[llvm-dev] Assigning constant value without alloca/load/store

[David Majnemer via llvm-dev]

On Sun, Feb 7, 2016 at 3:00 PM, Paul Peet via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
> I am currently trying to translate some custom IR to LLVM-IR and came
> across and issue.
> The custom IR has several registers and I am basically try to SSAfy it so
> it can be easily translated/converted to LLVM-IR.
>
> The problem:
>
> Since in my custom IR I can reassign every register I have to reassign
> every new expression with a new llvm Value. But my IR has something like
> this:
>
> REG A = VAR C + CONST 2
> REG A = CONST 12
>
> So my workaround looks like:
>
> ; I am returning the registers in an anonymous struct
> define { i32, i32, i32 } @test(i32 %var_c) {
>   ; Initializing registers
>   %reg_a_0 = select i1 true, i32 0, i32 0
>   %reg_b_0 = select i1 true, i32 0, i32 0
>   %reg_c_0 = select i1 true, i32 0, i32 0
>
>   ; Translated instructions
>   %reg_a_1 = add i32 %var_c, 2
>   %reg_a_2 = select i1 true, i32 12, i32 0
>
>   ; Prepare return values
>   %ret_0 = insertvalue { i32, i32, i32 } undef, i32 %reg_a_2, 0
>   %ret_1 = insertvalue { i32, i32, i32 } %ret_0, i32 %reg_b_0, 1
>   %ret_2 = insertvalue { i32, i32, i32 } %ret_1, i32 %reg_c_0, 2
>
>   ret { i32, i32, i32 } %ret_2
> }
>
> I am basically using "select i1 true, i32 1, i32 0" so after optimization
> it gets:
> %val = i32 1
>
> But as I said this looks like a hack to me and I can't simply use "%val =
> i32 1".
> So what's the proper way to do this without actually using
> alloca/load/store.
>

You can use trivial bitcasts if you want to avoid load/store/alloca and you
want SSA variables for your constants:
  %val = bitcast i32 1 to i32


>
> Regards,
> Paul
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160207/fea322a7/attachment-0001.html>