diff --git a/llvm/include/llvm/IR/ModuleSummaryIndex.h b/llvm/include/llvm/IR/ModuleSummaryIndex.h
index 50def0eaf7886..39c60229aa1d8 100644
--- a/llvm/include/llvm/IR/ModuleSummaryIndex.h
+++ b/llvm/include/llvm/IR/ModuleSummaryIndex.h
@@ -1463,7 +1463,7 @@ class ModuleSummaryIndex {
   // in the way some record are interpreted, like flags for instance.
   // Note that incrementing this may require changes in both BitcodeReader.cpp
   // and BitcodeWriter.cpp.
-  static constexpr uint64_t BitcodeSummaryVersion = 11;
+  static constexpr uint64_t BitcodeSummaryVersion = 12;
 
   // Regular LTO module name for ASM writer
   static constexpr const char *getRegularLTOModuleName() {
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 9ca76b54a88d9..3e6abacac2726 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -7997,7 +7997,16 @@ Error ModuleSummaryIndexBitcodeReader::parseEntireSummary(unsigned ID) {
     case bitc::FS_STACK_IDS: { // [n x stackid]
       // Save stack ids in the reader to consult when adding stack ids from the
       // lists in the stack node and alloc node entries.
-      StackIds = ArrayRef<uint64_t>(Record);
+      if (Version <= 11) {
+        StackIds = ArrayRef<uint64_t>(Record);
+        break;
+      }
+      // This is an array of 32-bit fixed-width values, holding each 64-bit
+      // context id as a pair of adjacent (most significant first) 32-bit words.
+      assert(Record.size() % 2 == 0);
+      StackIds.reserve(Record.size() / 2);
+      for (auto R = Record.begin(); R != Record.end(); R += 2)
+        StackIds.push_back(*R << 32 | *(R + 1));
       break;
     }
 
diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 5829af39cf5e2..24a4c2e8303d5 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -4429,12 +4429,17 @@ void ModuleBitcodeWriterBase::writePerModuleGlobalValueSummary() {
     StackIdAbbv->Add(BitCodeAbbrevOp(bitc::FS_STACK_IDS));
     // numids x stackid
     StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
-    // FIXME: The stack ids are hashes that are close to 64 bits in size, so
-    // emitting as a pair of 32-bit fixed-width values, as we do for context
-    // ids, would be more efficient.
-    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
+    // The stack ids are hashes that are close to 64 bits in size, so emitting
+    // as a pair of 32-bit fixed-width values is more efficient than a VBR.
+    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32));
     unsigned StackIdAbbvId = Stream.EmitAbbrev(std::move(StackIdAbbv));
-    Stream.EmitRecord(bitc::FS_STACK_IDS, Index->stackIds(), StackIdAbbvId);
+    SmallVector<uint32_t> Vals;
+    Vals.reserve(Index->stackIds().size() * 2);
+    for (auto Id : Index->stackIds()) {
+      Vals.push_back(static_cast<uint32_t>(Id >> 32));
+      Vals.push_back(static_cast<uint32_t>(Id));
+    }
+    Stream.EmitRecord(bitc::FS_STACK_IDS, Vals, StackIdAbbvId);
   }
 
   // n x context id
@@ -4624,9 +4629,17 @@ void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
     StackIdAbbv->Add(BitCodeAbbrevOp(bitc::FS_STACK_IDS));
     // numids x stackid
     StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
-    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
+    // The stack ids are hashes that are close to 64 bits in size, so emitting
+    // as a pair of 32-bit fixed-width values is more efficient than a VBR.
+    StackIdAbbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32));
     unsigned StackIdAbbvId = Stream.EmitAbbrev(std::move(StackIdAbbv));
-    Stream.EmitRecord(bitc::FS_STACK_IDS, StackIds, StackIdAbbvId);
+    SmallVector<uint32_t> Vals;
+    Vals.reserve(StackIds.size() * 2);
+    for (auto Id : StackIds) {
+      Vals.push_back(static_cast<uint32_t>(Id >> 32));
+      Vals.push_back(static_cast<uint32_t>(Id));
+    }
+    Stream.EmitRecord(bitc::FS_STACK_IDS, Vals, StackIdAbbvId);
   }
 
   // Abbrev for FS_COMBINED_PROFILE.
diff --git a/llvm/test/Bitcode/summary_version.ll b/llvm/test/Bitcode/summary_version.ll
index c8d36f812c208..c95c145a08788 100644
--- a/llvm/test/Bitcode/summary_version.ll
+++ b/llvm/test/Bitcode/summary_version.ll
@@ -2,7 +2,7 @@
 ; RUN: opt  -module-summary  %s -o - | llvm-bcanalyzer -dump | FileCheck %s
 
 ; CHECK: <GLOBALVAL_SUMMARY_BLOCK
-; CHECK: <VERSION op0=11/>
+; CHECK: <VERSION op0=12/>
 
 
 
diff --git a/llvm/test/ThinLTO/X86/Inputs/memprof-old-stackid-summary.bc b/llvm/test/ThinLTO/X86/Inputs/memprof-old-stackid-summary.bc
new file mode 100644
index 0000000000000..78b134cb44216
Binary files /dev/null and b/llvm/test/ThinLTO/X86/Inputs/memprof-old-stackid-summary.bc differ
diff --git a/llvm/test/ThinLTO/X86/memprof-old-stackid-summary.ll b/llvm/test/ThinLTO/X86/memprof-old-stackid-summary.ll
new file mode 100644
index 0000000000000..10048f8674a08
--- /dev/null
+++ b/llvm/test/ThinLTO/X86/memprof-old-stackid-summary.ll
@@ -0,0 +1,20 @@
+;; Check that we can read the old STACK_ID summary format that encoded the id as
+;; a VBR8 instead of as a pair of 32-bit fixed-width values.
+;;
+;; The old bitcode was generated by the older compiler from `opt -thinlto-bc`
+;; on the following LLVM assembly:
+;;
+;; target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+;; target triple = "x86_64-unknown-linux-gnu"
+;;
+;; define void @bar() {
+;;   call void @foo(), !callsite !0
+;;   ret void
+;; }
+;;
+;; declare void @foo()
+;;
+;; !0 = !{i64 9086428284934609951}
+
+; RUN: llvm-dis %S/Inputs/memprof-old-stackid-summary.bc -o - | FileCheck %s
+; CHECK: stackIds: (9086428284934609951)