[PATCH] D67280: [Mips][msa] Fix inifinite loop for mips.nori.b intrinsic

[Mirko Brkusanin via Phabricator via llvm-commits]

mbrkusanin added a comment.

More detailed explanation follows:

Expansion for mips.nori.b goes something like this:

  mips.nori.b %dst, %val, imm
  %dst = not (or %value, imm)
  %dst = xor (or %value, imm), -1


For -1 as immediate we should get:

  %dst = xor (or %value, -1), -1
  %dst = xor -1, -1
  %dst = 0

When value of immediate is 255 (-1 for i8, so all bits are 1) issue shows up in DAGCombiner when it runs into node for XOR:

SelectionDAG:

  ...
                t31: v2i64 = BUILD_VECTOR Constant:i64<0>, Constant:i64<0>
              t2: i64,ch = CopyFromReg t0, Register:i64 %0
            t28: v2i64 = insert_vector_elt t31, t2, Constant:i32<0>
            t4: i64,ch = CopyFromReg t0, Register:i64 %1
          t30: v2i64 = insert_vector_elt t28, t4, Constant:i32<1>
        t6: v16i8 = bitcast t30
      t24: v16i8 = or t6, t23
    t25: v16i8 = xor t24, t23
  ...
  t23: v16i8 = BUILD_VECTOR Constant:i32<255>, Constant:i32<255>, ...

-DAGCombiner::visitXOR() is called which in turns calles SimplifyDemandedBits().
--SimplifyDemandedBits() for XOR will call SimplifyDemandedBits() for OR.
--SimplifyDemandedBits() for OR  will call SimplifyDemandedBits() for Operand(0)
--SimplifyDemandedBits() for Operand(0) will make a new UNDEF node.
-Legalizer for Mips will replace UNDEF node with a new Constant<0> node.
-DAGCombiner::visitXOR() is called which again makes a new UNDEF node (this time from Constant<0>).
Legalizer for Mips will replace UNDEF node with a new Constant<0> node.
...
now we are in a loop.

Making UNDEF legal avoids making a loop. It also produces shorter code which is why there are changes to other tests.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D67280/new/

https://reviews.llvm.org/D67280