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

[Jeremy Lakeman via llvm-dev]

IMHO, don't try to tell llvm about your registers, instead create a mapping
between the last Value* assigned to each register for each basic block.

For example, here's a crude suggestion.

First pass, identify all of the branch destinations from your custom IR,
where you will need to create an llvm basic block.

Second pass, translate IR instructions into llvm IR for each block.
 - keep track of the last Value* stored in each register.
 - if you need to load a Value* from a register that hasn't been assigned
yet in this block, create a phi node and insert it at the start of the
block.

Third pass, follow branches and link up phi nodes.


Or you could simply generate code as if each register is on the stack.
Generating alloca, load & store. Then run the mem2reg pass to promote
everything back to virtual registers for you automatically. That's probably
easier to get right.






On Mon, Feb 8, 2016 at 9:30 AM, 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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