[Tablegen] Don't emit decoder tables with islands larger than 64 bits (#179651)

I have a downstream target which has 128-bit instructions where some
instructions can have large sections of encoding to be determined ahead
of time. This results in the island calculations for decoder tables to
emit checks over 64-bits.

This change will emit multiple separate checks when the island exceeds
64-bits.

llvm/test/TableGen/DecoderEmitter/large-islands.td

@@ -0,0 +1,38 @@
+// RUN: llvm-tblgen -gen-disassembler -I %p/../../../include %s | FileCheck %s
+
+include "llvm/Target/Target.td"
+
+class I<dag outs, dag ins, bits<5> opcode> : Instruction {
+  let InOperandList = ins;
+  let OutOperandList = outs;
+
+  bits<5> dst;
+  bits<5> src0;
+  bits<5> src1;
+
+  int Size = 16;
+  bits<128> Inst;
+
+  let Inst{4...0} = opcode;
+  let Inst{9...5} = dst;
+  let Inst{14...10} = src0;
+  let Inst{19...15} = src1;
+
+  let Inst{127...20} = 0xdeadbeef;
+}
+
+def Reg : Register<"reg">;
+
+def Regs : RegisterClass<"foo", [i32], 0, (add Reg)>;
+
+def IAdd : I<(outs Regs:$dst), (ins Regs:$src0, Regs:$src1), 0b10101>;
+
+// CHECK-LABEL: static const uint8_t DecoderTable128[20] = {
+// CHECK-NEXT:   OPC_CheckField, 84, 44, 0,                 //  0: check Inst[127:84] == 0x0
+// CHECK-NEXT:   OPC_CheckField, 20, 64, 239, 253, 182, 245, 13, 
+
+def II : InstrInfo;
+
+def MyTarget : Target {
+  let InstructionSet = II;
+}

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -656,8 +656,14 @@ static std::vector<EncodingIsland> getIslands(const KnownBits &EncodingBits,
     if (!IsFiltered && IsKnown) {
       if (OnIsland) {
         // Accumulate island bits.
-        FieldVal |= static_cast<uint64_t>(EncodingBits.One[I])
-                    << (I - StartBit);
+        const unsigned BitNo = I - StartBit;
+        FieldVal |= static_cast<uint64_t>(EncodingBits.One[I]) << (BitNo);
+        // If island becomes larger than 64-bits complete the island and start a
+        // new one
+        if (BitNo >= 63) {
+          Islands.push_back({StartBit, 64, FieldVal});
+          OnIsland = false;
+        }
       } else {
         // Onto an island.
         StartBit = I;