[LLVMdev] Simple question

[Bill Wendling]

On Mar 15, 2013, at 3:08 PM, James Courtier-Dutton <james.dutton at gmail.com> wrote:

> Hi,
> 
> I think this is a very simple question, and it must just be missing something.
> 
> I am looking for find out how to assign a constant integer value to
> the variable in llvm ir.
> 
> The following returns 12, and %var2 = 12.
> ; ModuleID = 't.c'
> target datalayout =
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target triple = "x86_64-pc-linux-gnu"
> 
> define i32 @test() nounwind readnone {
>  %var1 = xor i32 0, 0
>  %var2 = add i32 %var1, 12
>  ret i32 %var2
> }
> 
> Why can't I do?:
> ; ModuleID = 't.c'
> target datalayout =
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target triple = "x86_64-pc-linux-gnu"
> 
> define i32 @test() nounwind readnone {
>  %var2 = 12
>  ret i32 %var2
> }
> 
Because an assignment of a constant to a variable isn't an instruction. Or more to the point, the right-hand-side of "%var2 = 12" isn't an instruction.

> What is the simplest way to make %var2 = 12   ?
> 
I think that this should work:

define i32 @test() {
  %v = bitcast i32 42 to i32
  ret i32 %v
}

-bw