llvm dev - Oct 2012 - [LLVMdev] Immediate instructions / register allocator

Hi,

I tried transforming code that loads an immediate into a virtual register, and
then uses that virtual register in various ways, into code that uses immediate
operands instead.

To my surprise, this made the register allocator produce worse code in many
cases. It seems that it is splitting live intervals and spilling more. In some
cases the code was better.

I wonder how this could be? Generally, using the immediates directly as constant
operands should decrease regpressure, right? How could the RA-algorithm have
been mislead by this?

Thanks,

Jonas

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On Mon, Oct 22, 2012 at 3:38 AM, Jonas Paulsson
<jonas.paulsson at ericsson.com> wrote:
> Hi,
>
>
>
> I tried transforming code that loads an immediate into a virtual register,
> and then uses that virtual register in various ways, into code that uses
> immediate operands instead.
I suspect a concrete example might be helpful in demonstrating the
specific issue.
> To my surprise, this made the register allocator produce worse code in many
> cases. It seems that it is splitting live intervals and spilling more. In
> some cases the code was better.
>
>
>
> I wonder how this could be? Generally, using the immediates directly as
> constant operands should decrease regpressure, right? How could the
> RA-algorithm have been mislead by this?
>
>
>
> Thanks,
>
>
>
> Jonas
>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

Samsung R&D center at San Jose, CA, has a full time position open.  The
focus will be to work on a Translator/Instrumenter/Validator for WebCL
kernels.  The job description is attached for reference.  People interested
in the position can send their information to me.

WebCL provides JavaScript bindings to OpenCL APIs, and the standard is
currently being defined by Khronos, under the WebCK working group.  There
are currently three open source implementations of WebCL.

Noted below are some WebCL related resources:
The WebCL public draft can be found at:
https://cvs.khronos.org/svn/repos/registry/trunk/public/webcl/spec/latest/in
dex.html
Samsung's prototype (BSD license) source code:
http://code.google.com/p/webcl
Public WebCL distribution list: public_webcl at khronos.org

Regards,
Tasneem Brutch

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