llvm dev - May 2013 - [LLVMdev] Undoing DAG Combiner patterns

Hi all,

It's the first LLVM backend we do for our asynchronous DSP. So, I apologize
if this is a trivial question!

The target-independent DAG combiner performs the following transformation:

sub  n, c                ->  add n, -c

For our target, negative constants are more costly to encode.  What is the best
place to revert to a sub instruction?

Kind regards,

-- Martin

www.octasic.com<http://www.octasic.com>



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On Thu, May 16, 2013 at 02:03:14AM +0000, Martin Filteau
wrote:
> Hi all,
> 
> It's the first LLVM backend we do for our asynchronous DSP. So, I
apologize if this is a trivial question!
> 
> The target-independent DAG combiner performs the following transformation:
> 
> sub  n, c                ->  add n, -c
> 
It looks to me like this transformation would happen during the
legalization phase.  Is sub legal on your target?  If not, you
can custom lower it e.g.:

setOperationAction(ISD::SUB, MVT::i32, Custom);

and then handle ISD::SUB in the TargetLowering::LowerOperation()
function.

-Tom
> For our target, negative constants are more costly to encode.  What is the
best place to revert to a sub instruction?
> 
> Kind regards,
> 
> -- Martin
> 
> www.octasic.com<http://www.octasic.com>
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

A better way to handle this is to a td pattern to match "add n, -c" to
a subtraction. I believe several targets do something similar to this.

Evan

On May 16, 2013, at 7:12 AM, Tom Stellard <tom at stellard.net> wrote:
> On Thu, May 16, 2013 at 02:03:14AM +0000, Martin Filteau wrote:
>> Hi all,
>> 
>> It's the first LLVM backend we do for our asynchronous DSP. So, I
apologize if this is a trivial question!
>> 
>> The target-independent DAG combiner performs the following
transformation:
>> 
>> sub  n, c                ->  add n, -c
>> 
> 
> It looks to me like this transformation would happen during the
> legalization phase.  Is sub legal on your target?  If not, you
> can custom lower it e.g.:
> 
> setOperationAction(ISD::SUB, MVT::i32, Custom);
> 
> and then handle ISD::SUB in the TargetLowering::LowerOperation()
> function.
> 
> -Tom
> 
>> For our target, negative constants are more costly to encode.  What is
the best place to revert to a sub instruction?
>> 
>> Kind regards,
>> 
>> -- Martin
>> 
>> www.octasic.com<http://www.octasic.com>
>> 
>> 
>> 
> 
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev