[LLVMdev] Illegal optimization in LLVM 2.8 during SelectionDAG

[Evan Cheng]

Please test if r115571 has fixed it.

Evan

On Oct 4, 2010, at 5:00 AM, Heikki Kultala wrote:

> Bill Wendling wrote:
>> On Sep 30, 2010, at 2:13 AM, Heikki Kultala wrote:
>> 
>>> Bill Wendling wrote:
>>>> On Sep 29, 2010, at 12:36 AM, Heikki Kultala wrote:
>>>> 
>>>>> On 29 Sep 2010, at 06:25, Heikki Kultala wrote:
>>>>> 
>>>>>> Our architecture has 1-bit boolean predicate registers.
>>>>>> 
>>>>>> I've defined comparison
>>>>>> 
>>>>>> def NErrb : InstTCE<(outs I1Regs:$op3), (ins I32Regs:$op1,I32Regs:$op2), "", [(set I1Regs:$op3, (setne I32Regs:$op1, I32Regs:$op2))]>;
>>>>>> 
>>>>>> But then I end up having the following bug:
>>>>>> 
>>>>>> Code
>>>>>> 
>>>>>> %0 = zext i8 %data to i32
>>>>>> %1 = zext i16 %crc to i32
>>>>>> %2 = xor i32 %1, %0
>>>>>> %3 = and i32 %2, 1
>>>>>> %4 = icmp eq i32 %3, 0
>>>>>> 
>>>>>> which compares the lowest bits of the 2 variables
>>>>>> ends up being compiled as
>>>>>> 
>>>>>>     %reg16384<def> = LDWi <fi#-2>, 0; mem:LD4[FixedStack-2] I32Regs:%reg16384
>>>>>>     %reg16385<def> = LDWi <fi#-1>, 0; mem:LD4[FixedStack-1] I32Regs:%reg16385
>>>>>>     %reg16386<def> = COPY %reg16384; I32Regs:%reg16386,16384
>>>>>>     %reg16390<def> = NErrb %reg16384, %reg16385; I1Regs:%reg16390 I32Regs:%reg16384,16385
>>>>>> 
>>>>>> which just compares ALL BITS of the variables.
>>>>> I also have a pattern:
>>>>> 
>>>>> def XORrrb : InstTCE<(outs I1Regs:$op3), (ins I32Regs:$op1,I32Regs:$op2), "", [(set I1Regs:$op3, (trunc (xor I32Regs:$op1, I32Regs:$op2)))]>;
>>>>> 
>>>>> Which can do the whole 3-operation code sequence correctly with one operation.
>>>>> 
>>>>> With LLVM 2.7 this correct operation is selected, with LLVM 2.8 the wrong operation(which compares all bits) is chosen
>>>>> 
>>>>> So this looks like a bug in LLVM 2.8 isel?
>>>>> 
>>>> Hi Heikki,
>>>> 
>>>> We need a better example of what's going on. What's the original code? Also, I don't have access to your back-end's code so it's hard to tell just from these snippets what's going on. For instance, it's not clear whether it's the instruction selector that's at fault or if your .td files have a bug in them somewhere.
>>> The original code is:
>> 
>> [snip]
>> 
>>> where the interesting lines are lines 12-13:
>>> 
>>>                x16 = (e_u8)(((data) ^ ((e_u8)crc))&1);
>>>                if (x16 == 1)
>>> 
>>> The code which goes into isel is:
>>> 
>>> bb.nph:
>>>  %0 = zext i8 %data to i32
>>>  %1 = zext i16 %crc to i32
>>>  %2 = xor i32 %1, %0
>>>  %3 = and i32 %2, 1
>>>  %4 = icmp eq i32 %3, 0
>>>  br i1 %4, label %bb.nph._crit_edge, label %5
>>> 
>>> inside selectiondag this becomes:
>>> 
>>> Legalized selection DAG:
>> 
>> [snip]
>> 
>>>        0x248d280: <multiple use>
>>>        0x248d980: <multiple use>
>>>      0x25bb7f0: i32 = xor 0x248d280, 0x248d980 [ORD=3] [ID=15]
>>> 
>>>    0x25bbbf0: i1 = truncate 0x25bb7f0 [ID=18]
>> 
>> This truncate is weird to me. If anything, it should be an "and" instruction. I have a feeling that your back-end is telling instruction selection and the type legalizer that it's okay to replace the normal "and" with this truncate call, which leads to your troubles later on.
> 
> It would seem that the truncate is created by:
> 
> TargetLowering::SimplifySetCC
> 
> ...
> 
> 
>      if (N0.getOpcode() == ISD::SETCC &&
>           isTypeLegal(VT) && VT.bitsLE(N0.getValueType())) {
>         bool TrueWhenTrue = (Cond == ISD::SETEQ) ^ 
> (N1C->getAPIntValue() != 1);
>         if (TrueWhenTrue)
>           return DAG.getNode(ISD::TRUNCATE, dl, VT, N0);
> 
>         // Invert the condition.
>         ISD::CondCode CC = cast<CondCodeSDNode>(N0.getOperand(2))->get();
>         CC = ISD::getSetCCInverse(CC,
> 
> N0.getOperand(0).getValueType().isInteger());
>         return DAG.getSetCC(dl, VT, N0.getOperand(0), N0.getOperand(1), 
> CC);
>       }
> 
> 
> and the AND is then dropped by
> 
> TargetLowering::SimplifyDemandedBits
> 
> ...
> 
> 
>   switch (Op.getOpcode()) {
> ...
>   case ISD::AND:
>     // If the RHS is a constant, check to see if the LHS would be zero 
> without
>     // using the bits from the RHS.  Below, we use knowledge about the 
> RHS to
>     // simplify the LHS, here we're using information from the LHS to 
> simplify
>     // the RHS.
>     if (ConstantSDNode *RHSC = 
> dyn_cast<ConstantSDNode>(Op.getOperand(1))) {
>       APInt LHSZero, LHSOne;
>       TLO.DAG.ComputeMaskedBits(Op.getOperand(0), NewMask,
>                                 LHSZero, LHSOne, Depth+1);
>       // If the LHS already has zeros where RHSC does, this and is dead.
>       if ((LHSZero & NewMask) == (~RHSC->getAPIntValue() & NewMask))
> 	  return TLO.CombineTo(Op, Op.getOperand(0));
> 
> 
> 
> 
> 
> As neither of these are virtual functions, we cannot create an 
> workaround hack for our backend to easily circumvent this bug.
> 
> 
> 
> 
> It would now seem that TCE users cannot use the default LLVM 2.8 but 
> we'll have to distribute our own patch to disable the invalid dropping 
> of the trunc and make all our users compile LLVM themselves with the 
> patch :(
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