# Indentifying pathological cases in IR form

**URL:** https://discourse.julialang.org/t/indentifying-pathological-cases-in-ir-form/48746
**Category:** General Usage
**Tags:** metaprogramming
**Created:** [October 21, 2020, 1:33pm UTC](https://discourse.julialang.org/t/indentifying-pathological-cases-in-ir-form/48746 "2020-10-21T13:33:54Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![wulpuqu](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wulpuqu/32/17640_2.png) [@wulpuqu](https://discourse.julialang.org/u/wulpuqu)
#### Post date: [October 21, 2020, 1:33pm UTC](https://discourse.julialang.org/t/indentifying-pathological-cases-in-ir-form/48746/1 "2020-10-21T13:33:54Z")

</div>

Hello everyone,

I am currently playing with lowered code and I would like to understand how to identify pathological cases. In the following example, a variable `b` gets conditionnaly assigned at step `1` of the loop and then used to double the value of `a` at other steps.

When we look at the lowered code below (with `Meta.lower(Main,e)`), we see at step `15` that a `SlotNumber` for the variable `b` is assigned and used at step `17`, so by looking at the lowered code it seems that we shouldn’t get an error when executing it.

I understand why we get an error during execution since simply changing the condition to `i!=1` gives a pathological case. So in general this code is wrong since we need execution to get the value for the branching at step 12.

So my question is: Does Julia provide another representation showing that this code is pathological ?

I tried looking at `code_typed(f,())` or `code_llvm(f,();optimize=false)` but before going further I would like to know your thoughts about this.

```julia
julia> e=quote function f()
           a=1
           for i in 1:10
               if i==1
                   a+=1
                   b=2
               else
                   a*=b
               end
           end
           a
       end end;

julia> eval(e)();
ERROR: UndefVarError: b not defined
Stacktrace:
 [1] f() at .\REPL[43]:8
 [2] top-level scope at REPL[44]:1

julia> collect(enumerate(getci(e).code)) 
23-element Array{Tuple{Int64,Any},1}:
 (1, :(a = 1))
 (2, :(1:10))
 (3, :(_1 = Base.iterate(%2)))
 (4, :(_1 === nothing))
 (5, :(Base.not_int(%4)))
 (6, :(unless %5 goto %23))
 (7, Core.NewvarNode(:(_3)))
 (8, Core.NewvarNode(:(_4)))
 (9, :(_1))
 (10, :(_2 = Core.getfield(%9, 1)))
 (11, :(Core.getfield(%9, 2)))
 (12, :(_2 == 1))
 (13, :(unless %12 goto %17))
 (14, :(_4 = _4 + 1))
 (15, :(_3 = 2))
 (16, :(goto %18))
 (17, :(_4 = _4 * _3))
 (18, :(_1 = Base.iterate(%2, %11)))
 (19, :(_1 === nothing))
 (20, :(Base.not_int(%19)))
 (21, :(unless %20 goto %23))
 (22, :(goto %7))
 (23, :(return a))

```