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

[Paul Peet via llvm-dev]

This looks much better than using "select i1 true, i32 1, i32 0".
But since something like that (bitcast constant) is allowed why isn't it
possible to simply do this:
%val = i32 1
?

2016-02-08 7:45 GMT+01:00 David Majnemer <david.majnemer at gmail.com>:

>
>
> 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
>>
>>
>
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