diff --git a/mlir/docs/BytecodeFormat.md b/mlir/docs/BytecodeFormat.md
--- a/mlir/docs/BytecodeFormat.md
+++ b/mlir/docs/BytecodeFormat.md
@@ -154,6 +154,7 @@
 dialect_section {
   numDialects: varint,
   dialectNames: varint[],
+  numTotalOpNames: varint,
   opNames: op_name_group[]
 }
 
@@ -444,8 +445,8 @@
 }
 
 block_argument {
-  typeIndex: varint,
-  location: varint
+  typeAndLocation: varint, // (type << 1) | (hasLocation)
+  location: varint? // Optional, else unknown location
 }
 ```
 
diff --git a/mlir/include/mlir/Bytecode/Encoding.h b/mlir/include/mlir/Bytecode/Encoding.h
--- a/mlir/include/mlir/Bytecode/Encoding.h
+++ b/mlir/include/mlir/Bytecode/Encoding.h
@@ -29,7 +29,7 @@
   kMinSupportedVersion = 0,
 
   /// The current bytecode version.
-  kVersion = 3,
+  kVersion = 4,
 
   /// An arbitrary value used to fill alignment padding.
   kAlignmentByte = 0xCB,
diff --git a/mlir/lib/Bytecode/Reader/BytecodeReader.cpp b/mlir/lib/Bytecode/Reader/BytecodeReader.cpp
--- a/mlir/lib/Bytecode/Reader/BytecodeReader.cpp
+++ b/mlir/lib/Bytecode/Reader/BytecodeReader.cpp
@@ -1589,6 +1589,14 @@
     opNames.emplace_back(dialect, opName);
     return success();
   };
+  // Avoid re-allocation in bytecode version > 3 where the number of ops are
+  // known.
+  if (version > 3) {
+    uint64_t numOps;
+    if (failed(sectionReader.parseVarInt(numOps)))
+      return failure();
+    opNames.reserve(numOps);
+  }
   while (!sectionReader.empty())
     if (failed(parseDialectGrouping(sectionReader, dialects, parseOpName)))
       return failure();
@@ -2149,13 +2157,25 @@
   argTypes.reserve(numArgs);
   argLocs.reserve(numArgs);
 
+  Location unknownLoc = UnknownLoc::get(config.getContext());
   while (numArgs--) {
     Type argType;
-    LocationAttr argLoc;
-    if (failed(parseType(reader, argType)) ||
-        failed(parseAttribute(reader, argLoc)))
-      return failure();
-
+    LocationAttr argLoc = unknownLoc;
+    if (version > 3) {
+      // Parse the type with hasLoc flag to determine if it has type.
+      uint64_t typeIdx;
+      bool hasLoc;
+      if (failed(reader.parseVarIntWithFlag(typeIdx, hasLoc)) ||
+          !(argType = attrTypeReader.resolveType(typeIdx)))
+        return failure();
+      if (hasLoc && failed(parseAttribute(reader, argLoc)))
+        return failure();
+    } else {
+      // All args has type and location.
+      if (failed(parseType(reader, argType)) ||
+          failed(parseAttribute(reader, argLoc)))
+        return failure();
+    }
     argTypes.push_back(argType);
     argLocs.push_back(argLoc);
   }
diff --git a/mlir/lib/Bytecode/Writer/BytecodeWriter.cpp b/mlir/lib/Bytecode/Writer/BytecodeWriter.cpp
--- a/mlir/lib/Bytecode/Writer/BytecodeWriter.cpp
+++ b/mlir/lib/Bytecode/Writer/BytecodeWriter.cpp
@@ -585,6 +585,9 @@
                                  std::move(versionEmitter));
   }
 
+  if (config.bytecodeVersion > 3)
+    dialectEmitter.emitVarInt(size(numberingState.getOpNames()));
+
   // Emit the referenced operation names grouped by dialect.
   auto emitOpName = [&](OpNameNumbering &name) {
     dialectEmitter.emitVarInt(stringSection.insert(name.name.stripDialect()));
@@ -670,8 +673,16 @@
   if (hasArgs) {
     emitter.emitVarInt(args.size());
     for (BlockArgument arg : args) {
-      emitter.emitVarInt(numberingState.getNumber(arg.getType()));
-      emitter.emitVarInt(numberingState.getNumber(arg.getLoc()));
+      Location argLoc = arg.getLoc();
+      if (config.bytecodeVersion > 3) {
+        emitter.emitVarIntWithFlag(numberingState.getNumber(arg.getType()),
+                                   !isa<UnknownLoc>(argLoc));
+        if (!isa<UnknownLoc>(argLoc))
+          emitter.emitVarInt(numberingState.getNumber(argLoc));
+      } else {
+        emitter.emitVarInt(numberingState.getNumber(arg.getType()));
+        emitter.emitVarInt(numberingState.getNumber(argLoc));
+      }
     }
     if (config.bytecodeVersion > 2) {
       uint64_t maskOffset = emitter.size();
@@ -755,7 +766,7 @@
 
     for (Region &region : op->getRegions()) {
       // If the region is not isolated from above, or we are emitting bytecode
-      // targetting version <2, we don't use a section.
+      // targeting version <2, we don't use a section.
       if (!isIsolatedFromAbove || config.bytecodeVersion < 2) {
         writeRegion(emitter, &region);
         continue;
diff --git a/mlir/test/Bytecode/general.mlir b/mlir/test/Bytecode/general.mlir
--- a/mlir/test/Bytecode/general.mlir
+++ b/mlir/test/Bytecode/general.mlir
@@ -32,7 +32,7 @@
   }
   "bytecode.branch"()[^secondBlock] : () -> ()
 
-^secondBlock(%arg1: i32, %arg2: !bytecode.int, %arg3: !pdl.operation):
+^secondBlock(%arg1: i32 loc(unknown), %arg2: !bytecode.int, %arg3: !pdl.operation loc(unknown)):
   "bytecode.regions"() ({
     "bytecode.operands"(%arg1, %arg2, %arg3) : (i32, !bytecode.int, !pdl.operation) -> ()
     "bytecode.return"() : () -> ()
diff --git a/mlir/test/Bytecode/invalid/invalid-structure.mlir b/mlir/test/Bytecode/invalid/invalid-structure.mlir
--- a/mlir/test/Bytecode/invalid/invalid-structure.mlir
+++ b/mlir/test/Bytecode/invalid/invalid-structure.mlir
@@ -9,7 +9,7 @@
 //===--------------------------------------------------------------------===//
 
 // RUN: not mlir-opt %S/invalid-structure-version.mlirbc 2>&1 | FileCheck %s --check-prefix=VERSION
-// VERSION: bytecode version 127 is newer than the current version 3
+// VERSION: bytecode version 127 is newer than the current version
 
 //===--------------------------------------------------------------------===//
 // Producer