/* -- ARM64 Relocations -------------------------------------------------------------------- */

/*
 * What Apple says is a lie.
 *
 * They claim in $WHEREEVERTHEYHIDEIT/usr/include/mach-o/arm64/reloc.h:
 *
 * | ARM64_RELOC_PAGEOFF12                // offset within page, scaled by r_length
 *
 * That's not true. I see r_length = 2 always. But some instructions are ADD
 * instruction needing a scale of 1 while some are LDR instructions needing
 * a scale of 8.
 *
 * So we need to look at the opcode. Below is a table that I gained by
 * measuring ld(1).
 */

static int arm64_insn_scale (uint32_t insn) {
    /* Measure from what Apple's ld does */
    switch (insn & 0xFFC00000) {
    case 0x39000000: /* strb     w0, [x0, #0x20]      */ return 1;
    case 0x39400000: /* ldrb     w0, [x0, #0x20]      */ return 1;
    case 0x39800000: /* ldrsb    x0, [x0, #0x20]      */ return 1;
    case 0x39c00000: /* ldrsb    w0, [x0, #0x20]      */ return 1;
    case 0x3d000000: /* str      b0, [x0, #0x20]      */ return 1;
    case 0x3d400000: /* ldr      b0, [x0, #0x20]      */ return 1;
    case 0x3d800000: /* str      q0, [x0, #0x20]      */ return 16;
    case 0x3dc00000: /* ldr      q0, [x0, #0x20]      */ return 16;
    case 0x79000000: /* strh     w0, [x0, #0x20]      */ return 2;
    case 0x79400000: /* ldrh     w0, [x0, #0x20]      */ return 2;
    case 0x79800000: /* ldrsh    x0, [x0, #0x20]      */ return 2;
    case 0x79c00000: /* ldrsh    w0, [x0, #0x20]      */ return 2;
    case 0x7d000000: /* str      h0, [x0, #0x20]      */ return 2;
    case 0x7d400000: /* ldr      h0, [x0, #0x20]      */ return 2;
    case 0x7d800000: /* .long    0x7d804000           */ return 2;
    case 0x7dc00000: /* .long    0x7dc04000           */ return 2;
    case 0x91000000: /* add      x0, x0, #0x20        */ return 1;
    case 0xb9000000: /* str      w0, [x0, #0x20]      */ return 4;
    case 0xb9400000: /* ldr      w0, [x0, #0x20]      */ return 4;
    case 0xb9800000: /* ldrsw    x0, [x0, #0x20]      */ return 4;
    case 0xbd000000: /* str      s0, [x0, #0x20]      */ return 4;
    case 0xbd400000: /* ldr      s0, [x0, #0x20]      */ return 4;
    case 0xbd800000: /* .long    0xbd802000           */ return 4;
    case 0xf9000000: /* str      x0, [x0, #0x20]      */ return 8;
    case 0xf9400000: /* ldr      x0, [x0, #0x20]      */ return 8;
    case 0xfd000000: /* str      d0, [x0, #0x20]      */ return 8;
    case 0xfd400000: /* ldr      d0, [x0, #0x20]      */ return 8;
    default: return -1;
    }
}

static natural arm64_handle_reloc (struct relfile_cx *cx,
                                   natural P, natural S, natural A, 
                                   struct relocation_info *reloc) {
#define PAGE(X) ((X) >> 12)
    natural new_addend = 0;
    int new_has_addend = 0;

    natural V = A;              /* what we come up with */

    /* Care for PC relative or not and the addend. */
    natural PC = reloc->r_pcrel ? P : 0;
    S += cx->addend;

    switch (reloc->r_type) {
    case ARM64_RELOC_UNSIGNED: {
        V = S + A - PC;
    } break;

    case ARM64_RELOC_BRANCH26: {
        /* Addend really first? */
        V = S - PC;
        if (sex(28,V) != (snatural)V) {
            S = find_trampo(cx, S); }
        V = S - PC;
        if (sex(28,V) != (int64_t)V) goto range_blame;
        V = dpb(ldb(26,2, V), 26, 0, A);
    } break;

    case ARM64_RELOC_PAGEOFF12: {
        /* Measure says r_pcrel = 0, r_length = 2, r_extern = 1 is the only supported combination. */
        V = S - PC;
        int sc = arm64_insn_scale(A);
        if (sc <= 0) goto scale_blame;
        V = (V & ((1<<12)-1)) / sc;
        V = dpb(ldb(12, 0, V), 12, 10, A);
    } break;

    case ARM64_RELOC_PAGE21: {
        assert(reloc->r_pcrel);
        V = PAGE(S) - PAGE(PC);
        if (sex(21, V) != (snatural)V) goto range_blame;
        A = dpb(ldb(19, 2, V), 19, 5, A); /* immhi */
        A = dpb(ldb(2, 0, V), 2, 29, A);  /* immlo */
        V = A;
    } break;

    case ARM64_RELOC_GOT_LOAD_PAGEOFF12: {
        S = find_got(cx, S);
        V = S - PC;
        int sc = arm64_insn_scale(A);
        if (sc <= 0) goto scale_blame;
        V = (V & ((1<<12)-1)) / sc;
        V = dpb(ldb(12, 0, V), 12, 10, A);
    } break;

    case ARM64_RELOC_GOT_LOAD_PAGE21: {
        S = find_got(cx, S);
        V = PAGE(S) - PAGE(PC);
        A = dpb(ldb(19, 2, V), 19, 5, A); /* immhi */
        A = dpb(ldb(2, 0, V), 2, 29, A);  /* immlo */
        V = A;
    } break;

    case ARM64_RELOC_ADDEND: { /* xxx */
        new_addend = reloc->r_symbolnum;
        new_has_addend = 1;
    } break;

    case ARM64_RELOC_SUBTRACTOR: { /* xxx */
        /* This is a symbol rather! I wonder what r_extern is. */
        new_addend = -reloc->r_symbolnum;
        new_has_addend = 1;
    } break;

    default:
        goto punt;
    }

    cx->has_addend = new_has_addend;
    cx->addend = new_addend;

    return V;

 punt:
    error(cx, "\nRelocation type %d (%s) unbeknownst yet.",
          reloc->r_type, r_type_name(cx, reloc->r_type));

 scale_blame:
    error(cx, "\n **** Scale unknown for insn = %08"PRIx32"\n", (uint32_t)A);

 range_blame:
    error(cx, "\nRelocation value 0x%"PRIx64" too large for relocation of type %d (%s).",
          V, reloc->r_type, r_type_name(cx, reloc->r_type));
#undef PAGE
}