mgba_bl.c

#include <stdint.h>
#include <stdio.h>

union ARMOperand {
    uint8_t reg;
    int32_t immediate;
};

struct ARMInstructionInfo {
    uint32_t opcode;
    union ARMOperand op1;
    union ARMOperand op2;
    union ARMOperand op3;
};

enum {
    ARM_SP = 13,
    ARM_LR = 14,
    ARM_PC = 15
};

void BL1Decode(uint16_t opcode, struct ARMInstructionInfo* info) {
    int16_t immediate = (opcode & 0x07FF) << 5;
    info->op1.reg = ARM_LR;
    info->op2.reg = ARM_PC;
    info->op3.immediate = (((int32_t) immediate) << 7);
}

void BL2Decode(uint16_t opcode, struct ARMInstructionInfo* info) {
    info->op1.reg = ARM_PC;
    info->op2.reg = ARM_LR;
    info->op3.immediate = (opcode & 0x07FF) << 1;
}

void faultyCombine(struct ARMInstructionInfo* info1, struct ARMInstructionInfo* info2, struct ARMInstructionInfo* out) {
    if (info1->op1.reg != ARM_LR || info1->op2.reg != ARM_PC) {
        //return false;
    }
    if (info2->op1.reg != ARM_PC || info2->op2.reg != ARM_LR) {
        //return false;
    }
    out->op1.immediate = info1->op3.immediate | info2->op3.immediate;
    /* Skipped other variables */
    //return true;
}

void fixedCombine(struct ARMInstructionInfo* info1, struct ARMInstructionInfo* info2, struct ARMInstructionInfo* out) {
    if (info1->op1.reg != ARM_LR || info1->op2.reg != ARM_PC) {
        //return false;
    }
    if (info2->op1.reg != ARM_PC || info2->op2.reg != ARM_LR) {
        //return false;
    }

    uint32_t S = (uint32_t)(info1->op3.immediate) >> 31;

    uint32_t I1 = ((info2->opcode >> 13) & 1) ^ S ^ 1;
    uint32_t I2 = ((info2->opcode >> 11) & 1) ^ S ^ 1;

    out->op1.immediate = info1->op3.immediate | info2->op3.immediate;
    out->op1.immediate = (out->op1.immediate & 0xff3fffff) | (I2 << 22) | (I1 << 23);

    /* Skipped other variables */
    //return true;
}

struct TestCase {
    uint32_t opcode;
    int32_t expected;
};

struct TestCase testCases[] = {
    { .opcode = 0xf8fef000, .expected = 0x200},
    { .opcode = 0xfefef7ff, .expected = -0x200},
    { .opcode = 0xf810f000, .expected = 0x24},
    { .opcode = 0xff05f000, .expected = 0xe0e},
    { .opcode = 0xf691f183, .expected = 0x583d26},
    { .opcode = 0xf16bf67c, .expected = -0x583d26},
};

static int testFaulty(struct TestCase testCase) {
    struct ARMInstructionInfo info1;
    struct ARMInstructionInfo info2;
    struct ARMInstructionInfo out;

    info1.opcode = testCase.opcode & 0xffff;
    info2.opcode = testCase.opcode >> 16;

    BL1Decode(info1.opcode, &info1);
    BL2Decode(info2.opcode, &info2);

    faultyCombine(&info1, &info2, &out);

    if((out.op1.immediate + 4)!= testCase.expected) {
        printf("faultyCombine: Expected 0x%08x but got 0x%08x\n", testCase.expected, out.op1.immediate);
        return 0;
    }

    return 1;
}


static int testFixed(struct TestCase testCase) {
    struct ARMInstructionInfo info1;
    struct ARMInstructionInfo info2;
    struct ARMInstructionInfo out;

    info1.opcode = testCase.opcode & 0xffff;
    info2.opcode = testCase.opcode >> 16;

    BL1Decode(info1.opcode, &info1);
    BL2Decode(info2.opcode, &info2);

    fixedCombine(&info1, &info2, &out);

    if((out.op1.immediate + 4) != testCase.expected) {
        printf("fixedCombine: Expected 0x%08x but got 0x%08x\n", testCase.expected, out.op1.immediate);
        return 0;
    }

    return 1;
}

int main() {
    size_t faultyCount = 0;
    size_t fixedCount = 0;
    size_t testCasesCount = sizeof(testCases) / sizeof(struct TestCase);

    for(size_t i = 0; i < testCasesCount; i++) {
        struct TestCase c = testCases[i];
        testFaulty(c);
        testFixed(c);
    }
}