nextInner

fn nextInner(unwinder: *SelfUnwinder, gpa: Allocator, cache_entry: *const CacheEntry) !usize {
    const format = cache_entry.cie.format;

    const cfa = switch (cache_entry.cfa_rule) {
        .none => return error.InvalidDebugInfo,
        .reg_off => |ro| cfa: {
            const ptr = try regNative(&unwinder.cpu_state, ro.register);
            break :cfa try applyOffset(@intCast(ptr.*), ro.offset);
        },
        .expression => |expr| cfa: {
            // On most implemented architectures, the CFA is defined to be the previous frame's SP.
            //
            // On s390x, it's defined to be SP + 160 (ELF ABI s390x Supplement §1.6.3); however,
            // what this actually means is that there will be a `def_cfa r15 + 160`, so nothing
            // special for us to do.
            const prev_cfa_val = (try regNative(&unwinder.cpu_state, sp_reg_num)).*;
            unwinder.expr_vm.reset();
            const value = try unwinder.expr_vm.run(expr, gpa, .{
                .format = format,
                .cpu_context = &unwinder.cpu_state,
            }, @intCast(prev_cfa_val)) orelse return error.InvalidDebugInfo;
            switch (value) {
                .generic => |g| break :cfa g,
                else => return error.InvalidDebugInfo,
            }
        },
    };

    // Create a copy of the CPU state, to which we will apply the new rules.
    var new_cpu_state = unwinder.cpu_state;

    // On all implemented architectures, the CFA is defined to be the previous frame's SP
    (try regNative(&new_cpu_state, sp_reg_num)).* = cfa;

    const return_address_register = cache_entry.cie.return_address_register;
    var has_return_address = true;

    const rules_len = cache_entry.num_rules;
    for (cache_entry.rules_regs[0..rules_len], cache_entry.rules[0..rules_len]) |register, rule| {
        const new_val: union(enum) {
            same,
            undefined,
            val: std.debug.cpu_context.Native.Gpr,
            bytes: []const u8,
        } = switch (rule) {
            .default => val: {
                // The way things are supposed to work is that `.undefined` is the default rule
                // unless an ABI says otherwise (e.g. aarch64, s390x).
                //
                // Unfortunately, at some point, a decision was made to have libgcc's unwinder
                // assume `.same` as the default for all registers. Compilers then started depending
                // on this, and the practice was carried forward to LLVM's libunwind and some of its
                // backends.
                break :val .same;
            },
            .undefined => .undefined,
            .same_value => .same,
            .offset => |offset| val: {
                const ptr: *const std.debug.cpu_context.Native.Gpr = @ptrFromInt(try applyOffset(cfa, offset));
                break :val .{ .val = ptr.* };
            },
            .val_offset => |offset| .{ .val = try applyOffset(cfa, offset) },
            .register => |r| .{ .bytes = try unwinder.cpu_state.dwarfRegisterBytes(r) },
            .expression => |expr| val: {
                unwinder.expr_vm.reset();
                const value = try unwinder.expr_vm.run(expr, gpa, .{
                    .format = format,
                    .cpu_context = &unwinder.cpu_state,
                }, cfa) orelse return error.InvalidDebugInfo;
                const ptr: *const usize = switch (value) {
                    .generic => |addr| @ptrFromInt(addr),
                    else => return error.InvalidDebugInfo,
                };
                break :val .{ .val = ptr.* };
            },
            .val_expression => |expr| val: {
                unwinder.expr_vm.reset();
                const value = try unwinder.expr_vm.run(expr, gpa, .{
                    .format = format,
                    .cpu_context = &unwinder.cpu_state,
                }, cfa) orelse return error.InvalidDebugInfo;
                switch (value) {
                    .generic => |val| break :val .{ .val = val },
                    else => return error.InvalidDebugInfo,
                }
            },
        };
        switch (new_val) {
            .same => {},
            .undefined => {
                const dest = try new_cpu_state.dwarfRegisterBytes(@intCast(register));
                @memset(dest, undefined);

                // If the return address register is explicitly set to `.undefined`, it means that
                // there are no more frames to unwind.
                if (register == return_address_register) {
                    has_return_address = false;
                }
            },
            .val => |val| (try regNative(&new_cpu_state, register)).* = val,
            .bytes => |src| {
                const dest = try new_cpu_state.dwarfRegisterBytes(@intCast(register));
                if (dest.len != src.len) return error.InvalidDebugInfo;
                @memcpy(dest, src);
            },
        }
    }

    const return_address = if (has_return_address)
        stripInstructionPtrAuthCode(@intCast((try regNative(&new_cpu_state, return_address_register)).*))
    else
        0;

    (try regNative(&new_cpu_state, ip_reg_num)).* = return_address;

    // The new CPU state is complete; flush changes.
    unwinder.cpu_state = new_cpu_state;

    // The caller will subtract 1 from the return address to get an address corresponding to the
    // function call. However, if this is a signal frame, that's actually incorrect, because the
    // "return address" we have is the instruction which triggered the signal (if the signal
    // handler returned, the instruction would be re-run). Compensate for this by incrementing
    // the address in that case.
    const adjusted_ret_addr = if (cache_entry.cie.is_signal_frame) return_address +| 1 else return_address;

    // We also want to do that same subtraction here to get the PC for the next frame's FDE.
    // This is because if the callee was noreturn, then the function call might be the caller's
    // last instruction, so `return_address` might actually point outside of it!
    unwinder.pc = adjusted_ret_addr -| 1;

    return adjusted_ret_addr;
}