尤其是那些虚线，是什么意思？
julia> @code_warntype gcd(4,5)
Body::Int64
31 1 ── %1 = %new(Base.:(#throw1#307))::Core.Compiler.Const(getfield(Base, Symbol("#throw1#307"))(), false)
32 │ %2 = (a === 0)::Bool │╻ ==
└─── goto #3 if not %2 │
2 ── %4 = (Base.flipsign_int)(b, b)::Int64 ││╻ flipsign
└─── return %4 │
33 3 ── %6 = (b === 0)::Bool │╻ ==
└─── goto #5 if not %6 │
4 ── %8 = (Base.flipsign_int)(a, a)::Int64 ││╻ flipsign
└─── return %8 │
34 5 ── %10 = (Base.cttz_int)(a)::Int64 │╻ trailing_zeros
35 │ %11 = (Base.cttz_int)(b)::Int64 │╻ trailing_zeros
36 │ %12 = (Base.slt_int)(%11, %10)::Bool │╻╷ min
│ %13 = (Base.ifelse)(%12, %11, %10)::Int64 ││
37 │ %14 = (Base.sle_int)(0, %10)::Bool │╻╷ >>
│ %15 = (Base.bitcast)(UInt64, %10)::UInt64 ││╻ unsigned
│ %16 = (Base.ashr_int)(a, %15)::Int64 ││╻ >>
│ %17 = (Base.neg_int)(%10)::Int64 ││╻ -
│ %18 = (Base.bitcast)(UInt64, %17)::UInt64 │││╻ reinterpret
│ %19 = (Base.shl_int)(a, %18)::Int64 ││╻ <<
│ %20 = (Base.ifelse)(%14, %16, %19)::Int64 ││
│ %21 = (Base.flipsign_int)(%20, %20)::Int64 ││╻ flipsign
│ %22 = (Base.bitcast)(UInt64, %21)::UInt64 ││╻ reinterpret
38 │ %23 = (Base.sle_int)(0, %11)::Bool │╻╷ >>
│ %24 = (Base.bitcast)(UInt64, %11)::UInt64 ││╻ unsigned
│ %25 = (Base.ashr_int)(b, %24)::Int64 ││╻ >>
│ %26 = (Base.neg_int)(%11)::Int64 ││╻ -
│ %27 = (Base.bitcast)(UInt64, %26)::UInt64 │││╻ reinterpret
│ %28 = (Base.shl_int)(b, %27)::Int64 ││╻ <<
│ %29 = (Base.ifelse)(%23, %25, %28)::Int64 ││
│ %30 = (Base.flipsign_int)(%29, %29)::Int64 ││╻ flipsign
└─── %31 = (Base.bitcast)(UInt64, %30)::UInt64 ││╻ reinterpret
39 6 ┄─ %32 = φ (#5 => %22, #15 => %40)::UInt64 │
│ %33 = φ (#5 => %31, #15 => %61)::UInt64 │
│ %34 = (%32 === %33)::Bool │╻ !=
│ %35 = (Base.not_int)(%34)::Bool ││╻ !
└─── goto #16 if not %35 │
40 7 ── %37 = (Base.ult_int)(%33, %32)::Bool │╻╷ >
└─── goto #9 if not %37 │
8 ── nothing │
43 9 ── %40 = φ (#8 => %33, #7 => %32)::UInt64 │
│ %41 = φ (#8 => %32, #7 => %33)::UInt64 │
│ %42 = (Base.sub_int)(%41, %40)::UInt64 │╻ -
44 │ %43 = (Base.cttz_int)(%42)::UInt64 │╻ trailing_zeros
│ %44 = (Core.lshr_int)(%43, 63)::UInt64 ││╻╷╷ Type
│ %45 = (Core.trunc_int)(Core.UInt8, %44)::UInt8 │││┃││ toInt64
│ %46 = (Core.eq_int)(%45, 0x01)::Bool ││││┃│ check_top_bit
└─── goto #11 if not %46 │││││
10 ─ invoke Core.throw_inexacterror(:check_top_bit::Symbol, UInt64::Any, %43::UInt64)
└─── $(Expr(:unreachable)) │││││
11 ─ goto #12 │││││
12 ─ %51 = (Core.bitcast)(Core.Int64, %43)::Int64 ││││
└─── goto #13 ││││
13 ─ goto #14 │││
14 ─ goto #15 ││
15 ─ %55 = (Base.sle_int)(0, %51)::Bool ││╻ <=
│ %56 = (Base.bitcast)(UInt64, %51)::UInt64 │││╻ reinterpret
│ %57 = (Base.lshr_int)(%42, %56)::UInt64 ││╻ >>
│ %58 = (Base.neg_int)(%51)::Int64 ││╻ -
│ %59 = (Base.bitcast)(UInt64, %58)::UInt64 │││╻ reinterpret
│ %60 = (Base.shl_int)(%42, %59)::UInt64 ││╻ <<
│ %61 = (Base.ifelse)(%55, %57, %60)::UInt64 ││
└─── goto #6 │
46 16 ─ %63 = (Base.sle_int)(0, %13)::Bool │╻╷ <<
│ %64 = (Base.bitcast)(UInt64, %13)::UInt64 ││╻ unsigned
│ %65 = (Base.shl_int)(%32, %64)::UInt64 ││╻ <<
│ %66 = (Base.neg_int)(%13)::Int64 ││╻ -
│ %67 = (Base.bitcast)(UInt64, %66)::UInt64 │││╻ reinterpret
│ %68 = (Base.lshr_int)(%32, %67)::UInt64 ││╻ >>
│ %69 = (Base.ifelse)(%63, %65, %68)::UInt64 ││
48 │ %70 = (Base.ult_int)(0x7fffffffffffffff, %69)::Bool╷ >
│ %71 = (Base.or_int)(false, %70)::Bool ││╻ <
└─── goto #18 if not %71 │
17 ─ invoke %1(_2::Int64, _3::Int64) │
└─── $(Expr(:unreachable)) │
49 18 ─ %75 = (Base.bitcast)(Int64, %69)::Int64 │╻ rem
└─── return %75 │
源代码：
30 function gcd(a::T, b::T) where T<:Union{Int8,UInt8,Int16,UInt16,Int32,UInt32,Int64,UInt64,Int128,UInt128}
31 @noinline throw1(a, b) = throw(OverflowError("gcd($a, $b) overflows"))
32 a == 0 && return abs(b)
33 b == 0 && return abs(a)
34 za = trailing_zeros(a)
35 zb = trailing_zeros(b)
36 k = min(za, zb)
37 u = unsigned(abs(a >> za))
38 v = unsigned(abs(b >> zb))
39 while u != v
40 if u > v
41 u, v = v, u
42 end
43 v -= u
44 v >>= trailing_zeros(v)
45 end
46 r = u << k
47 # T(r) would throw InexactError; we want OverflowError instead
48 r > typemax(T) && throw1(a, b)
49 r % T
50 end
答案 0 :(得分：2)
https://github.com/JuliaLang/julia/blob/master/base/compiler/ssair/show.jl#L170对此进行了详细说明。
总结：