llvm dev - Oct 2012 - [LLVMdev] Does LLVM optimize recursive call?

Hi list,

I have a simple question about LLVM.

I learned that we need to use iterations than recursions in C programming.
That is because recursion is expensive. It can easily consume out all
the stack given to a program. And the call/return consumes time much
more.

But I've read a sentence that nowadays compilers can optimize
recursions in C or whatever languages not to consume heavy stack and
call/return.

I wonder what kind of optimization this is and if LLVM support this
kind of optimization.

I am not a compiler expert. Please consider this. ^^;

Thank you in advance

-- 
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Journeyer J. Joh
o o s a p r o g r a m m e r
a t
g m a i l  d o t  c o m
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On Tue, Oct 2, 2012 at 11:44 PM, Journeyer J. Joh
<oosaprogrammer at gmail.com> wrote:
> Hi list,
>
> I have a simple question about LLVM.
>
> I learned that we need to use iterations than recursions in C programming.
> That is because recursion is expensive. It can easily consume out all
> the stack given to a program. And the call/return consumes time much
> more.
>
> But I've read a sentence that nowadays compilers can optimize
> recursions in C or whatever languages not to consume heavy stack and
> call/return.
>
> I wonder what kind of optimization this is and if LLVM support this
> kind of optimization.
>
> I am not a compiler expert. Please consider this. ^^;
The short answer is: sometimes.

Some recursive code may be transformed into something similar to a
loop, but sometimes the compiler won't be able to figure it out &
it'll remain recursive.

- David

Hi David Blaikie and others who might be interested on this

Thank you very much!

#1. Then I'd like to know, to make Clang/LLVM optimize a recursion
into an iteration, how a recursion has to be implemented with any
compiler option? (if the language is C/C++)

Clang uses recursions, especially it uses recursive decent and
operator-precedence parser.
#2. I wonder if this kind of recursion is optimized to a iteration.

Sorry my question is getting deeper. ^^;

Thank you very much in advance

Journeyer J. Joh



2012/10/3 David Blaikie <dblaikie at gmail.com>:
> On Tue, Oct 2, 2012 at 11:44 PM, Journeyer J. Joh
> <oosaprogrammer at gmail.com> wrote:
>> Hi list,
>>
>> I have a simple question about LLVM.
>>
>> I learned that we need to use iterations than recursions in C
programming.
>> That is because recursion is expensive. It can easily consume out all
>> the stack given to a program. And the call/return consumes time much
>> more.
>>
>> But I've read a sentence that nowadays compilers can optimize
>> recursions in C or whatever languages not to consume heavy stack and
>> call/return.
>>
>> I wonder what kind of optimization this is and if LLVM support this
>> kind of optimization.
>>
>> I am not a compiler expert. Please consider this. ^^;
>
> The short answer is: sometimes.
>
> Some recursive code may be transformed into something similar to a
> loop, but sometimes the compiler won't be able to figure it out &
> it'll remain recursive.
>
> - David


-- 
----------------------------------------
Journeyer J. Joh
o o s a p r o g r a m m e r
a t
g m a i l  d o t  c o m
----------------------------------------