gem5/arch/mips/isa/formats/branch.isa

// -*- mode:c++ -*-

////////////////////////////////////////////////////////////////////
//
// Control transfer instructions
//

output header {{

    /**
     * Base class for instructions whose disassembly is not purely a
     * function of the machine instruction (i.e., it depends on the
     * PC).  This class overrides the disassemble() method to check
     * the PC and symbol table values before re-using a cached
     * disassembly string.  This is necessary for branches and jumps,
     * where the disassembly string includes the target address (which
     * may depend on the PC and/or symbol table).
     */
    class PCDependentDisassembly : public MipsStaticInst
    {
      protected:
        /// Cached program counter from last disassembly
        mutable Addr cachedPC;

        /// Cached symbol table pointer from last disassembly
        mutable const SymbolTable *cachedSymtab;

        /// Constructor
        PCDependentDisassembly(const char *mnem, MachInst _machInst,
                               OpClass __opClass)
            : MipsStaticInst(mnem, _machInst, __opClass),
              cachedPC(0), cachedSymtab(0)
        {
        }

        const std::string &
        disassemble(Addr pc, const SymbolTable *symtab) const;
    };

    /**
     * Base class for branches (PC-relative control transfers),
     * conditional or unconditional.
     */
    class Branch : public PCDependentDisassembly
    {
      protected:
        /// target address (signed) Displacement .
        int32_t targetOffset;

        /// Constructor.
        Branch(const char *mnem, MachInst _machInst, OpClass __opClass)
            : PCDependentDisassembly(mnem, _machInst, __opClass),
              targetOffset(OFFSET << 2)
        {
        }

        Addr branchTarget(Addr branchPC) const;

        std::string
        generateDisassembly(Addr pc, const SymbolTable *symtab) const;
    };

    /**
     * Base class for branches (PC-relative control transfers),
     * conditional or unconditional.
     */
    class BranchLikely : public PCDependentDisassembly
    {
      protected:
        /// target address (signed) Displacement .
        int32_t targetOffset;

        /// Constructor.
        Branch(const char *mnem, MachInst _machInst, OpClass __opClass)
            : PCDependentDisassembly(mnem, _machInst, __opClass),
              targetOffset(OFFSET << 2)
        {
        }

        Addr branchTarget(Addr branchPC) const;

        std::string
        generateDisassembly(Addr pc, const SymbolTable *symtab) const;
    };

    /**
     * Base class for jumps (register-indirect control transfers).  In
     * the Mips ISA, these are always unconditional.
     */
    class Jump : public PCDependentDisassembly
    {
      protected:

        /// Displacement to target address (signed).
        int32_t disp;

      public:
        /// Constructor
        Jump(const char *mnem, MachInst _machInst, OpClass __opClass)
            : PCDependentDisassembly(mnem, _machInst, __opClass),
              disp(OFFSET)
        {
        }

        Addr branchTarget(ExecContext *xc) const;

        std::string
        generateDisassembly(Addr pc, const SymbolTable *symtab) const;
    };
}};

output decoder {{
    Addr
    Branch::branchTarget(Addr branchPC) const
    {
        return branchPC + 4 + disp;
    }

    Addr
    Jump::branchTarget(ExecContext *xc) const
    {
        Addr NPC = xc->readPC() + 4;
        uint64_t Rb = xc->readIntReg(_srcRegIdx[0]);
        return (Rb & ~3) | (NPC & 1);
    }

    const std::string &
    PCDependentDisassembly::disassemble(Addr pc,
                                        const SymbolTable *symtab) const
    {
        if (!cachedDisassembly ||
            pc != cachedPC || symtab != cachedSymtab)
        {
            if (cachedDisassembly)
                delete cachedDisassembly;

            cachedDisassembly =
                new std::string(generateDisassembly(pc, symtab));
            cachedPC = pc;
            cachedSymtab = symtab;
        }

        return *cachedDisassembly;
    }

    std::string
    Branch::generateDisassembly(Addr pc, const SymbolTable *symtab) const
    {
        std::stringstream ss;

        ccprintf(ss, "%-10s ", mnemonic);

        // There's only one register arg (RA), but it could be
        // either a source (the condition for conditional
        // branches) or a destination (the link reg for
        // unconditional branches)
        if (_numSrcRegs > 0) {
            printReg(ss, _srcRegIdx[0]);
            ss << ",";
        }
        else if (_numDestRegs > 0) {
            printReg(ss, _destRegIdx[0]);
            ss << ",";
        }

#ifdef SS_COMPATIBLE_DISASSEMBLY
        if (_numSrcRegs == 0 && _numDestRegs == 0) {
            printReg(ss, 31);
            ss << ",";
        }
#endif

        Addr target = pc + 4 + disp;

        std::string str;
        if (symtab && symtab->findSymbol(target, str))
            ss << str;
        else
            ccprintf(ss, "0x%x", target);

        return ss.str();
    }

    std::string
    Jump::generateDisassembly(Addr pc, const SymbolTable *symtab) const
    {
        std::stringstream ss;

        ccprintf(ss, "%-10s ", mnemonic);

#ifdef SS_COMPATIBLE_DISASSEMBLY
        if (_numDestRegs == 0) {
            printReg(ss, 31);
            ss << ",";
        }
#endif

        if (_numDestRegs > 0) {
            printReg(ss, _destRegIdx[0]);
            ss << ",";
        }

        ccprintf(ss, "(r%d)", RB);

        return ss.str();
    }
}};

def template JumpOrBranchDecode {{
    return (RD == 0)
        ? (StaticInst<MipsISA> *)new %(class_name)s(machInst)
        : (StaticInst<MipsISA> *)new %(class_name)sAndLink(machInst);
}};

def format Branch(code) {{
    code = 'bool cond;\n' + code + '\nif (cond) NPC = NPC + disp;\n';
    iop = InstObjParams(name, Name, 'Branch', CodeBlock(code),
                        ('IsDirectControl', 'IsCondControl'))
    header_output = BasicDeclare.subst(iop)
    decoder_output = BasicConstructor.subst(iop)
    decode_block = BasicDecode.subst(iop)
    exec_output = BasicExecute.subst(iop)
}};


def format BranchLikely(code) {{
    code = 'bool cond;\n' + code + '\nif (cond) NPC = NPC + disp;\n';
    iop = InstObjParams(name, Name, 'Branch', CodeBlock(code),
                        ('IsDirectControl', 'IsCondControl'))
    header_output = BasicDeclare.subst(iop)
    decoder_output = BasicConstructor.subst(iop)
    decode_block = BasicDecode.subst(iop)
    exec_output = BasicExecute.subst(iop)
}};


def format UncondBranch(*flags) {{
    flags += ('IsUncondControl', 'IsDirectControl')
    (header_output, decoder_output, decode_block, exec_output) = \
        UncondCtrlBase(name, Name, 'Branch', 'NPC + disp', flags)
}};

def format Jump(*flags) {{
    flags += ('IsUncondControl', 'IsIndirectControl')
    (header_output, decoder_output, decode_block, exec_output) = \
        UncondCtrlBase(name, Name, 'Jump', '(Rb & ~3) | (NPC & 1)', flags)
}};