[llvm-dev] LLVM IR temporary variable reuse

[Ansar K.A. via llvm-dev]

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> wrote:

>
>
> On 18 March 2016 at 06:31, Ansar K.A. via llvm-dev <
> 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> 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> 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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