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

[Daniel Sanders via llvm-dev]

> On 15 May 2017, at 17:29, Benoit Vey via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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.

I think the confusion stems from there being multiple kinds of constants. The switch instruction wants to know the value of the constant at compile-time but while the address of a global is constant, it isn't known until link-time.

> 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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