[llvm-dev] LLVM IR temporary variable reuse

Fri Mar 18 03:11:34 PDT 2016

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 <mailto: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
>> <mailto:mats at planetcatfish.com>> wrote:
>>
>>
>>
>>     On 18 March 2016 at 06:31, Ansar K.A. via llvm-dev
>>     <llvm-dev at lists.llvm.org <mailto: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
>>         <mailto: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
>>             <mailto: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 ?/ /
>>
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>>
>>
>>
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