llvm dev - Mar 2019 - Must the MatcherTable start with an OPC_SwitchOpcode?

Hi all,

There is a cache OpcodeOffset in class SelectionDAGISel to accelerate ISEL.
But OpcodeOffset can only be filled when MatcherTable[0] is
OPC_SwitchOpcode.

Code for the check:

  } else if (MatcherTable[0] == OPC_SwitchOpcode) {
    // Otherwise, the table isn't computed, but the state machine does
start
    // with an OPC_SwitchOpcode instruction.  Populate the table now, since
this
    // is the first time we're selecting an instruction.

According to the comment, I guess it is a must have condition? If it is a
must have condition, can we add assert for it in someplace like 1: where we
generate the MatcherTable in TableGen or 2: the place when we do ISEL in
function SelectionDAGISel::SelectCodeCommon.

If MatcherTable[0] is not OPC_SwitchOpcode, no test case will capture this
issue and it will leads to a significant increase in isel match failures.

Thanks.

BRS//
Chen Zheng
Power Compiler Backend Developer
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