Index: lib/sanitizer_common/sanitizer_allocator_primary64.h
===================================================================
--- lib/sanitizer_common/sanitizer_allocator_primary64.h
+++ lib/sanitizer_common/sanitizer_allocator_primary64.h
@@ -94,6 +94,13 @@
     return P >= SpaceBeg() && P < SpaceEnd();
   }
 
+  uptr GetRegionBegin(const void *p) {
+    if (kUsingConstantSpaceBeg)
+      return reinterpret_cast<uptr>(p) & ~(kRegionSize - 1);
+    uptr space_beg = SpaceBeg();
+    return ((reinterpret_cast<uptr>(p)  - space_beg) & ~(kRegionSize - 1)) + space_beg;
+  }
+
   uptr GetSizeClass(const void *p) {
     if (kUsingConstantSpaceBeg && (kSpaceBeg % kSpaceSize) == 0)
       return ((reinterpret_cast<uptr>(p)) / kRegionSize) % kNumClassesRounded;
@@ -106,7 +113,7 @@
     uptr size = SizeClassMap::Size(class_id);
     if (!size) return nullptr;
     uptr chunk_idx = GetChunkIdx((uptr)p, size);
-    uptr reg_beg = (uptr)p & ~(kRegionSize - 1);
+    uptr reg_beg = GetRegionBegin(p);    
     uptr beg = chunk_idx * size;
     uptr next_beg = beg + size;
     if (class_id >= kNumClasses) return nullptr;
@@ -258,7 +265,10 @@
     return &regions[class_id];
   }
 
-  static uptr GetChunkIdx(uptr chunk, uptr size) {
+  uptr GetChunkIdx(uptr chunk, uptr size) {
+    if (!kUsingConstantSpaceBeg)
+      chunk -= SpaceBeg();
+
     uptr offset = chunk % kRegionSize;
     // Here we divide by a non-constant. This is costly.
     // size always fits into 32-bits. If the offset fits too, use 32-bit div.
Index: lib/sanitizer_common/sanitizer_win.cc
===================================================================
--- lib/sanitizer_common/sanitizer_win.cc
+++ lib/sanitizer_common/sanitizer_win.cc
@@ -221,8 +221,12 @@
 }
 
 void *MmapNoAccess(uptr size) {
-  // FIXME: unsupported.
-  return nullptr;
+  void *res = VirtualAlloc(nullptr, size, MEM_RESERVE, PAGE_NOACCESS);
+  if (res == 0)
+    Report("WARNING: %s failed to "
+           "mprotect %p (%zd) bytes (error code: %d)\n",
+           SanitizerToolName, size, size, GetLastError());
+  return res;
 }
 
 bool MprotectNoAccess(uptr addr, uptr size) {
Index: lib/sanitizer_common/tests/sanitizer_allocator_test.cc
===================================================================
--- lib/sanitizer_common/tests/sanitizer_allocator_test.cc
+++ lib/sanitizer_common/tests/sanitizer_allocator_test.cc
@@ -30,7 +30,10 @@
 
 #if SANITIZER_CAN_USE_ALLOCATOR64
 #if SANITIZER_WINDOWS
-static const uptr kAllocatorSpace = 0x10000000000ULL;
+// On Windows 64-bit there is no easy way to find a large enough fixed address
+// space that is always available. Thus, a dynamically allocated address space
+// is used instead (i.e. ~(uptr)0).
+static const uptr kAllocatorSpace = ~(uptr)0;
 static const uptr kAllocatorSize  =  0x10000000000ULL;  // 1T.
 static const u64 kAddressSpaceSize = 1ULL << 40;
 #else
@@ -41,6 +44,8 @@
 
 typedef SizeClassAllocator64<
   kAllocatorSpace, kAllocatorSize, 16, DefaultSizeClassMap> Allocator64;
+typedef SizeClassAllocator64<
+  ~(uptr)0, kAllocatorSize, 16, DefaultSizeClassMap> Allocator64Dynamic;
 
 typedef SizeClassAllocator64<
   kAllocatorSpace, kAllocatorSize, 16, CompactSizeClassMap> Allocator64Compact;
@@ -158,6 +163,10 @@
   TestSizeClassAllocator<Allocator64>();
 }
 
+TEST(SanitizerCommon, SizeClassAllocator64Dynamic) {
+  TestSizeClassAllocator<Allocator64Dynamic>();
+}
+
 TEST(SanitizerCommon, SizeClassAllocator64Compact) {
   TestSizeClassAllocator<Allocator64Compact>();
 }
@@ -202,6 +211,10 @@
   SizeClassAllocatorMetadataStress<Allocator64>();
 }
 
+TEST(SanitizerCommon, SizeClassAllocator64DynamicMetadataStress) {
+  SizeClassAllocatorMetadataStress<Allocator64Dynamic>();
+}
+
 TEST(SanitizerCommon, SizeClassAllocator64CompactMetadataStress) {
   SizeClassAllocatorMetadataStress<Allocator64Compact>();
 }
@@ -238,6 +251,9 @@
 TEST(SanitizerCommon, SizeClassAllocator64GetBlockBegin) {
   SizeClassAllocatorGetBlockBeginStress<Allocator64>();
 }
+TEST(SanitizerCommon, SizeClassAllocator64DynamicGetBlockBegin) {
+  SizeClassAllocatorGetBlockBeginStress<Allocator64Dynamic>();
+}
 TEST(SanitizerCommon, SizeClassAllocator64CompactGetBlockBegin) {
   SizeClassAllocatorGetBlockBeginStress<Allocator64Compact>();
 }
@@ -484,6 +500,12 @@
       SizeClassAllocatorLocalCache<Allocator64> > ();
 }
 
+TEST(SanitizerCommon, CombinedAllocator64Dynamic) {
+  TestCombinedAllocator<Allocator64Dynamic,
+      LargeMmapAllocator<>,
+      SizeClassAllocatorLocalCache<Allocator64Dynamic> > ();
+}
+
 TEST(SanitizerCommon, CombinedAllocator64Compact) {
   TestCombinedAllocator<Allocator64Compact,
       LargeMmapAllocator<>,
@@ -537,6 +559,11 @@
       SizeClassAllocatorLocalCache<Allocator64> >();
 }
 
+TEST(SanitizerCommon, SizeClassAllocator64DynamicLocalCache) {
+  TestSizeClassAllocatorLocalCache<
+      SizeClassAllocatorLocalCache<Allocator64Dynamic> >();
+}
+
 TEST(SanitizerCommon, SizeClassAllocator64CompactLocalCache) {
   TestSizeClassAllocatorLocalCache<
       SizeClassAllocatorLocalCache<Allocator64Compact> >();
@@ -710,6 +737,9 @@
 TEST(SanitizerCommon, SizeClassAllocator64Iteration) {
   TestSizeClassAllocatorIteration<Allocator64>();
 }
+TEST(SanitizerCommon, SizeClassAllocator64DynamicIteration) {
+  TestSizeClassAllocatorIteration<Allocator64Dynamic>();
+}
 #endif
 
 TEST(SanitizerCommon, SizeClassAllocator32Iteration) {