[LLVMdev] Undoing DAG Combiner patterns

[Martin Filteau]

Thanks all,

I've found exactly what I was looking for in the ARM backend. My early attempt was missing the "AddedComplexity" setting.

// (sub X, imm) gets canonicalized to (add X, -imm).  Match this form.
// The assume-no-carry-in form uses the negation of the input since add/sub
// assume opposite meanings of the carry flag (i.e., carry == !borrow).
// See the definition of AddWithCarry() in the ARM ARM A2.2.1 for the gory
// details.
// The AddedComplexity preferences the first variant over the others since
// it can be shrunk to a 16-bit wide encoding, while the others cannot.
let AddedComplexity = 1 in
def : T2Pat<(add        GPR:$src, imm1_255_neg:$imm),
            (t2SUBri    GPR:$src, imm1_255_neg:$imm)>;
def : T2Pat<(add        GPR:$src, t2_so_imm_neg:$imm),
            (t2SUBri    GPR:$src, t2_so_imm_neg:$imm)>;
def : T2Pat<(add        GPR:$src, imm0_4095_neg:$imm),
            (t2SUBri12  GPR:$src, imm0_4095_neg:$imm)>;
def : T2Pat<(add        GPR:$src, imm0_65535_neg:$imm),
            (t2SUBrr    GPR:$src, (t2MOVi16 (imm_neg_XFORM imm:$imm)))>;

Regards,

-Martin

-----Original Message-----
From: Evan Cheng [mailto:evan.cheng at apple.com] 
Sent: Thursday, May 16, 2013 1:49 PM
To: Tom Stellard
Cc: Martin Filteau; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Undoing DAG Combiner patterns

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>
>>
>>
>>
>
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