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

[George Burgess IV via llvm-dev]

Hi!

I don't know what "the right way" to do this is, but is there any reason
you're against just using alloca/load/store? That's what clang emits for
most locals/parameters/..., and LLVM tends to do a very good job of
SSAifying that where it can. :)

George

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.
>
> Regards,
> Paul
>
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>
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