Fix TableGen -gen-disassembler output for certain bit field definitions.

[srking]

Summary

TableGen -gen-disassembler extracts incorrect operand values for certain bit fields with an offset. The result is that disassembly silently fails with incorrect operand values. Bit assignments like:
Inst{7-0} = Foo{9-2}
are vulnerable to this problem.

[craig.topper]

LGTM. Though I wonder if the if and else couldn't be better combined.

[srking]

Changes from previous patch:

1. Use more succinct test file name "BitOffsetDecoder.td"
2. Update test file comments to use "offset" instead of "shift"

Verified all tests pass.

[srking]

Operator error: uploaded old patch again. Here is the new patch with renamed regression test file.

[craig.topper]

Commited in r218560.

test/TableGen/BitOffsetDecoder.td

+// RUN: llvm-tblgen -gen-disassembler -I %p/../../include %s | FileCheck %s
+
+include "llvm/Target/Target.td"
+
+def archInstrInfo : InstrInfo { }
+
+def arch : Target {
+    let InstructionSet = archInstrInfo;
+}
+
+def  Myi32  : Operand<i32> {
+  let DecoderMethod = "DecodeMyi32";
+}
+
+
+let OutOperandList = (outs), Size = 2 in {
+
+def foo : Instruction {
+    let InOperandList = (ins i32imm:$factor);
+    field bits<16> Inst;
+    bits<32> factor;
+    let Inst{7-0} = 0xAA;
+    let Inst{14-8} = factor{6-0}; // no offset
+    let AsmString = "foo  $factor";
+    field bits<16> SoftFail = 0;
+    }
+
+def bar : Instruction {
+    let InOperandList = (ins i32imm:$factor);
+    field bits<16> Inst;
+    bits<32> factor;
+    let Inst{7-0} = 0xBB;
+    let Inst{15-8} = factor{10-3}; // offset by 3
+    let AsmString = "bar  $factor";
+    field bits<16> SoftFail = 0;
+    }
+
+def biz : Instruction {
+    let InOperandList = (ins i32imm:$factor);
+    field bits<16> Inst;
+    bits<32> factor;
+    let Inst{7-0} = 0xCC;
+    let Inst{11-8,15-12} = factor{10-3}; // offset by 3, multipart
+    let AsmString = "biz  $factor";
+    field bits<16> SoftFail = 0;
+    }
+
+def baz : Instruction {
+    let InOperandList = (ins Myi32:$factor);
+    field bits<16> Inst;
+    bits<32> factor;
+    let Inst{7-0} = 0xDD;
+    let Inst{15-8} = factor{11-4}; // offset by 4 + custom decode
+    let AsmString = "baz  $factor";
+    field bits<16> SoftFail = 0;
+    }
+
+def bum : Instruction {
+    let InOperandList = (ins i32imm:$factor);
+    field bits<16> Inst;
+    bits<32> factor;
+    let Inst{7-0} = 0xEE;
+    let Inst{15-8} = !srl(factor,5);
+    let AsmString = "bum  $factor";
+    field bits<16> SoftFail = 0;
+    }
+}
+
+
+// CHECK: tmp = fieldFromInstruction(insn, 8, 7);
+// CHECK: tmp = fieldFromInstruction(insn, 8, 8) << 3;
+// CHECK: tmp |= (fieldFromInstruction(insn, 8, 4) << 7);
+// CHECK: tmp |= (fieldFromInstruction(insn, 12, 4) << 3);
+// CHECK: tmp = fieldFromInstruction(insn, 8, 8) << 4;
 No newline at end of file

utils/TableGen/FixedLenDecoderEmitter.cpp

 void FilterChooser::emitBinaryParser(raw_ostream &o, unsigned &Indentation,
                                      const OperandInfo &OpInfo) const {
   const std::string &Decoder = OpInfo.Decoder;
 
   if (OpInfo.numFields() == 1) {
     OperandInfo::const_iterator OI = OpInfo.begin();
     o.indent(Indentation) << "tmp = fieldFromInstruction"
                           << "(insn, " << OI->Base << ", " << OI->Width
-                          << ");\n";
+                          << ")";
+    if (OI->Offset)
+      o << " << " << OI->Offset;
+    o << ";\n";
+
   } else {
     o.indent(Indentation) << "tmp = 0;\n";
     for (OperandInfo::const_iterator OI = OpInfo.begin(), OE = OpInfo.end();
          OI != OE; ++OI) {
       o.indent(Indentation) << "tmp |= (fieldFromInstruction"
                             << "(insn, " << OI->Base << ", " << OI->Width
                             << ") << " << OI->Offset << ");\n";
     }