diff --git a/compiler-rt/lib/asan/asan_internal.h b/compiler-rt/lib/asan/asan_internal.h
--- a/compiler-rt/lib/asan/asan_internal.h
+++ b/compiler-rt/lib/asan/asan_internal.h
@@ -118,8 +118,6 @@
 
 void *AsanDlSymNext(const char *sym);
 
-void ReserveShadowMemoryRange(uptr beg, uptr end, const char *name);
-
 // Returns `true` iff most of ASan init process should be skipped due to the
 // ASan library being loaded via `dlopen()`. Platforms may perform any
 // `dlopen()` specific initialization inside this function.
diff --git a/compiler-rt/lib/asan/asan_linux.cpp b/compiler-rt/lib/asan/asan_linux.cpp
--- a/compiler-rt/lib/asan/asan_linux.cpp
+++ b/compiler-rt/lib/asan/asan_linux.cpp
@@ -87,25 +87,12 @@
   return &_DYNAMIC;  // defined in link.h
 }
 
-static void UnmapFromTo(uptr from, uptr to) {
-  CHECK(to >= from);
-  if (to == from) return;
-  uptr res = internal_munmap(reinterpret_cast<void *>(from), to - from);
-  if (UNLIKELY(internal_iserror(res))) {
-    Report(
-        "ERROR: AddresSanitizer failed to unmap 0x%zx (%zd) bytes at address "
-        "%p\n",
-        to - from, to - from, from);
-    CHECK("unable to unmap" && 0);
-  }
-}
-
 #if ASAN_PREMAP_SHADOW
-uptr FindPremappedShadowStart() {
+uptr FindPremappedShadowStart(uptr shadow_size_bytes) {
   uptr granularity = GetMmapGranularity();
   uptr shadow_start = reinterpret_cast<uptr>(&__asan_shadow);
   uptr premap_shadow_size = PremapShadowSize();
-  uptr shadow_size = RoundUpTo(kHighShadowEnd, granularity);
+  uptr shadow_size = RoundUpTo(shadow_size_bytes, granularity);
   // We may have mapped too much. Release extra memory.
   UnmapFromTo(shadow_start + shadow_size, shadow_start + premap_shadow_size);
   return shadow_start;
@@ -113,25 +100,14 @@
 #endif
 
 uptr FindDynamicShadowStart() {
+  uptr shadow_size_bytes = MemToShadowSize(kHighMemEnd);
 #if ASAN_PREMAP_SHADOW
   if (!PremapShadowFailed())
-    return FindPremappedShadowStart();
+    return FindPremappedShadowStart(shadow_size_bytes);
 #endif
 
-  uptr granularity = GetMmapGranularity();
-  uptr alignment = granularity * 8;
-  uptr left_padding = granularity;
-  uptr shadow_size = RoundUpTo(kHighShadowEnd, granularity);
-  uptr map_size = shadow_size + left_padding + alignment;
-
-  uptr map_start = (uptr)MmapNoAccess(map_size);
-  CHECK_NE(map_start, ~(uptr)0);
-
-  uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
-  UnmapFromTo(map_start, shadow_start - left_padding);
-  UnmapFromTo(shadow_start + shadow_size, map_start + map_size);
-
-  return shadow_start;
+  return MapDynamicShadow(shadow_size_bytes, SHADOW_SCALE,
+                          /*min_shadow_base_alignment*/ 0, kHighMemEnd);
 }
 
 void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
diff --git a/compiler-rt/lib/asan/asan_mac.cpp b/compiler-rt/lib/asan/asan_mac.cpp
--- a/compiler-rt/lib/asan/asan_mac.cpp
+++ b/compiler-rt/lib/asan/asan_mac.cpp
@@ -55,46 +55,8 @@
 }
 
 uptr FindDynamicShadowStart() {
-  uptr granularity = GetMmapGranularity();
-  uptr alignment = 8 * granularity;
-  uptr left_padding = granularity;
-  uptr space_size = kHighShadowEnd + left_padding;
-
-  uptr largest_gap_found = 0;
-  uptr max_occupied_addr = 0;
-  VReport(2, "FindDynamicShadowStart, space_size = %p\n", space_size);
-  uptr shadow_start =
-      FindAvailableMemoryRange(space_size, alignment, granularity,
-                               &largest_gap_found, &max_occupied_addr);
-  // If the shadow doesn't fit, restrict the address space to make it fit.
-  if (shadow_start == 0) {
-    VReport(
-        2,
-        "Shadow doesn't fit, largest_gap_found = %p, max_occupied_addr = %p\n",
-        largest_gap_found, max_occupied_addr);
-    uptr new_max_vm = RoundDownTo(largest_gap_found << SHADOW_SCALE, alignment);
-    if (new_max_vm < max_occupied_addr) {
-      Report("Unable to find a memory range for dynamic shadow.\n");
-      Report(
-          "space_size = %p, largest_gap_found = %p, max_occupied_addr = %p, "
-          "new_max_vm = %p\n",
-          space_size, largest_gap_found, max_occupied_addr, new_max_vm);
-      CHECK(0 && "cannot place shadow");
-    }
-    RestrictMemoryToMaxAddress(new_max_vm);
-    kHighMemEnd = new_max_vm - 1;
-    space_size = kHighShadowEnd + left_padding;
-    VReport(2, "FindDynamicShadowStart, space_size = %p\n", space_size);
-    shadow_start = FindAvailableMemoryRange(space_size, alignment, granularity,
-                                            nullptr, nullptr);
-    if (shadow_start == 0) {
-      Report("Unable to find a memory range after restricting VM.\n");
-      CHECK(0 && "cannot place shadow after restricting vm");
-    }
-  }
-  CHECK_NE((uptr)0, shadow_start);
-  CHECK(IsAligned(shadow_start, alignment));
-  return shadow_start;
+  return MapDynamicShadow(MemToShadowSize(kHighMemEnd), SHADOW_SCALE,
+                          /*min_shadow_base_alignment*/ 0, kHighMemEnd);
 }
 
 // No-op. Mac does not support static linkage anyway.
diff --git a/compiler-rt/lib/asan/asan_mapping.h b/compiler-rt/lib/asan/asan_mapping.h
--- a/compiler-rt/lib/asan/asan_mapping.h
+++ b/compiler-rt/lib/asan/asan_mapping.h
@@ -304,6 +304,7 @@
 
 namespace __asan {
 
+static inline uptr MemToShadowSize(uptr size) { return size >> SHADOW_SCALE; }
 static inline bool AddrIsInLowMem(uptr a) {
   PROFILE_ASAN_MAPPING();
   return a <= kLowMemEnd;
diff --git a/compiler-rt/lib/asan/asan_premap_shadow.cpp b/compiler-rt/lib/asan/asan_premap_shadow.cpp
--- a/compiler-rt/lib/asan/asan_premap_shadow.cpp
+++ b/compiler-rt/lib/asan/asan_premap_shadow.cpp
@@ -32,22 +32,8 @@
 // Returns an address aligned to 8 pages, such that one page on the left and
 // PremapShadowSize() bytes on the right of it are mapped r/o.
 uptr PremapShadow() {
-  uptr granularity = GetMmapGranularity();
-  uptr alignment = granularity * 8;
-  uptr left_padding = granularity;
-  uptr shadow_size = PremapShadowSize();
-  uptr map_size = shadow_size + left_padding + alignment;
-
-  uptr map_start = (uptr)MmapNoAccess(map_size);
-  CHECK_NE(map_start, ~(uptr)0);
-
-  uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
-  uptr shadow_end = shadow_start + shadow_size;
-  internal_munmap(reinterpret_cast<void *>(map_start),
-                  shadow_start - left_padding - map_start);
-  internal_munmap(reinterpret_cast<void *>(shadow_end),
-                  map_start + map_size - shadow_end);
-  return shadow_start;
+  return MapDynamicShadow(PremapShadowSize(), /*mmap_alignment_scale*/ 3,
+                          /*min_shadow_base_alignment*/ 0, kHighMemEnd);
 }
 
 bool PremapShadowFailed() {
diff --git a/compiler-rt/lib/asan/asan_rtl.cpp b/compiler-rt/lib/asan/asan_rtl.cpp
--- a/compiler-rt/lib/asan/asan_rtl.cpp
+++ b/compiler-rt/lib/asan/asan_rtl.cpp
@@ -319,7 +319,7 @@
   kHighMemEnd = GetMaxUserVirtualAddress();
   // Increase kHighMemEnd to make sure it's properly
   // aligned together with kHighMemBeg:
-  kHighMemEnd |= SHADOW_GRANULARITY * GetMmapGranularity() - 1;
+  kHighMemEnd |= (GetMmapGranularity() << SHADOW_SCALE) - 1;
 #endif  // !ASAN_FIXED_MAPPING
   CHECK_EQ((kHighMemBeg % GetMmapGranularity()), 0);
 #endif  // !SANITIZER_MYRIAD2
diff --git a/compiler-rt/lib/asan/asan_shadow_setup.cpp b/compiler-rt/lib/asan/asan_shadow_setup.cpp
--- a/compiler-rt/lib/asan/asan_shadow_setup.cpp
+++ b/compiler-rt/lib/asan/asan_shadow_setup.cpp
@@ -22,24 +22,6 @@
 
 namespace __asan {
 
-// ---------------------- mmap -------------------- {{{1
-// Reserve memory range [beg, end].
-// We need to use inclusive range because end+1 may not be representable.
-void ReserveShadowMemoryRange(uptr beg, uptr end, const char *name) {
-  CHECK_EQ((beg % GetMmapGranularity()), 0);
-  CHECK_EQ(((end + 1) % GetMmapGranularity()), 0);
-  uptr size = end - beg + 1;
-  DecreaseTotalMmap(size);  // Don't count the shadow against mmap_limit_mb.
-  if (!MmapFixedSuperNoReserve(beg, size, name)) {
-    Report(
-        "ReserveShadowMemoryRange failed while trying to map 0x%zx bytes. "
-        "Perhaps you're using ulimit -v\n",
-        size);
-    Abort();
-  }
-  if (common_flags()->use_madv_dontdump) DontDumpShadowMemory(beg, size);
-}
-
 static void ProtectGap(uptr addr, uptr size) {
   if (!flags()->protect_shadow_gap) {
     // The shadow gap is unprotected, so there is a chance that someone
@@ -57,26 +39,8 @@
                              "unprotected gap shadow");
     return;
   }
-  void *res = MmapFixedNoAccess(addr, size, "shadow gap");
-  if (addr == (uptr)res) return;
-  // A few pages at the start of the address space can not be protected.
-  // But we really want to protect as much as possible, to prevent this memory
-  // being returned as a result of a non-FIXED mmap().
-  if (addr == kZeroBaseShadowStart) {
-    uptr step = GetMmapGranularity();
-    while (size > step && addr < kZeroBaseMaxShadowStart) {
-      addr += step;
-      size -= step;
-      void *res = MmapFixedNoAccess(addr, size, "shadow gap");
-      if (addr == (uptr)res) return;
-    }
-  }
-
-  Report(
-      "ERROR: Failed to protect the shadow gap. "
-      "ASan cannot proceed correctly. ABORTING.\n");
-  DumpProcessMap();
-  Die();
+  __sanitizer::ProtectGap(addr, size, kZeroBaseShadowStart,
+                          kZeroBaseMaxShadowStart);
 }
 
 static void MaybeReportLinuxPIEBug() {
@@ -99,8 +63,6 @@
   // |kDefaultShadowSentinel|.
   bool full_shadow_is_available = false;
   if (shadow_start == kDefaultShadowSentinel) {
-    __asan_shadow_memory_dynamic_address = 0;
-    CHECK_EQ(0, kLowShadowBeg);
     shadow_start = FindDynamicShadowStart();
     if (SANITIZER_LINUX) full_shadow_is_available = true;
   }
diff --git a/compiler-rt/lib/asan/asan_win.cpp b/compiler-rt/lib/asan/asan_win.cpp
--- a/compiler-rt/lib/asan/asan_win.cpp
+++ b/compiler-rt/lib/asan/asan_win.cpp
@@ -247,15 +247,8 @@
 }
 
 uptr FindDynamicShadowStart() {
-  uptr granularity = GetMmapGranularity();
-  uptr alignment = 8 * granularity;
-  uptr left_padding = granularity;
-  uptr space_size = kHighShadowEnd + left_padding;
-  uptr shadow_start = FindAvailableMemoryRange(space_size, alignment,
-                                               granularity, nullptr, nullptr);
-  CHECK_NE((uptr)0, shadow_start);
-  CHECK(IsAligned(shadow_start, alignment));
-  return shadow_start;
+  return MapDynamicShadow(MemToShadowSize(kHighMemEnd), SHADOW_SCALE,
+                          /*min_shadow_base_alignment*/ 0, kHighMemEnd);
 }
 
 void AsanCheckDynamicRTPrereqs() {}
diff --git a/compiler-rt/lib/hwasan/hwasan.h b/compiler-rt/lib/hwasan/hwasan.h
--- a/compiler-rt/lib/hwasan/hwasan.h
+++ b/compiler-rt/lib/hwasan/hwasan.h
@@ -75,7 +75,6 @@
 bool InitShadow();
 void InitPrctl();
 void InitThreads();
-void MadviseShadow();
 void InitializeInterceptors();
 
 void HwasanAllocatorInit();
diff --git a/compiler-rt/lib/hwasan/hwasan.cpp b/compiler-rt/lib/hwasan/hwasan.cpp
--- a/compiler-rt/lib/hwasan/hwasan.cpp
+++ b/compiler-rt/lib/hwasan/hwasan.cpp
@@ -286,8 +286,6 @@
   // initialized when InitInstrumentation() was called.
   GetCurrentThread()->InitRandomState();
 
-  MadviseShadow();
-
   SetPrintfAndReportCallback(AppendToErrorMessageBuffer);
   // This may call libc -> needs initialized shadow.
   AndroidLogInit();
diff --git a/compiler-rt/lib/hwasan/hwasan_dynamic_shadow.cpp b/compiler-rt/lib/hwasan/hwasan_dynamic_shadow.cpp
--- a/compiler-rt/lib/hwasan/hwasan_dynamic_shadow.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_dynamic_shadow.cpp
@@ -24,47 +24,6 @@
 // The code in this file needs to run in an unrelocated binary. It should not
 // access any external symbol, including its own non-hidden globals.
 
-namespace __hwasan {
-
-static void UnmapFromTo(uptr from, uptr to) {
-  if (to == from)
-    return;
-  CHECK(to >= from);
-  uptr res = internal_munmap(reinterpret_cast<void *>(from), to - from);
-  if (UNLIKELY(internal_iserror(res))) {
-    Report("ERROR: %s failed to unmap 0x%zx (%zd) bytes at address %p\n",
-           SanitizerToolName, to - from, to - from, from);
-    CHECK("unable to unmap" && 0);
-  }
-}
-
-// Returns an address aligned to kShadowBaseAlignment, such that
-// 2**kShadowBaseAlingment on the left and shadow_size_bytes bytes on the right
-// of it are mapped no access.
-static uptr MapDynamicShadow(uptr shadow_size_bytes) {
-  const uptr granularity = GetMmapGranularity();
-  const uptr min_alignment = granularity << kShadowScale;
-  const uptr alignment = 1ULL << kShadowBaseAlignment;
-  CHECK_GE(alignment, min_alignment);
-
-  const uptr left_padding = 1ULL << kShadowBaseAlignment;
-  const uptr shadow_size =
-      RoundUpTo(shadow_size_bytes, granularity);
-  const uptr map_size = shadow_size + left_padding + alignment;
-
-  const uptr map_start = (uptr)MmapNoAccess(map_size);
-  CHECK_NE(map_start, ~(uptr)0);
-
-  const uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
-
-  UnmapFromTo(map_start, shadow_start - left_padding);
-  UnmapFromTo(shadow_start + shadow_size, map_start + map_size);
-
-  return shadow_start;
-}
-
-}  // namespace __hwasan
-
 #if SANITIZER_ANDROID
 extern "C" {
 
@@ -82,7 +41,8 @@
 }
 
 static uptr PremapShadow() {
-  return MapDynamicShadow(PremapShadowSize());
+  return MapDynamicShadow(PremapShadowSize(), kShadowScale,
+                          kShadowBaseAlignment, kHighMemEnd);
 }
 
 static bool IsPremapShadowAvailable() {
@@ -146,7 +106,8 @@
 uptr FindDynamicShadowStart(uptr shadow_size_bytes) {
   if (IsPremapShadowAvailable())
     return FindPremappedShadowStart(shadow_size_bytes);
-  return MapDynamicShadow(shadow_size_bytes);
+  return MapDynamicShadow(shadow_size_bytes, kShadowScale, kShadowBaseAlignment,
+                          kHighMemEnd);
 }
 
 }  // namespace __hwasan
@@ -156,7 +117,8 @@
 void InitShadowGOT() {}
 
 uptr FindDynamicShadowStart(uptr shadow_size_bytes) {
-  return MapDynamicShadow(shadow_size_bytes);
+  return MapDynamicShadow(shadow_size_bytes, kShadowScale, kShadowBaseAlignment,
+                          kHighMemEnd);
 }
 
 }  // namespace __hwasan
diff --git a/compiler-rt/lib/hwasan/hwasan_linux.cpp b/compiler-rt/lib/hwasan/hwasan_linux.cpp
--- a/compiler-rt/lib/hwasan/hwasan_linux.cpp
+++ b/compiler-rt/lib/hwasan/hwasan_linux.cpp
@@ -57,56 +57,24 @@
 
 namespace __hwasan {
 
-static void ReserveShadowMemoryRange(uptr beg, uptr end, const char *name) {
-  CHECK_EQ((beg % GetMmapGranularity()), 0);
-  CHECK_EQ(((end + 1) % GetMmapGranularity()), 0);
-  uptr size = end - beg + 1;
-  DecreaseTotalMmap(size);  // Don't count the shadow against mmap_limit_mb.
-  if (!MmapFixedNoReserve(beg, size, name)) {
-    Report(
-        "ReserveShadowMemoryRange failed while trying to map 0x%zx bytes. "
-        "Perhaps you're using ulimit -v\n",
-        size);
-    Abort();
-  }
-}
+// With the zero shadow base we can not actually map pages starting from 0.
+// This constant is somewhat arbitrary.
+constexpr uptr kZeroBaseShadowStart = 0;
+constexpr uptr kZeroBaseMaxShadowStart = 1 << 18;
 
 static void ProtectGap(uptr addr, uptr size) {
-  if (!size)
-    return;
-  void *res = MmapFixedNoAccess(addr, size, "shadow gap");
-  if (addr == (uptr)res)
-    return;
-  // A few pages at the start of the address space can not be protected.
-  // But we really want to protect as much as possible, to prevent this memory
-  // being returned as a result of a non-FIXED mmap().
-  if (addr == 0) {
-    uptr step = GetMmapGranularity();
-    while (size > step) {
-      addr += step;
-      size -= step;
-      void *res = MmapFixedNoAccess(addr, size, "shadow gap");
-      if (addr == (uptr)res)
-        return;
-    }
-  }
-
-  Report(
-      "ERROR: Failed to protect shadow gap [%p, %p]. "
-      "HWASan cannot proceed correctly. ABORTING.\n", (void *)addr,
-      (void *)(addr + size));
-  DumpProcessMap();
-  Die();
+  __sanitizer::ProtectGap(addr, size, kZeroBaseShadowStart,
+                          kZeroBaseMaxShadowStart);
 }
 
-static uptr kLowMemStart;
-static uptr kLowMemEnd;
-static uptr kLowShadowEnd;
-static uptr kLowShadowStart;
-static uptr kHighShadowStart;
-static uptr kHighShadowEnd;
-static uptr kHighMemStart;
-static uptr kHighMemEnd;
+uptr kLowMemStart;
+uptr kLowMemEnd;
+uptr kLowShadowEnd;
+uptr kLowShadowStart;
+uptr kHighShadowStart;
+uptr kHighShadowEnd;
+uptr kHighMemStart;
+uptr kHighMemEnd;
 
 static void PrintRange(uptr start, uptr end, const char *name) {
   Printf("|| [%p, %p] || %.*s ||\n", (void *)start, (void *)end, 10, name);
@@ -242,24 +210,12 @@
   uptr thread_space_end =
       __hwasan_shadow_memory_dynamic_address - guard_page_size;
   ReserveShadowMemoryRange(thread_space_start, thread_space_end - 1,
-                           "hwasan threads");
+                           "hwasan threads", /*madvise_shadow*/ false);
   ProtectGap(thread_space_end,
              __hwasan_shadow_memory_dynamic_address - thread_space_end);
   InitThreadList(thread_space_start, thread_space_end - thread_space_start);
 }
 
-static void MadviseShadowRegion(uptr beg, uptr end) {
-  uptr size = end - beg + 1;
-  SetShadowRegionHugePageMode(beg, size);
-  if (common_flags()->use_madv_dontdump)
-    DontDumpShadowMemory(beg, size);
-}
-
-void MadviseShadow() {
-  MadviseShadowRegion(kLowShadowStart, kLowShadowEnd);
-  MadviseShadowRegion(kHighShadowStart, kHighShadowEnd);
-}
-
 bool MemIsApp(uptr p) {
   CHECK(GetTagFromPointer(p) == 0);
   return p >= kHighMemStart || (p >= kLowMemStart && p <= kLowMemEnd);
diff --git a/compiler-rt/lib/hwasan/hwasan_mapping.h b/compiler-rt/lib/hwasan/hwasan_mapping.h
--- a/compiler-rt/lib/hwasan/hwasan_mapping.h
+++ b/compiler-rt/lib/hwasan/hwasan_mapping.h
@@ -39,6 +39,15 @@
 
 namespace __hwasan {
 
+extern uptr kLowMemStart;
+extern uptr kLowMemEnd;
+extern uptr kLowShadowEnd;
+extern uptr kLowShadowStart;
+extern uptr kHighShadowStart;
+extern uptr kHighShadowEnd;
+extern uptr kHighMemStart;
+extern uptr kHighMemEnd;
+
 inline uptr MemToShadow(uptr untagged_addr) {
   return (untagged_addr >> kShadowScale) +
          __hwasan_shadow_memory_dynamic_address;
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_common.h b/compiler-rt/lib/sanitizer_common/sanitizer_common.h
--- a/compiler-rt/lib/sanitizer_common/sanitizer_common.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_common.h
@@ -121,6 +121,30 @@
 
 void MprotectMallocZones(void *addr, int prot);
 
+// Get the max address, taking into account alignment due to the mmap
+// granularity and shadow size.
+uptr GetHighMemEnd(uptr shadow_scale);
+
+// Maps shadow_size_bytes of shadow memory and returns shadow address. It will
+// be aligned to the mmap granularity * 2^shadow_scale, or to
+// 2^min_shadow_base_alignment if that is larger. The returned address will
+// have max(2^min_shadow_base_alignment, mmap granularity) on the left, and
+// shadow_size_bytes bytes on the right, mapped no access.
+// The high_mem_end may be updated if the original shadow size doesn't fit.
+uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
+                      uptr min_shadow_base_alignment, uptr &high_mem_end);
+
+// Reserve memory range [beg, end]. If madvise_shadow is true then apply
+// madvise (e.g. hugepages, core dumping) requested by options.
+void ReserveShadowMemoryRange(uptr beg, uptr end, const char *name,
+                              bool madvise_shadow = true);
+
+// Protect size bytes of memory starting at addr. Also try to protect
+// several pages at the start of the address space as specified by
+// zero_base_shadow_start, at most up to the size or zero_base_max_shadow_start.
+void ProtectGap(uptr addr, uptr size, uptr zero_base_shadow_start,
+                uptr zero_base_max_shadow_start);
+
 // Find an available address space.
 uptr FindAvailableMemoryRange(uptr size, uptr alignment, uptr left_padding,
                               uptr *largest_gap_found, uptr *max_occupied_addr);
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_common_libcdep.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_common_libcdep.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_common_libcdep.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_common_libcdep.cpp
@@ -139,6 +139,55 @@
   return start;
 }
 
+// Reserve memory range [beg, end].
+// We need to use inclusive range because end+1 may not be representable.
+void ReserveShadowMemoryRange(uptr beg, uptr end, const char *name,
+                              bool madvise_shadow) {
+  CHECK_EQ((beg % GetMmapGranularity()), 0);
+  CHECK_EQ(((end + 1) % GetMmapGranularity()), 0);
+  uptr size = end - beg + 1;
+  DecreaseTotalMmap(size);  // Don't count the shadow against mmap_limit_mb.
+  if (madvise_shadow ? !MmapFixedSuperNoReserve(beg, size, name)
+                     : !MmapFixedNoReserve(beg, size, name)) {
+    Report(
+        "ReserveShadowMemoryRange failed while trying to map 0x%zx bytes. "
+        "Perhaps you're using ulimit -v\n",
+        size);
+    Abort();
+  }
+  if (madvise_shadow && common_flags()->use_madv_dontdump)
+    DontDumpShadowMemory(beg, size);
+}
+
+void ProtectGap(uptr addr, uptr size, uptr zero_base_shadow_start,
+                uptr zero_base_max_shadow_start) {
+  if (!size)
+    return;
+  void *res = MmapFixedNoAccess(addr, size, "shadow gap");
+  if (addr == (uptr)res)
+    return;
+  // A few pages at the start of the address space can not be protected.
+  // But we really want to protect as much as possible, to prevent this memory
+  // being returned as a result of a non-FIXED mmap().
+  if (addr == zero_base_shadow_start) {
+    uptr step = GetMmapGranularity();
+    while (size > step && addr < zero_base_max_shadow_start) {
+      addr += step;
+      size -= step;
+      void *res = MmapFixedNoAccess(addr, size, "shadow gap");
+      if (addr == (uptr)res)
+        return;
+    }
+  }
+
+  Report(
+      "ERROR: Failed to protect the shadow gap. "
+      "%s cannot proceed correctly. ABORTING.\n",
+      SanitizerToolName);
+  DumpProcessMap();
+  Die();
+}
+
 }  // namespace __sanitizer
 
 SANITIZER_INTERFACE_WEAK_DEF(void, __sanitizer_sandbox_on_notify,
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp
@@ -841,6 +841,41 @@
 }
 #endif  // !SANITIZER_OPENBSD
 
+static void UnmapFromTo(uptr from, uptr to) {
+  if (to == from)
+    return;
+  CHECK(to >= from);
+  uptr res = internal_munmap(reinterpret_cast<void *>(from), to - from);
+  if (UNLIKELY(internal_iserror(res))) {
+    Report("ERROR: %s failed to unmap 0x%zx (%zd) bytes at address %p\n",
+           SanitizerToolName, to - from, to - from, (void *)from);
+    CHECK("unable to unmap" && 0);
+  }
+}
+
+uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
+                      uptr min_shadow_base_alignment,
+                      UNUSED uptr &high_mem_end) {
+  const uptr granularity = GetMmapGranularity();
+  const uptr alignment =
+      Max<uptr>(granularity << shadow_scale, 1ULL << min_shadow_base_alignment);
+  const uptr left_padding =
+      Max<uptr>(granularity, 1ULL << min_shadow_base_alignment);
+
+  const uptr shadow_size = RoundUpTo(shadow_size_bytes, granularity);
+  const uptr map_size = shadow_size + left_padding + alignment;
+
+  const uptr map_start = (uptr)MmapNoAccess(map_size);
+  CHECK_NE(map_start, ~(uptr)0);
+
+  const uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
+
+  UnmapFromTo(map_start, shadow_start - left_padding);
+  UnmapFromTo(shadow_start + shadow_size, map_start + map_size);
+
+  return shadow_start;
+}
+
 } // namespace __sanitizer
 
 #endif
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
@@ -1099,6 +1099,53 @@
   return GetMaxUserVirtualAddress();
 }
 
+uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
+                      uptr min_shadow_base_alignment, uptr &high_mem_end) {
+  const uptr granularity = GetMmapGranularity();
+  const uptr alignment =
+      Max<uptr>(granularity << shadow_scale, 1ULL << min_shadow_base_alignment);
+  const uptr left_padding =
+      Max<uptr>(granularity, 1ULL << min_shadow_base_alignment);
+
+  uptr space_size = shadow_size_bytes + left_padding;
+
+  uptr largest_gap_found = 0;
+  uptr max_occupied_addr = 0;
+  VReport(2, "FindDynamicShadowStart, space_size = %p\n", space_size);
+  uptr shadow_start =
+      FindAvailableMemoryRange(space_size, alignment, granularity,
+                               &largest_gap_found, &max_occupied_addr);
+  // If the shadow doesn't fit, restrict the address space to make it fit.
+  if (shadow_start == 0) {
+    VReport(
+        2,
+        "Shadow doesn't fit, largest_gap_found = %p, max_occupied_addr = %p\n",
+        largest_gap_found, max_occupied_addr);
+    uptr new_max_vm = RoundDownTo(largest_gap_found << shadow_scale, alignment);
+    if (new_max_vm < max_occupied_addr) {
+      Report("Unable to find a memory range for dynamic shadow.\n");
+      Report(
+          "space_size = %p, largest_gap_found = %p, max_occupied_addr = %p, "
+          "new_max_vm = %p\n",
+          space_size, largest_gap_found, max_occupied_addr, new_max_vm);
+      CHECK(0 && "cannot place shadow");
+    }
+    RestrictMemoryToMaxAddress(new_max_vm);
+    high_mem_end = new_max_vm - 1;
+    space_size = (high_mem_end >> shadow_scale) + left_padding;
+    VReport(2, "FindDynamicShadowStart, space_size = %p\n", space_size);
+    shadow_start = FindAvailableMemoryRange(space_size, alignment, granularity,
+                                            nullptr, nullptr);
+    if (shadow_start == 0) {
+      Report("Unable to find a memory range after restricting VM.\n");
+      CHECK(0 && "cannot place shadow after restricting vm");
+    }
+  }
+  CHECK_NE((uptr)0, shadow_start);
+  CHECK(IsAligned(shadow_start, alignment));
+  return shadow_start;
+}
+
 uptr FindAvailableMemoryRange(uptr size, uptr alignment, uptr left_padding,
                               uptr *largest_gap_found,
                               uptr *max_occupied_addr) {
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_win.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_win.cpp
--- a/compiler-rt/lib/sanitizer_common/sanitizer_win.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_win.cpp
@@ -348,6 +348,22 @@
   return true;
 }
 
+uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
+                      uptr min_shadow_base_alignment,
+                      UNUSED uptr &high_mem_end) {
+  const uptr granularity = GetMmapGranularity();
+  const uptr alignment =
+      Max<uptr>(granularity << shadow_scale, 1ULL << min_shadow_base_alignment);
+  const uptr left_padding =
+      Max<uptr>(granularity, 1ULL << min_shadow_base_alignment);
+  uptr space_size = shadow_size_bytes + left_padding;
+  uptr shadow_start = FindAvailableMemoryRange(space_size, alignment,
+                                               granularity, nullptr, nullptr);
+  CHECK_NE((uptr)0, shadow_start);
+  CHECK(IsAligned(shadow_start, alignment));
+  return shadow_start;
+}
+
 uptr FindAvailableMemoryRange(uptr size, uptr alignment, uptr left_padding,
                               uptr *largest_gap_found,
                               uptr *max_occupied_addr) {