diff --git a/llvm/docs/TableGen/LangIntro.rst b/llvm/docs/TableGen/LangIntro.rst
index 460ff9067f20..534597b041bc 100644
--- a/llvm/docs/TableGen/LangIntro.rst
+++ b/llvm/docs/TableGen/LangIntro.rst
@@ -205,6 +205,9 @@ supported include:
 ``!empty(a)``
     An integer {0,1} indicating whether list 'a' is empty.
 
+``!size(a)``
+    An integer indicating the number of elements in list 'a'.
+
 ``!if(a,b,c)``
   'b' if the result of 'int' or 'bit' operator 'a' is nonzero, 'c' otherwise.
 
diff --git a/llvm/docs/TableGen/LangRef.rst b/llvm/docs/TableGen/LangRef.rst
index 285572fa481c..5f7917a5b4bb 100644
--- a/llvm/docs/TableGen/LangRef.rst
+++ b/llvm/docs/TableGen/LangRef.rst
@@ -98,7 +98,7 @@ wide variety of meanings:
                :!eq     !if      !head    !tail      !con
                :!add    !shl     !sra     !srl       !and
                :!or     !empty   !subst   !foreach   !strconcat
-               :!cast   !listconcat
+               :!cast   !listconcat       !size
 
 
 Syntax
diff --git a/llvm/include/llvm/TableGen/Record.h b/llvm/include/llvm/TableGen/Record.h
index 0c27f395cfbd..a94f76d852f3 100644
--- a/llvm/include/llvm/TableGen/Record.h
+++ b/llvm/include/llvm/TableGen/Record.h
@@ -759,7 +759,7 @@ public:
 ///
 class UnOpInit : public OpInit, public FoldingSetNode {
 public:
-  enum UnaryOp : uint8_t { CAST, HEAD, TAIL, EMPTY };
+  enum UnaryOp : uint8_t { CAST, HEAD, TAIL, SIZE, EMPTY };
 
 private:
   Init *LHS;
diff --git a/llvm/lib/TableGen/Record.cpp b/llvm/lib/TableGen/Record.cpp
index f447b3b1b477..80c0973644bb 100644
--- a/llvm/lib/TableGen/Record.cpp
+++ b/llvm/lib/TableGen/Record.cpp
@@ -722,6 +722,11 @@ Init *UnOpInit::Fold(Record *CurRec, MultiClass *CurMultiClass) const {
     }
     break;
 
+  case SIZE:
+    if (ListInit *LHSl = dyn_cast<ListInit>(LHS))
+      return IntInit::get(LHSl->size());
+    break;
+
   case EMPTY:
     if (ListInit *LHSl = dyn_cast<ListInit>(LHS))
       return IntInit::get(LHSl->empty());
@@ -746,6 +751,7 @@ std::string UnOpInit::getAsString() const {
   case CAST: Result = "!cast<" + getType()->getAsString() + ">"; break;
   case HEAD: Result = "!head"; break;
   case TAIL: Result = "!tail"; break;
+  case SIZE: Result = "!size"; break;
   case EMPTY: Result = "!empty"; break;
   }
   return Result + "(" + LHS->getAsString() + ")";
diff --git a/llvm/lib/TableGen/TGLexer.cpp b/llvm/lib/TableGen/TGLexer.cpp
index 5d6f7c23e0b6..bc03055eb236 100644
--- a/llvm/lib/TableGen/TGLexer.cpp
+++ b/llvm/lib/TableGen/TGLexer.cpp
@@ -469,6 +469,7 @@ tgtok::TokKind TGLexer::LexExclaim() {
     .Case("if", tgtok::XIf)
     .Case("head", tgtok::XHead)
     .Case("tail", tgtok::XTail)
+    .Case("size", tgtok::XSize)
     .Case("con", tgtok::XConcat)
     .Case("add", tgtok::XADD)
     .Case("and", tgtok::XAND)
diff --git a/llvm/lib/TableGen/TGLexer.h b/llvm/lib/TableGen/TGLexer.h
index b5b58161878b..342eb76fc0f2 100644
--- a/llvm/lib/TableGen/TGLexer.h
+++ b/llvm/lib/TableGen/TGLexer.h
@@ -48,7 +48,7 @@ namespace tgtok {
 
     // !keywords.
     XConcat, XADD, XAND, XOR, XSRA, XSRL, XSHL, XListConcat, XStrConcat, XCast,
-    XSubst, XForEach, XHead, XTail, XEmpty, XIf, XEq,
+    XSubst, XForEach, XHead, XTail, XSize, XEmpty, XIf, XEq,
 
     // Integer value.
     IntVal,
diff --git a/llvm/lib/TableGen/TGParser.cpp b/llvm/lib/TableGen/TGParser.cpp
index 17c2c2ee6137..72ab8d3838d4 100644
--- a/llvm/lib/TableGen/TGParser.cpp
+++ b/llvm/lib/TableGen/TGParser.cpp
@@ -781,6 +781,7 @@ Init *TGParser::ParseOperation(Record *CurRec, RecTy *ItemType) {
     return nullptr;
   case tgtok::XHead:
   case tgtok::XTail:
+  case tgtok::XSize:
   case tgtok::XEmpty:
   case tgtok::XCast: {  // Value ::= !unop '(' Value ')'
     UnOpInit::UnaryOp Code;
@@ -808,6 +809,11 @@ Init *TGParser::ParseOperation(Record *CurRec, RecTy *ItemType) {
       Lex.Lex();  // eat the operation
       Code = UnOpInit::TAIL;
       break;
+    case tgtok::XSize:
+      Lex.Lex();
+      Code = UnOpInit::SIZE;
+      Type = IntRecTy::get();
+      break;
     case tgtok::XEmpty:
       Lex.Lex();  // eat the operation
       Code = UnOpInit::EMPTY;
@@ -842,12 +848,15 @@ Init *TGParser::ParseOperation(Record *CurRec, RecTy *ItemType) {
         }
       }
 
-      if (Code == UnOpInit::HEAD || Code == UnOpInit::TAIL) {
+      if (Code == UnOpInit::HEAD || Code == UnOpInit::TAIL ||
+          Code == UnOpInit::SIZE) {
         if (!LHSl && !LHSt) {
           TokError("expected list type argument in unary operator");
           return nullptr;
         }
+      }
 
+      if (Code == UnOpInit::HEAD || Code == UnOpInit::TAIL) {
         if (LHSl && LHSl->empty()) {
           TokError("empty list argument in unary operator");
           return nullptr;
@@ -1453,6 +1462,7 @@ Init *TGParser::ParseSimpleValue(Record *CurRec, RecTy *ItemType,
 
   case tgtok::XHead:
   case tgtok::XTail:
+  case tgtok::XSize:
   case tgtok::XEmpty:
   case tgtok::XCast:  // Value ::= !unop '(' Value ')'
   case tgtok::XConcat:
diff --git a/llvm/test/TableGen/size.td b/llvm/test/TableGen/size.td
new file mode 100644
index 000000000000..f7061e110790
--- /dev/null
+++ b/llvm/test/TableGen/size.td
@@ -0,0 +1,34 @@
+// RUN: llvm-tblgen %s | FileCheck %s
+// XFAIL: vg_leak
+
+// CHECK: --- Defs ---
+
+// CHECK: def A1 {
+// CHECK:   int Val = 0;
+// CHECK: }
+
+// CHECK: def A2 {
+// CHECK:   int Val = 3;
+// CHECK: }
+
+// CHECK: def B1 {
+// CHECK:   int Val = 0;
+// CHECK: }
+
+// CHECK: def B2 {
+// CHECK:   int Val = 2;
+// CHECK: }
+
+class A<list<int> L> {
+  int Val = !size(L);
+}
+
+class B<list<string> L> {
+  int Val = !size(L);
+}
+
+def A1 : A<[]>;
+def A2 : A<[1, 1, 2]>;
+
+def B1 : B<[]>;
+def B2 : B<["a", "b"]>;