diff --git a/llvm/include/llvm/ExecutionEngine/Orc/MapperJITLinkMemoryManager.h b/llvm/include/llvm/ExecutionEngine/Orc/MapperJITLinkMemoryManager.h
--- a/llvm/include/llvm/ExecutionEngine/Orc/MapperJITLinkMemoryManager.h
+++ b/llvm/include/llvm/ExecutionEngine/Orc/MapperJITLinkMemoryManager.h
@@ -13,6 +13,7 @@
 #ifndef LLVM_EXECUTIONENGINE_ORC_MAPPERJITLINKMEMORYMANAGER_H
 #define LLVM_EXECUTIONENGINE_ORC_MAPPERJITLINKMEMORYMANAGER_H
 
+#include "llvm/ADT/IntervalMap.h"
 #include "llvm/ExecutionEngine/JITLink/JITLinkMemoryManager.h"
 #include "llvm/ExecutionEngine/Orc/MemoryMapper.h"
 
@@ -52,8 +53,12 @@
 
   // We reserve multiples of this from the executor address space
   size_t ReservationUnits;
+
   // Ranges that have been reserved in executor but not yet allocated
-  std::vector<ExecutorAddrRange> AvailableMemory;
+  using AvailableMemoryMap = IntervalMap<ExecutorAddr, bool>;
+  AvailableMemoryMap::Allocator AMAllocator;
+  IntervalMap<ExecutorAddr, bool> AvailableMemory;
+
   // Ranges that have been reserved in executor and already allocated
   DenseMap<ExecutorAddr, ExecutorAddrDiff> UsedMemory;
 
diff --git a/llvm/lib/ExecutionEngine/Orc/MapperJITLinkMemoryManager.cpp b/llvm/lib/ExecutionEngine/Orc/MapperJITLinkMemoryManager.cpp
--- a/llvm/lib/ExecutionEngine/Orc/MapperJITLinkMemoryManager.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/MapperJITLinkMemoryManager.cpp
@@ -57,7 +57,8 @@
 
 MapperJITLinkMemoryManager::MapperJITLinkMemoryManager(
     size_t ReservationGranularity, std::unique_ptr<MemoryMapper> Mapper)
-    : ReservationUnits(ReservationGranularity), Mapper(std::move(Mapper)) {}
+    : ReservationUnits(ReservationGranularity), AvailableMemory(AMAllocator),
+      Mapper(std::move(Mapper)) {}
 
 void MapperJITLinkMemoryManager::allocate(const JITLinkDylib *JD, LinkGraph &G,
                                           OnAllocatedFunction OnAllocated) {
@@ -72,20 +73,6 @@
 
   auto TotalSize = SegsSizes->total();
 
-  Mutex.lock();
-
-  // find an already reserved range that is large enough
-  ExecutorAddrRange SelectedRange{};
-  std::vector<ExecutorAddrRange>::iterator SelectedRangeIt;
-  SelectedRangeIt =
-      llvm::find_if(AvailableMemory, [TotalSize](ExecutorAddrRange Range) {
-        return TotalSize < Range.size();
-      });
-  if (SelectedRangeIt != AvailableMemory.end()) {
-    SelectedRange = *SelectedRangeIt;
-    AvailableMemory.erase(SelectedRangeIt);
-  }
-
   auto CompleteAllocation = [this, &G, BL = std::move(BL),
                              OnAllocated = std::move(OnAllocated)](
                                 Expected<ExecutorAddrRange> Result) mutable {
@@ -123,8 +110,7 @@
 
     if (NextSegAddr < Result->End) {
       // Save the remaining memory for reuse in next allocation(s)
-      auto RemainingRange = ExecutorAddrRange(NextSegAddr, Result->End);
-      AvailableMemory.push_back(RemainingRange);
+      AvailableMemory.insert(NextSegAddr, Result->End - 1, true);
     }
     Mutex.unlock();
 
@@ -137,6 +123,20 @@
                                                 std::move(SegInfos)));
   };
 
+  Mutex.lock();
+
+  // find an already reserved range that is large enough
+  ExecutorAddrRange SelectedRange{};
+
+  for (AvailableMemoryMap::iterator It = AvailableMemory.begin();
+       It != AvailableMemory.end(); It++) {
+    if (It.stop() - It.start() + 1 >= TotalSize) {
+      SelectedRange = ExecutorAddrRange(It.start(), It.stop() + 1);
+      It.erase();
+      break;
+    }
+  }
+
   if (SelectedRange.empty()) { // no already reserved range was found
     auto TotalAllocation = alignTo(TotalSize, ReservationUnits);
     Mapper->reserve(TotalAllocation, std::move(CompleteAllocation));
@@ -168,7 +168,7 @@
         ExecutorAddrDiff Size = UsedMemory[Addr];
 
         UsedMemory.erase(Addr);
-        AvailableMemory.push_back({Addr, Addr + Size});
+        AvailableMemory.insert(Addr, Addr + Size - 1, true);
 
         FA.release();
       }
diff --git a/llvm/unittests/ExecutionEngine/Orc/MapperJITLinkMemoryManagerTest.cpp b/llvm/unittests/ExecutionEngine/Orc/MapperJITLinkMemoryManagerTest.cpp
--- a/llvm/unittests/ExecutionEngine/Orc/MapperJITLinkMemoryManagerTest.cpp
+++ b/llvm/unittests/ExecutionEngine/Orc/MapperJITLinkMemoryManagerTest.cpp
@@ -1,4 +1,4 @@
-//===-------------- MapperJITLinkMemoryManagerTest.cpp ----------------===//
+//===---------------- MapperJITLinkMemoryManagerTest.cpp ------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -131,4 +131,46 @@
   EXPECT_EQ(Counter->DeinitCount, 2);
 }
 
+TEST(MapperJITLinkMemoryManagerTest, Coalescing) {
+  auto Mapper = cantFail(InProcessMemoryMapper::Create());
+  auto MemMgr = std::make_unique<MapperJITLinkMemoryManager>(16 * 1024 * 1024,
+                                                             std::move(Mapper));
+
+  auto SSA1 = jitlink::SimpleSegmentAlloc::Create(
+      *MemMgr, nullptr, {{jitlink::MemProt::Read, {1024, Align(1)}}});
+  EXPECT_THAT_EXPECTED(SSA1, Succeeded());
+  auto SegInfo1 = SSA1->getSegInfo(jitlink::MemProt::Read);
+  ExecutorAddr TargetAddr1(SegInfo1.Addr);
+  auto FA1 = SSA1->finalize();
+  EXPECT_THAT_EXPECTED(FA1, Succeeded());
+
+  auto SSA2 = jitlink::SimpleSegmentAlloc::Create(
+      *MemMgr, nullptr, {{jitlink::MemProt::Read, {1024, Align(1)}}});
+  EXPECT_THAT_EXPECTED(SSA2, Succeeded());
+  auto FA2 = SSA2->finalize();
+  EXPECT_THAT_EXPECTED(FA2, Succeeded());
+
+  auto Err2 = MemMgr->deallocate(std::move(*FA1));
+  EXPECT_THAT_ERROR(std::move(Err2), Succeeded());
+
+  auto Err3 = MemMgr->deallocate(std::move(*FA2));
+  EXPECT_THAT_ERROR(std::move(Err3), Succeeded());
+
+  auto SSA3 = jitlink::SimpleSegmentAlloc::Create(
+      *MemMgr, nullptr, {{jitlink::MemProt::Read, {2048, Align(1)}}});
+  EXPECT_THAT_EXPECTED(SSA3, Succeeded());
+
+  auto SegInfo3 = SSA3->getSegInfo(jitlink::MemProt::Read);
+  ExecutorAddr TargetAddr3(SegInfo3.Addr);
+
+  auto FA3 = SSA3->finalize();
+  EXPECT_THAT_EXPECTED(FA3, Succeeded());
+
+  // previous two freed 1024 blocks should be fused to form a 2048 block
+  EXPECT_EQ(TargetAddr1, TargetAddr3);
+
+  auto Err4 = MemMgr->deallocate(std::move(*FA3));
+  EXPECT_THAT_ERROR(std::move(Err4), Succeeded());
+}
+
 } // namespace