[llvm-dev] optimisation issue in an llvm IR pass

Craig Topper via llvm-dev
Thu, 04 Jul 2019 01:53:52 -0000

Patch posted for review here https://reviews.llvm.org/D64190

~Craig

On Wed, Jul 3, 2019 at 5:28 PM Craig Topper <craig.topper at gmail.com>
wrote:

> This patch fixes the issue. I haven't checked our lit tests yet to see
> what effect this has and if we can commit it.
>
> lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>
> @@ -2967,7 +2967,8 @@ SDValue DAGCombiner::visitADDCARRYLike(SDValue N0,
> SDValue N1, SDValue CarryIn,
>
>    // Iff the flag result is dead:
>
>    // (addcarry (add|uaddo X, Y), 0, Carry) -> (addcarry X, Y, Carry)
>
>    if ((N0.getOpcode() == ISD::ADD ||
>
> -       (N0.getOpcode() == ISD::UADDO && N0.getResNo() == 0))
&&
>
> +       (N0.getOpcode() == ISD::UADDO && N0.getResNo() == 0
&&
>
> +        N0.getValue(1) != CarryIn)) &&
>
>        isNullConstant(N1) && !N->hasAnyUseOfValue(1))
>
>      return DAG.getNode(ISD::ADDCARRY, SDLoc(N), N->getVTList(),
>
>                         N0.getOperand(0), N0.getOperand(1), CarryIn);
>
> ~Craig
>
>
> On Wed, Jul 3, 2019 at 9:40 AM Marc <mh at mh-sec.de> wrote:
>
>> Hi Craig,
>>
>> On 03.07.19 17:33, Craig Topper wrote:
>> > Don't the CreateICmp calls return a Value* with an i1 type?
But then
>> > they are added to an i8 type? Not sure that works.
>>
>> I had that initially:
>>  auto cf = IRB.CreateICmpULT(Incr, ConstantInt::get(Int8Ty, 1));
>>  auto carry = IRB.CreateZExt(cf, Int8Ty);
>>  Incr = IRB.CreateAdd(Incr, carry);
>> it makes no difference to the generated assembly
>>
>>
>> > Have you tried using the llvm.uadd.with.overflow.i8 intrinsic?
>>
>> we have tried this:
>>
>>  CallInst *AddOv >>
IRB.CreateBinaryIntrinsic(Intrinsic::uadd_with_overflow, Counter,
>> ConstantInt::get(Int8Ty, 1));
>>  AddOv->setMetadata(M.getMDKindID("nosanitize"),
MDNode::get(C, None));
>>  Value *SumWithOverflowBit = AddOv;
>>  Incr = IRB.CreateAdd(IRB.CreateExtractValue(SumWithOverflowBit, 0),
>>  IRB.CreateZExt(IRB.CreateExtractValue(SumWithOverflowBit, 1),
Int8Ty));
>>
>> but that generated exactly the same as
>>
>>  auto cf = IRB.CreateICmpULT(Incr, ConstantInt::get(Int8Ty, 1));
>>  Incr = IRB.CreateAdd(Incr, cf);
>>
>>
>> All the above create (with llvm 6.0, 7 and 8):
>>
>>  mov    dl,BYTE PTR [rsi+rdi*1]
>>  mov    ecx,edx // <- not needed
>>  add    cl,0x1  // <- should be done to dl instead
>>  adc    dl,0x1
>>  mov    BYTE PTR [rsi+rdi*1],dl
>>
>>
>> Thanks!
>>
>> Regards,
>> Marc
>>
>> >
>> > ~Craig
>> >
>> >
>> > On Wed, Jul 3, 2019 at 4:01 AM Marc via llvm-dev
>> > <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>> >
>> >     Hello,
>> >
>> >     I have an optimisation issue in an llvm IR pass - the issue
being
>> that
>> >     unnecessary instructions are generated in the final assembly
(with
>> -O3).
>> >
>> >     I want to create the following assembly snippet:
>> >
>> >       mov    dl,BYTE PTR [rsi+rdi*1]
>> >       add    dl,0x1
>> >       adc    dl,0x0
>> >       mov    BYTE PTR [rsi+rdi*1],dl
>> >
>> >     however what is created is (variant #1):
>> >
>> >       mov    dl,BYTE PTR [rsi+rdx*1]
>> >       add    dl,0x1
>> >       cmp    dl,0x1 // <- not needed
>> >       adc    dl,0x0
>> >       mov    BYTE PTR [rsi+rdi*1],dl
>> >
>> >     in variant #2 the following is created:
>> >
>> >      mov    dl,BYTE PTR [rsi+rdi*1]
>> >      mov    ecx,edx // <- not needed
>> >      add    cl,0x1  // <- should be done to dl instead
>> >      adc    dl,0x1
>> >      mov    BYTE PTR [rsi+rdi*1],dl
>> >
>> >     Far below are both variants with the full code around it,
however
>> the
>> >     difference in both variants is this:
>> >
>> >     //variant1
>> >     auto cf = IRB.CreateICmpEQ(Incr, ConstantInt::get(Int8Ty, 0));
>> >     Incr = IRB.CreateAdd(Incr, cf);
>> >
>> >     //variant2
>> >     auto cf = IRB.CreateICmpULT(Incr, ConstantInt::get(Int8Ty,
1));
>> >     Incr = IRB.CreateAdd(Incr, cf);
>> >
>> >     //interestingly this totally different approach creates the
same
>> >     instructions as variant2
>> >     CallInst *AddOv >>
IRB.CreateBinaryIntrinsic(Intrinsic::uadd_with_over
>> >     flow, Counter, ConstantInt::get(Int8Ty, 1));
>> >     AddOv->setMetadata(M.getMDKindID("nosanitize"),
MDNode::get(C,
>> None));
>> >     Value *SumWithOverflowBit = AddOv;
>> >     Incr =
IRB.CreateAdd(IRB.CreateExtractValue(SumWithOverflowBit, 0),
>> >     IRB.CreateZExt(IRB.CreateExtractValue(SumWithOverflowBit, 1),
>> Int8Ty));
>> >
>> >     So - How do I have to write the code that the target code has
a
>> chance
>> >     of being generated?
>> >     For me its the same result on LLVM 6.0 and 7.
>> >
>> >     Alternatively
>> >       add    BYTE PTR [rsi+rdi*1],0x1
>> >       adc    BYTE PTR [rsi+rdi*1],0x0
>> >      would work as well.
>> >
>> >     Thank you very much!
>> >
>> >     Regards,
>> >     Marc
>> >
>> >     -> Below now is the full llvm IR pass code
>> >
>> >     // vvv same code before both variants
>> >     LoadInst *PrevLoc = IRB.CreateLoad(AFLPrevLoc);
>> >     PrevLoc->setMetadata(M.getMDKindID("nosanitize"),
MDNode::get(C,
>> None));
>> >     Value *PrevLocCasted = IRB.CreateZExt(PrevLoc,
IRB.getInt32Ty());
>> >
>> >      LoadInst *MapPtr = IRB.CreateLoad(AFLMapPtr);
>> >     MapPtr->setMetadata(M.getMDKindID("nosanitize"),
MDNode::get(C,
>> None));
>> >     Value *MapPtrIdx = IRB.CreateGEP(MapPtr,
>> IRB.CreateXor(PrevLocCasted,
>> >     CurLoc));
>> >
>> >     LoadInst *Counter = IRB.CreateLoad(MapPtrIdx);
>> >     Counter->setMetadata(M.getMDKindID("nosanitize"),
MDNode::get(C,
>> None));
>> >
>> >     Value *Incr = IRB.CreateAdd(Counter, ConstantInt::get(Int8Ty,
1));
>> >     // ^^^ same code before both variants
>> >
>> >     // CODE FOR VARIANT #1
>> >     auto cf = IRB.CreateICmpEQ(Incr, ConstantInt::get(Int8Ty, 0));
>> >     Incr = IRB.CreateAdd(Incr, cf);
>> >
>> >     // CODE FOR VARIANT #2
>> >     auto cf = IRB.CreateICmpULT(Incr, ConstantInt::get(Int8Ty,
1));
>> >     Incr = IRB.CreateAdd(Incr, cf);
>> >
>> >     // vvv same code after both variants
>> >     IRB.CreateStore(Incr,
>> >
MapPtrIdx)->setMetadata(M.getMDKindID("nosanitize"), MDNode::get(C,
>> >     None));
>> >     StoreInst *Store = IRB.CreateStore(ConstantInt::get(Int32Ty,
>> cur_loc >>
>> >     1), AFLPrevLoc);
>> >     Store->setMetadata(M.getMDKindID("nosanitize"),
MDNode::get(C,
>> None));
>> >     // ^^^ same code after both variants
>> >
>> >
>> >     --
>> >     Marc Heuse
>> >     www.mh-sec.de <http://www.mh-sec.de>
>> >
>> >     PGP: AF3D 1D4C D810 F0BB 977D  3807 C7EE D0A0 6BE9 F573
>> >     _______________________________________________
>> >     LLVM Developers mailing list
>> >     llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>> >     https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>>
>> --
>> Marc Heuse
>> Mobil: +49 177 9611560
>> Fax: +49 30 37309726
>> www.mh-sec.de
>>
>> Marc Heuse - IT-Security Consulting
>> Winsstr. 68
>> 10405 Berlin
>>
>> Ust.-Ident.-Nr.: DE244222388
>> PGP: AF3D 1D4C D810 F0BB 977D  3807 C7EE D0A0 6BE9 F573
>>
>
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