Fix the dependency of `CodeGenDAGPatterns::ParseDefaultOperands()` on
the particular order of SDNode definitions. Implicit usage of the first
definition as a placeholder makes `llvm-tblgen -gen-dag-isel` fail if
that SDNode is not usable as an output pattern operator and an instance
of `OperandWithDefaultOps` is used in a pattern.
Presently, each `OperandWithDefaultOps` record is processed by
constructing an instance of TreePattern from its `DefaultOps` argument
that has the form `(ops ...)`. Even though the result of processing the
root operator of that DAG is not inspected by `ParseDefaultOperands()`
function itself, that operator has to be supported by the underlying
`TreePattern::ParseTreePattern()` function. For that reason, a temporary
DAG is created by replacing the root operator of `DefaultOps` argument
with the first SDNode defined, which is usually `def imm : ...` defined
in `TargetSelectionDAG.td` file.
This results in misleading errors being reported when implementing new
`SDNode` types, if the new definition happens to be added before the
`def imm : ...` line. The error is reported by several test cases
executed by `check-llvm` target, as well as by the regular build, if one
of the enabled targets inherit one of its operand types from
`OperandWithDefaultOps`:
OptionalIntOperand: ../llvm/test/TableGen/DAGDefaultOps.td:28:5: error: In OptionalIntOperand: Cannot use 'unexpected_node' in an output pattern!
def OptionalIntOperand: OperandWithDefaultOps<i32, (ops (i32 0))>;
This commit implements a dedicated constructor of `TreePattern` to be
used if the caller does not care about the particular root operator of
the pattern being processed.
d416289417