[llvm-dev] LLVM IR temporary variable reuse

[Ansar K.A. via llvm-dev]

Yes. Thank You.

On 18 March 2016 at 15:41, Stefan Gränitz <llvm-dev at lists.llvm.org> wrote:

> Hi Ansar
>
> But, If in case LLVM is reusing *%sub *in between to hold some other
> computations, then there will be some errors in my computation. So I want
> to make sure that LLVM won't reuse any of these virtual registers.
>
> The simple answer is: yes, you can be sure that %sub will not be
> reassigned, it's immutable.
> Anyway, as Mehdi mentioned it's good to read the docs.
>
> Best,
> Stefan
>
> Am 18.03.16 um 11:06 schrieb Mehdi Amini via llvm-dev:
>
> Have you read the SSA link that David sent earlier? (i.e.
> https://en.wikipedia.org/wiki/Static_single_assignment_form )
>
> There is no such thing as a "LLVM register", or "variables".
> The main concepts are the memory, and  *values* (like %sub in your
> example). The latter are *immutable*.
>
> --
> Mehdi
>
> On Mar 18, 2016, at 2:49 AM, Ansar K.A. via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> For eg:
>
> c=a-b is represented using the following statements in IR:
>
>   %tmp = load i32, i32* %a, align 4
>   %tmp1 = load i32, i32* %b, align 4
>   %sub = sub nsw i32 %tmp, %tmp1
>   store i32 %sub, i32* %c, align 4
>
>
> whenever there is a redundant computation of the same, say k=a-b (if the
> value of *a* and *b* are not changed) the code will be like this:
>
>   %tmp2 = load i32, i32* %a, align 4
>   %tmp3 = load i32, i32* %b, align 4
>   %sub1 = sub nsw i32 %tmp2, %tmp3
>   store i32 %sub1, i32* %k, align 4
>
> But this is not required, Since *a-b* is already there in that virtual
> register *%sub. *So I can rewrite the highlighted code ( eliminating
> redundancy ) as
>
> * store i32 %sub, i32* %k, align 4*
>
> and can delete the statement *%sub1 = sub nsw i32 %tmp2, %tmp3 *
>
> But, If in case LLVM is reusing *%sub *in between to hold some other
> computations, then there will be some errors in my computation. So I want
> to make sure that LLVM won't reuse any of these virtual registers.
>
> I hope it clarifies.
>
>
>
> On 18 March 2016 at 13:39, mats petersson < <mats at planetcatfish.com>
> mats at planetcatfish.com> wrote:
>
>>
>>
>> On 18 March 2016 at 06:31, Ansar K.A. via llvm-dev <
>> <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org> wrote:
>>
>>> I meant LLVM registers. Thanks for the correcting me.
>>>
>>
>> But registers in LLVM-IR are in infinite supply, since the are virtual
>> registers. Real registers appear when the IR is translated to machine
>> instructions, and this is based on the live-range of the actual variables,
>> so the same register will be re-used within the machine code. Reusing a
>> virtual register is BAD because it will confuse the IR into thinking that
>> your code is using the same thing for longer, and thus, potentially, make
>> it use the same hardware register when two different ones could have been
>> used.
>>
>> Can you please explain what it is you want to achieve, as a bigger
>> picture? Ideally with some example of some source, its generated IR and how
>> you want it to be different?
>>
>> --
>> Mats
>>
>>>
>>> On 18 March 2016 at 10:42, David Blaikie < <dblaikie at gmail.com>
>>> dblaikie at gmail.com> wrote:
>>>
>>>> Question is hard to understand - the registers in LLVM are in Static
>>>> Single Assignment form
>>>> <https://en.wikipedia.org/wiki/Static_single_assignment_form>, they're
>>>> not variables that can be assigned and reassigned values (so the answer to
>>>> your question is probably "no"). It's best to look at what Clang does to
>>>> see how IR can be used to represent constructs in C you may be more
>>>> familiar with.
>>>>
>>>> On Thu, Mar 17, 2016 at 10:05 PM, Ansar K.A. via llvm-dev <
>>>> <llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> In any case, Is there any chance for reusing *temporary variable *used
>>>>> in it's IR by LLVM ?
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>
>>>
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>>>
>>
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