[RISCV]Add support for resolving encoding conflicts among vendor specific CSRs

This patch adds the framework for resolving encoding conflicts among CSRs.

Specifically, this patch adds a support for emitting a new lookup function for
the primary key which return a pair of iterators pointing to first and last
value hence giving a range of values which satisfies the query.

While printing the CSR name during objdump, iterate over the range and print the
name of only that CSR which satisifes the feature requirement of subtarget.

Below is the signature of the new function that will be emitted for primary key:

```
llvm::iterator_range<const SysReg *> lookupSysRegByEncoding(uint16_t Encoding) {
  struct KeyType {
    uint16_t Encoding;
  };
  KeyType Key = {Encoding};
  struct Comp {
    bool operator()(const SysReg &LHS, const KeyType &RHS) const {
      if (LHS.Encoding < RHS.Encoding)
        return true;
      if (LHS.Encoding > RHS.Encoding)
        return false;
      return false;
    }
    bool operator()(const KeyType &LHS, const SysReg &RHS) const {
      if (LHS.Encoding < RHS.Encoding)
        return true;
      if (LHS.Encoding > RHS.Encoding)
        return false;
      return false;
    }
  };
  auto Table = ArrayRef(SysRegsList);
  auto It = std::equal_range(Table.begin(), Table.end(), Key, Comp());
  return llvm::make_range(It.first, It.second);
}
```

NOTE: Emitting a different signature for returning a range of results is only
supported by primary key.

Author: quic-garvgupt

llvm/docs/TableGen/BackEnds.rst

@@ -717,6 +717,12 @@ This class provides six fields.
 
 * ``bit PrimaryKeyEarlyOut``. See the third example below.
 
+* ``bit PrimaryKeyReturnRange``. when set to 1, modifies the lookup function’s
+  definition to return a range of results rather than a single pointer to the
+  object. This feature proves useful when multiple objects meet the criteria
+  specified by the lookup function. Currently, it is supported only for primary
+  lookup functions. Refer to the second example below for further details.
+
 TableGen attempts to deduce the type of each of the table fields so that it
 can format the C++ initializers in the emitted table. It can deduce ``bit``,
 ``bits<n>``, ``string``, ``Intrinsic``, and ``Instruction``.  These can be
@@ -883,6 +889,84 @@ Here is the generated C++ code.
     return &*Idx;
   }
 
+In the above example, lets add one more record with encoding same as that of
+record ``CEntry<"Pear",  CBaz, 15>``.
+
+.. code-block:: text
+
+  def CFoobar : CEnum;
+  def : CEntry<"Banana", CFoobar, 15>;
+
+Below is the new generated ``CTable``
+
+.. code-block:: text
+
+  #ifdef GET_Table_IMPL
+  constexpr CEntry Table[] = {
+    { "Apple", CFoo, 0xA }, // 0
+    { "Apple", CBar, 0xD }, // 1
+    { "Banana", CFoobar, 0xF }, // 2
+    { "Pear", CBaz, 0xF }, // 3
+  };
+
+Since ``Banana`` lexicographically appears first, therefore in the ``CEntry``
+table, record with name ``Banana`` will come before the record with name
+``Pear``. Because of this, the ``lookupCEntryByEncoding`` function will always
+return a pointer to the record with name ``Banana`` even though in some cases
+the correct result can be the record with name ``Pear``. Such kind of scenario
+makes the exisitng lookup function insufficient because they always return a
+pointer to a single entry from the table, but instead it should return a range
+of results because multiple entries match the criteria sought by the lookup
+function. In this case, the definition of the lookup function needs to be
+modified to return a range of results which can be done by setting
+``PrimaryKeyReturnRange``.
+
+.. code-block:: text
+
+  def CTable : GenericTable {
+    let FilterClass = "CEntry";
+    let Fields = ["Name", "Kind", "Encoding"];
+    string TypeOf_Kind = "CEnum";
+    let PrimaryKey = ["Encoding"];
+    let PrimaryKeyName = "lookupCEntryByEncoding";
+    let PrimaryKeyReturnRange = true;
+  }
+
+Here is the modified lookup function.
+
+.. code-block:: text
+
+  llvm::iterator_range<const CEntry *> lookupCEntryByEncoding(uint16_t Encoding) {
+    struct KeyType {
+      uint16_t Encoding;
+    };
+    KeyType Key = {Encoding};
+    struct Comp {
+      bool operator()(const CEntry &LHS, const KeyType &RHS) const {
+        if (LHS.Encoding < RHS.Encoding)
+          return true;
+        if (LHS.Encoding > RHS.Encoding)
+          return false;
+        return false;
+      }
+      bool operator()(const KeyType &LHS, const CEntry &RHS) const {
+        if (LHS.Encoding < RHS.Encoding)
+          return true;
+        if (LHS.Encoding > RHS.Encoding)
+          return false;
+        return false;
+      }
+    };
+    auto Table = ArrayRef(Table);
+    auto It = std::equal_range(Table.begin(), Table.end(), Key, Comp());
+    return llvm::make_range(It.first, It.second);
+  }
+
+The new lookup function will return an iterator range with first pointer to the
+first result and the last pointer to the last matching result from the table.
+However, please note that the support for emitting modified definition exists
+for ``PrimaryKeyName`` only.
+
 The ``PrimaryKeyEarlyOut`` field, when set to 1, modifies the lookup
 function so that it tests the first field of the primary key to determine
 whether it is within the range of the collected records' primary keys. If
@@ -987,6 +1071,8 @@ function. This class provides three fields.
 
 * ``bit EarlyOut``. See the third example in `Generic Tables`_.
 
+* ``bit ReturnRange``. See the second example in `Generic Tables`_.
+
 Here is an example of a secondary key added to the ``CTable`` above. The
 generated function looks up entries based on the ``Name`` and ``Kind`` fields.
 

llvm/include/llvm/TableGen/SearchableTable.td

@@ -114,6 +114,9 @@ class GenericTable {
 
   // See SearchIndex.EarlyOut
   bit PrimaryKeyEarlyOut = false;
+
+  // See SearchIndex.ReturnRange
+  bit PrimaryKeyReturnRange = false;
 }
 
 // Define a record derived from this class to generate an additional search
@@ -135,6 +138,12 @@ class SearchIndex {
   //
   // Can only be used when the first field is an integral (non-string) type.
   bit EarlyOut = false;
+
+  // If true, will generate a different function signature which will return an
+  // iterator range of pointers giving the starting and end value of the range.
+  // This feature is only supported for primary key only.
+  // e.g. lookupSysRegByEncoding returns multiple CSRs for same encoding.
+  bit ReturnRange = false;
 }
 
 // Legacy table type with integrated enum.

llvm/test/TableGen/ReturnRange.td

@@ -0,0 +1,108 @@
+// RUN: llvm-tblgen -gen-searchable-tables -I %p/../../include %s | FileCheck %s
+// RUN: not llvm-tblgen -DERROR -gen-searchable-tables -I %p/../../include %s 2>&1 \
+// RUN:         | FileCheck  --check-prefix=ERROR %s
+
+include "llvm/TableGen/SearchableTable.td"
+
+class SysReg<string name, bits<12> op> {
+  string Name = name;
+  bits<12> Encoding = op;
+  code FeaturesRequired = [{ {} }];
+}
+
+def List1 : GenericTable {
+  let FilterClass = "SysReg";
+  let Fields = [
+     "Name", "Encoding", "FeaturesRequired",
+  ];
+
+  let PrimaryKey = [ "Encoding" ];
+  let PrimaryKeyName = "lookupSysRegByEncoding";
+  let PrimaryKeyReturnRange = true;
+}
+
+let FeaturesRequired = [{ {Feature1} }] in {
+def : SysReg<"csr1", 0x7C0>;
+}
+
+let FeaturesRequired = [{ {Feature2} }] in {
+def : SysReg<"csr2", 0x7C0>;
+}
+
+def lookupSysRegByName : SearchIndex {
+  let Table = List1;
+  let Key = [ "Name" ];
+#ifdef ERROR
+  // ERROR: Emitting different signature for returning a range of results is only supported for Primary Key.
+  let ReturnRange = true;
+#endif
+}
+
+// CHECK: #ifdef GET_List1_DECL
+// CHECK-NEXT: llvm::iterator_range<const SysReg *> lookupSysRegByEncoding(uint16_t Encoding);
+// CHECK-NEXT: onst SysReg *lookupSysRegByName(StringRef Name);
+// CHECK-NEXT: #endif
+
+// CHECK: #ifdef GET_List1_IMPL
+// CHECK-NEXT: constexpr SysReg List1[] = {
+// CHECK-NEXT:   { "csr1", 0x7C0,  {Feature1}  }, // 0
+// CHECK-NEXT:   { "csr2", 0x7C0,  {Feature2}  }, // 1
+// CHECK-NEXT:  };
+
+// CHECK: llvm::iterator_range<const SysReg *> lookupSysRegByEncoding(uint16_t Encoding) {
+// CHECK-NEXT: struct KeyType {
+// CHECK-NEXT:    uint16_t Encoding;
+// CHECK-NEXT:  };
+// CHECK-NEXT:  KeyType Key = {Encoding};
+// CHECK-NEXT:  struct Comp {
+// CHECK-NEXT:    bool operator()(const SysReg &LHS, const KeyType &RHS) const {
+// CHECK-NEXT:      if (LHS.Encoding < RHS.Encoding)
+// CHECK-NEXT:        return true;
+// CHECK-NEXT:      if (LHS.Encoding > RHS.Encoding)
+// CHECK-NEXT:        return false;
+// CHECK-NEXT:      return false;
+// CHECK-NEXT:    }
+// CHECK-NEXT:    bool operator()(const KeyType &LHS, const SysReg &RHS) const {
+// CHECK-NEXT:      if (LHS.Encoding < RHS.Encoding)
+// CHECK-NEXT:        return true;
+// CHECK-NEXT:      if (LHS.Encoding > RHS.Encoding)
+// CHECK-NEXT:        return false;
+// CHECK-NEXT:      return false;
+// CHECK-NEXT:    }
+// CHECK-NEXT:  };
+// CHECK-NEXT:  auto Table = ArrayRef(List1);
+// CHECK-NEXT:  auto It = std::equal_range(Table.begin(), Table.end(), Key, Comp());
+// CHECK-NEXT:  return llvm::make_range(It.first, It.second);
+// CHECK-NEXT: }
+
+// CHECK: const SysReg *lookupSysRegByName(StringRef Name) {
+// CHECK-NEXT:   struct IndexType {
+// CHECK-NEXT:     const char * Name;
+// CHECK-NEXT:     unsigned _index;
+// CHECK-NEXT:   };
+// CHECK-NEXT:   static const struct IndexType Index[] = {
+// CHECK-NEXT:     { "CSR1", 0 },
+// CHECK-NEXT:     { "CSR2", 1 },
+// CHECK-NEXT:   };
+
+// CHECK:   struct KeyType {
+// CHECK-NEXT:     std::string Name;
+// CHECK-NEXT:   };
+// CHECK-NEXT:   KeyType Key = {Name.upper()};
+// CHECK-NEXT:   struct Comp {
+// CHECK-NEXT:     bool operator()(const IndexType &LHS, const KeyType &RHS) const {
+// CHECK-NEXT:       int CmpName = StringRef(LHS.Name).compare(RHS.Name);
+// CHECK-NEXT:       if (CmpName < 0) return true;
+// CHECK-NEXT:       if (CmpName > 0) return false;
+// CHECK-NEXT:       return false;
+// CHECK-NEXT:     }
+// CHECK-NEXT:   };
+// CHECK-NEXT:   auto Table = ArrayRef(Index);
+// CHECK-NEXT:   auto Idx = std::lower_bound(Table.begin(), Table.end(), Key, Comp());
+// CHECK-NEXT:   if (Idx == Table.end() ||
+// CHECK-NEXT:       Key.Name != Idx->Name)
+// CHECK-NEXT:     return nullptr;
+
+// CHECK:  return &List1[Idx->_index];
+// CHECK-NEXT: }
+// CHECK-NEXT: #endif