[llvm-dev] [IR question] Switching on pointers

[Benoit Vey via llvm-dev]

Hi.

First of all, some context. I'm trying to implement a new  
functionality in an LLVM-based compiler and I need to take various  
actions based on the value of a given pointer, the possible values  
being the addresses of various global constants. I tried to use a  
`switch` instruction but I encountered several problems.

The "ideal switch" I'd like to have would look something like this  
(with way more cases):

     ; ModuleID = 'module'
     source_filename = "module"

     @var0 = global i32 0
     @var1 = global i32 0

     define i32 @func(i32*) {
     entry:
       switch i32* %0, label %unreachable [
         i32* @var0, label %var0
         i32* @var1, label %var1
       ]

     var0:                                             ; preds = %entry
       br label %post

     var1:                                             ; preds = %entry
       br label %post

     unreachable:                                      ; preds = %entry
       unreachable

     post:                                             ; preds = %var1, %var0
       %1 = phi i32 [ 0, %var0 ], [ 1, %var1 ]
       ret i32 %1
     }

This example is impossible because a `switch` cannot have pointer  
operands. So I tried with `ptrtoint`, turning the `switch` into this:

       %1 = ptrtoint i32* %0 to i64
       switch i64 %1, label %unreachable [
         i64 ptrtoint (i32* @var0 to i64), label %var0
         i64 ptrtoint (i32* @var1 to i64), label %var1
       ]

I'm a bit baffled by that one. According to the documentation the  
address of a global value is a constant, yet this `switch` is  
impossible because the result of the `ptrtoint` isn't a constant  
integer.

At that point, I don't really know how to proceed. My questions are:

- Why is `switch` restricted to integers?
- What is the result of a constant `ptrtoint` and why isn't it a  
constant integer?
- Is there a way to switch on pointers despite these restrictions?

Thanks.

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