi3_cst1" >> [(set (match_operand:HI 0 "d_operand" "=d") >> (rotatert:HI (match_operand:HI 1 "d_operand" "0") >> (const_int 1))) >> (use (match_operand:HI 2 "d_operand" "+&d"))] >> "" >> "rrc.w\t%2\t| \trrc.w\t%0 ; rotr1") > > I don't know what these instructions do, but if the temporary register > is only used by this insn you should use match_scratch. If the register > is used by other subsequent instructions then you need to set it, not > just use it. > >> So it uses the same register for the use operand %2 and for the operands >> %0/%1 despite the +& modifers... > > It's a bit confusing, but the +& modifiers are only for register > allocation. The CSE pass is before register allocation, and it is only > looking at what the RTL says. The RTL says that the register is used > but not set, so the compiler feels free to feed in the same register. > >> The doc says about use: >> "use can only be used to describe that the register is live. You >> should think twice before adding use statements, more often you will >> want to use unspec instead." >> >> Does it mean that it's not possible using "use" and the only way to do >> this is to create an unspec insn ? > > You can use a "use" statement as long as you know what it means. The > comment in the doc is there because what it means is not what most > people actually want. > > Ian Ok, it's very clear. In my case, the %2 reg is used by other subsequent insn, but some bits in this reg clobber (the MSB for a rotatert in fact), or set wrt the cc0. I think I need to set it with an unspec. Thanks for your help ! Aurélien