[LLVMdev] Call to address 0 gets removed

[Dale Johannesen]

On Jun 8, 2009, at 4:48 PMPDT, Marius Wachtler wrote:

> Hello
>
> If I optimize (opt -std-compile-opts ) the following .ll
>
> ; ModuleID = 'call0.ll'
> target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32- 
> i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64- 
> f80:32:32"
> target triple = "i386-mingw32"
>
> define i32 @t(i32 %a) nounwind {
> entry:
>     %0 = tail call i32 inttoptr (i32 0 to i32 (i32)*)(i32 %a)  
> nounwind        ; <i32> [#uses=1]
>     ret i32 %0
> }
>
> The call to address 0 gets removed.
>
> define i32 @t(i32 %a) noreturn nounwind readnone {
> entry:
>     unreachable
> }
>
> How can I prevent that the call is removed, without making the  
> function addr volatile?
> Does anyone know which optimization removes it?

Calling 0 is undefined behavior; the optimizer is within its rights to  
remove this.  Why do you want to call 0?