Optimize subtraction by small immediate

eace45a744258390b6b287b413a4c8733a1e8cedc54cc8f624de1ecea2ef789f 
Use ADDI/ADDIW with negated value instead of LI+SUB.
Alexis Sellier committed ago 1 parent 6cd41da3
lib/std/arch/rv64/isel.rad +12 -0
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            } else {
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                selectBinOp(s, rd, rs1, b, BinOp::Add, super::SCRATCH2);
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            }
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        }
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        case il::BinOp::Sub => {
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            // Optimize subtraction by small immediate: use ADDI with negated value.
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            if let case il::Val::Imm(imm) = b {
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                let neg = -imm;
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                if neg >= super::MIN_IMM as i64 and neg <= super::MAX_IMM as i64 {
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                    emit::emit(s.e,
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                        encode::addiw(rd, rs1, neg as i32)
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                            if typ == il::Type::W32 else
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                        encode::addi(rd, rs1, neg as i32));
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                    return;
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                }
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            }
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            let rs2 = resolveVal(s, super::SCRATCH2, b);
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            emit::emit(s.e,

    

  
    

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                encode::subw(rd, rs1, rs2)

    

  
    

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                    if typ == il::Type::W32 else

    

  
    

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                encode::sub(rd, rs1, rs2));

    

  
    

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        }