UInt can't hold (2^63)+1

run.sh

amos@localhost option-test $ swiftc uint.swift && ./main
2^62 = 4611686018427387904
2^63 - 1 = 9223372036854775807
Illegal instruction

# Running with -Ounchecked gives wrong result.. (last two numbers are same)
amos@localhost option-test $ swiftc tx1-uint.swift -Ounchecked  && ./main
2^62 = 4611686018427387904
2^63 - 1 = 9223372036854775807
2^63 = 9223372036854775807

uint.s

// Cleaned up assembly, with no prints
main:
    .cfi_startproc
    pushq   %rbp
    movq    %rsp, %rbp
    pushq   %rbx
    pushq   %rax
    movq    %rsi, %rbx
    movq    _TZvOs7Process5_argcVs5Int32@GOTPCREL(%rip), %rax
    movl    %edi, (%rax)
    movq    globalinit_33_1BDF70FFC18749BAB495A73B459ED2F0_token4@GOTPCREL(%rip), %rdi
    movq    globalinit_33_1BDF70FFC18749BAB495A73B459ED2F0_func4@GOTPCREL(%rip), %rsi
    callq   swift_once@PLT
    movq    _TZvOs7Process11_unsafeArgvGSpGSpVs4Int8__@GOTPCREL(%rip), %rax
    movq    %rbx, (%rax)
    xorl    %ecx, %ecx
    movl    $1, %eax
    movq    _Tv4main3numSi@GOTPCREL(%rip), %rdx
    movq    _Tv4main2itSi@GOTPCREL(%rip), %rsi
.LBB0_1:
    addq    %rax, %rax
// Jump if addq overflows
// This treats rax as a signed int64, so going from 2^63 - 1 to 2^63 is treated as an overflow
    jo  .LBB0_8
    movq    %rax, (%rdx)
    incq    %rcx
    jo  .LBB0_8
    movq    %rcx, (%rsi)
    cmpq    $62, %rcx
    jl  .LBB0_1
    movq    %rax, %rcx
    decq    %rcx
// the overflow flag will be set for 2^63 - 1, but not for 0 - 1.
    jo  .LBB0_8
    movq    _Tv4main13num_minus_oneSi@GOTPCREL(%rip), %rdx
    movq    %rcx, (%rdx)
    addq    %rcx, %rax
    jo  .LBB0_8
    movq    _Tv4main12num_one_lessSi@GOTPCREL(%rip), %rcx
    movq    %rax, (%rcx)
    incq    %rax
    jo  .LBB0_8
    movq    _Tv4main6num_63Si@GOTPCREL(%rip), %rcx
    movq    %rax, (%rcx)
    xorl    %eax, %eax
    addq    $8, %rsp
    popq    %rbx
    popq    %rbp
    retq
// "Illegal instruction"
.LBB0_8:
    ud2

uint.swift

var it = 0
var num = 1
while it < 62 {
    num = num * 2
    it = it + 1
}
print("2^62 = \(num)")

var num_minus_one = num - 1
var num_one_less = num + num_minus_one

print("2^63 - 1 = \(num_one_less)")

var num_63 = num_one_less + 1

print("2^63 = \(num_63)")