[llvm] [TableGen][Decoder] Decode operands with zero width or all bits known (PR #156358)

[Rahul Joshi via llvm-commits]

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@@ -1922,15 +1946,48 @@ static void debugDumpRecord(const Record &Rec) {
 /// insert from the decoded instruction.
 static void addOneOperandFields(const Record *EncodingDef, const BitsInit &Bits,
                                 std::map<StringRef, StringRef> &TiedNames,
-                                StringRef OpName, OperandInfo &OpInfo) {
-  // Some bits of the operand may be required to be 1 depending on the
-  // instruction's encoding. Collect those bits.
-  if (const RecordVal *EncodedValue = EncodingDef->getValue(OpName))
-    if (const BitsInit *OpBits = dyn_cast<BitsInit>(EncodedValue->getValue()))
-      for (unsigned I = 0; I < OpBits->getNumBits(); ++I)
-        if (const BitInit *OpBit = dyn_cast<BitInit>(OpBits->getBit(I)))
-          if (OpBit->getValue())
-            OpInfo.InitValue |= 1ULL << I;
+                                const Record *OpRec, StringRef OpName,
+                                OperandInfo &OpInfo) {
+  // Find a field with the operand's name.
+  const RecordVal *OpEncodingField = EncodingDef->getValue(OpName);
+
+  // If there is no such field, try tied operand's name.
+  if (!OpEncodingField) {
+    if (auto I = TiedNames.find(OpName); I != TiedNames.end())
+      OpEncodingField = EncodingDef->getValue(I->second);
+
+    // If still no luck, the old behavior is to not decode this operand
+    // automatically and let the target do it. This is error-prone, so
+    // the new behavior is to report an error.
+    if (!OpEncodingField) {
+      if (!IgnoreNonDecodableOperands)
+        PrintError(EncodingDef->getLoc(),
+                   "could not find field for operand '" + OpName + "'");
+      return;
+    }
+  }
+
+  // Some or all bits of the operand may be required to be 0 or 1 depending
+  // on the instruction's encoding. Collect those bits.
+  if (const auto *OpBit = dyn_cast<BitInit>(OpEncodingField->getValue())) {
+    OpInfo.InitValue = OpBit->getValue();
+    return;
+  }
+  if (const auto *OpBits = dyn_cast<BitsInit>(OpEncodingField->getValue())) {
+    if (OpBits->getNumBits() == 0) {
+      if (OpInfo.Decoder.empty()) {
+        PrintError(EncodingDef->getLoc(), "operand '" + OpName + "' of type '" +
+                                              OpRec->getName() +
+                                              "' must have a decoder method");
+      }
+      return;
+    }
+    for (unsigned I = 0; I < OpBits->getNumBits(); ++I) {
+      if (const auto *OpBit = dyn_cast<BitInit>(OpBits->getBit(I)))
+        OpInfo.InitValue = OpInfo.InitValue.value_or(0) |
+                           static_cast<uint64_t>(OpBit->getValue()) << I;
+    }
+  }
 
   for (unsigned I = 0, J = 0; I != Bits.getNumBits(); I = J) {
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jurahul wrote:

not related to this PR directly, but this loop to collect spans of initializations based on the same variable can use some comments to make it easy to understand. 

https://github.com/llvm/llvm-project/pull/156358