[LLVMdev] [LLVM, loop-unswitch tests] preserve-analyses.ll, strange PHI instruction.

[Stepan Dyatkovskiy]

OK. Thanks. And if I removed "case 4:" for example. Need I scan all phi 
nodes and fix it, by removing one predecessor (if it exists) to switch's 
parent block?

-Stepan.

Nick Lewycky wrote:
> Stepan Dyatkovskiy wrote:
>> Hi all.
>>
>> Looking at test/Transforms/LoopUnswitch/preserve-analyses.ll, I found
>> improper phi instruction at string #122:
>> %call.i25.lcssa48 = phi i8* [ %call.i25, %if.then31.i.i ], [ %call.i25,
>> %if.then31.i.i ] ;<i8*> [#uses=0]
>> Is it trick or mistake?
>
> What's improper about it?
>
> The block may have two predecessors, both of which are the same block
> (consider a switch statement with "case 3: case 4: code;"). This is
> called a critical edge. The requirement in the IR is that a PHI node
> have an entry for each pred (including duplicates for critical edges)
> and that the incoming value must be the same.
>
> The best test is to ask the verifier (opt -verify). It knows all the
> special rules, like why this code is legal:
>
> define i32 @test() {
> unreachable
>
> %a = add i32 %a, %a
> ret i32 %a
> }
>
> Surprise! That one's my favourite example.
>
> Nick