llvm dev - Oct 2014 - [LLVMdev] Switch instruction lowering

Hi,

I'm interested in any information about implementations of switch
instruction and its runtime cost. If it's very target dependent, I'm
mostly
care about X86. Pointing some LLVM code is also good.

- Paweł
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On Mon, Oct 27, 2014 at 10:54:12AM +0100, Paweł Bylica
wrote:
> Hi,
> 
> I'm interested in any information about implementations of switch
instruction
> and its runtime cost. If it's very target dependent, I'm mostly
care about X86.
> Pointing some LLVM code is also good.
  `grep -r "switch instruction"` under directory llvm/lib shows a lot
of stuff you
might have interest. Most of the code related to switch instructions are under 
subdirectories IR and Transformation/Utils.

HTH,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Homepage: http://people.cs.nctu.edu.tw/~chenwj

On Mon, Oct 27, 2014 at 2:54 AM, Paweł Bylica <chfast at gmail.com>
wrote:
> Hi,
>
> I'm interested in any information about implementations of switch
> instruction and its runtime cost. If it's very target dependent,
I'm mostly
> care about X86. Pointing some LLVM code is also good.
I don't think there's any documentation besides the comments in the
code, but switch lowering (to binary tree or jump table, etc.) happens
in SelectionDAGBuilder::visitSwitch()

For switches which are used to select from a set of constant values,
SimplifyCFG will attempt to build a lookup table.

 - Hans

Some of the early ideas were actually described in
http://llvm.org/pubs/2007-05-31-Switch-Lowering.pdf

On Wed, Oct 29, 2014 at 3:52 AM, Hans Wennborg <hans at chromium.org>
wrote:
> On Mon, Oct 27, 2014 at 2:54 AM, Paweł Bylica <chfast at gmail.com>
wrote:
>> Hi,
>>
>> I'm interested in any information about implementations of switch
>> instruction and its runtime cost. If it's very target dependent,
I'm mostly
>> care about X86. Pointing some LLVM code is also good.
>
> I don't think there's any documentation besides the comments in the
> code, but switch lowering (to binary tree or jump table, etc.) happens
> in SelectionDAGBuilder::visitSwitch()
>
> For switches which are used to select from a set of constant values,
> SimplifyCFG will attempt to build a lookup table.
>
>  - Hans
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

Some more general information on switch lowering can be found in the
paper "A Superoptimizer Analysis of Multiway Branch Code Generation"
in
https://gcc.gnu.org/wiki/HomePage?action=AttachFile&do=get&target=gcc-2008-proceedings.pdf

-- Sean Silva

On Mon, Oct 27, 2014 at 2:54 AM, Paweł Bylica <chfast at gmail.com> wrote:
> Hi,
>
> I'm interested in any information about implementations of switch
> instruction and its runtime cost. If it's very target dependent,
I'm mostly
> care about X86. Pointing some LLVM code is also good.
>
> - Paweł
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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