[llvm-dev] Issue with DAG legalization of brcond, setcc, xor

[Dilan Manatunga via llvm-dev]

Hi,

I am having some issues with how some of the instructions are being
legalized.
So this is my intial basic block. The area of concern is the last three
instructions. I will pick and choose debug output to keep this small.

SelectionDAG has 36 nodes:
  t0: ch = EntryToken
          t6: i32,ch = CopyFromReg t0, Register:i32 %vreg507
              t2: i32,ch = CopyFromReg t0, Register:i32 %vreg17
            t4: i32 = or t2, Constant:i32<256>
          t9: i32 = shl t4, Constant:i32<2>
        t10: i32 = add t6, t9
        t12: i32,ch = CopyFromReg t0, Register:i32 %vreg79
        t15: i32,ch = CopyFromReg t0, Register:i32 %vreg1
      t16: ch = llvm.tpu.dma.write.1KB.async t0,
TargetConstant:i32<4602>, t10, t12, t15
              t18: i32,ch = CopyFromReg t0, Register:i32 %vreg166
            t20: i32 = AssertZext t18, ValueType:ch:i1
          t23: i1 = setcc t20, Constant:i32<0>, seteq:ch
            t25: i32,ch = CopyFromReg t0, Register:i32 %vreg396
          t28: i1 = setcc t25, Constant:i32<255>, setugt:ch
        t29: i1 = and t23, t28
      t37: i1 = setcc t29, Constant:i1<-1>, setne:ch
    t33: ch = brcond t16, t37, BasicBlock:ch<if.end65.1 0x7097330>
  t35: ch = br t33, BasicBlock:ch<if.then64.1 0x7097280>
 Here we see that the settcc has been legalized to xor, which I am fine with..
Legalizing: t37: i1 = setcc t29, Constant:i1<-1>, setne:ch

Combining: t37: i1 = setcc t29, Constant:i1<-1>, setne:ch
 ... into: t38: i1 = xor t29, Constant:i1<-1>

However, the issue I have is that we than try to legalize and combin
the brcond again.

Legalizing: t33: ch = brcond t16, t38, BasicBlock:ch<if.end65.1 0x7097330>
Combining: t33: ch = brcond t16, t38, BasicBlock:ch<if.end65.1 0x7097330>

What the debug output doesn't show you is that when try to combine the
brcond again, we end up modifying the xor we created to setcc again.

      t40: i32 = setcc t29, Constant:i1<-1>, setne:ch

But the issue is now the setcc returns a 32-bit type.

I did some tracing and from what I can tell the visitBRCOND function
in DAGCombiner is responsible for this change. It sees that the brcond
has only one use that is an xor. Later on, it also sees that the two
operands to the xor are neither setcc, so it converts the xor to setcc
again. I would be fine with this, but when it tries to find the output
type of the setcc, it chooses i32 instead of i1. Again, it seems this
happens from the line of code that is:

 SetCCVT = getSetCCResultType(SetCCVT)

All that function does though is return the data type for pointers.

Well, to my questions I guess. Why is this happening? Isn't it kinda
silly to convert a setcc to an xor, and then convert it back to a
slightly different output setcc. And how can I prevent this, or fix
it, since my conditional branches only support having single bit
inputs.

Thanks,

-Dilan

PS Here is optimized DAG in case you were wondering.

Combining: t0: ch = EntryToken Optimized legalized selection DAG: BB#212
'merge_sort:for.cond.cleanup39.1' SelectionDAG has 36 nodes: t0: ch =
EntryToken t6: i32,ch = CopyFromReg t0, Register:i32 %vreg507 t2: i32,ch =
CopyFromReg t0, Register:i32 %vreg17 t4: i32 = or t2, Constant:i32<256> t9:
i32 = shl t4, Constant:i32<2> t10: i32 = add t6, t9 t12: i32,ch =
CopyFromReg t0, Register:i32 %vreg79 t15: i32,ch = CopyFromReg t0,
Register:i32 %vreg1 t16: ch = llvm.tpu.dma.write.1KB.async t0,
TargetConstant:i32<4602>, t10, t12, t15 t18: i32,ch = CopyFromReg t0,
Register:i32 %vreg166 t20: i32 = AssertZext t18, ValueType:ch:i1 t23: i1 =
setcc t20, Constant:i32<0>, seteq:ch t25: i32,ch = CopyFromReg t0,
Register:i32 %vreg396 t28: i1 = setcc t25, Constant:i32<255>, setugt:ch
t29: i1 = and t23, t28 t40: i32 = setcc t29, Constant:i1<-1>, setne:ch t33:
ch = brcond t16, t40, BasicBlock:ch<if.end65.1 0x7097330> t35: ch = br t33,
BasicBlock:ch<if.then64.1 0x7097280>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170720/a221223f/attachment.html>