diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
index d4811ff4ed217..c4a6bf5bcbbee 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp
@@ -22,6 +22,11 @@
 #  endif
 #  include <stdio.h>
 
+// Start searching for available memory region past PAGEZERO, which is
+// 4KB on 32-bit and 4GB on 64-bit.
+#  define GAP_SEARCH_START_ADDRESS \
+    ((SANITIZER_WORDSIZE == 32) ? 0x000000001000 : 0x000100000000)
+
 #  include "sanitizer_common.h"
 #  include "sanitizer_file.h"
 #  include "sanitizer_flags.h"
@@ -58,6 +63,7 @@ extern char ***_NSGetArgv(void);
 #  include <dlfcn.h>  // for dladdr()
 #  include <errno.h>
 #  include <fcntl.h>
+#  include <inttypes.h>
 #  include <libkern/OSAtomic.h>
 #  include <mach-o/dyld.h>
 #  include <mach/mach.h>
@@ -1106,6 +1112,67 @@ static void StripEnv() {
 }
 #endif  // SANITIZER_GO
 
+// Prints out a consolidated memory map: contiguous regions
+// are merged together.
+static void PrintVmmap() {
+  const mach_vm_address_t max_vm_address = GetMaxVirtualAddress() + 1;
+  mach_vm_address_t address = GAP_SEARCH_START_ADDRESS;
+  kern_return_t kr = KERN_SUCCESS;
+
+  Report("Memory map:\n");
+  mach_vm_address_t last = 0;
+  mach_vm_address_t lastsz = 0;
+
+  while (1) {
+    mach_vm_size_t vmsize = 0;
+    natural_t depth = 0;
+    vm_region_submap_short_info_data_64_t vminfo;
+    mach_msg_type_number_t count = VM_REGION_SUBMAP_SHORT_INFO_COUNT_64;
+    kr = mach_vm_region_recurse(mach_task_self(), &address, &vmsize, &depth,
+                                (vm_region_info_t)&vminfo, &count);
+
+    if (kr == KERN_DENIED) {
+      Report(
+          "ERROR: mach_vm_region_recurse got KERN_DENIED when printing memory "
+          "map.\n");
+      Report(
+          "HINT: Check whether mach_vm_region_recurse is allowed by "
+          "sandbox.\n");
+    }
+
+    if (kr == KERN_SUCCESS && address < max_vm_address) {
+      if (last + lastsz == address) {
+        // This region is contiguous with the last; merge together.
+        lastsz += vmsize;
+      } else {
+        if (lastsz)
+          Printf("|| `[%p, %p]` || size=0x%016" PRIx64 " ||\n", last,
+                 last + lastsz, lastsz);
+
+        last = address;
+        lastsz = vmsize;
+      }
+      address += vmsize;
+    } else {
+      // We've reached the end of the memory map. Print the last remaining
+      // region, if there is one.
+      if (lastsz)
+        Printf("|| `[%p, %p]` || size=0x%016" PRIx64 " ||\n", last,
+               last + lastsz, lastsz);
+
+      break;
+    }
+  }
+}
+
+static void ReportShadowAllocFail(uptr shadow_size_bytes, uptr alignment) {
+  Report(
+      "FATAL: Failed to allocate shadow memory. Tried to allocate %p bytes "
+      "(alignment=%p).\n",
+      shadow_size_bytes, alignment);
+  PrintVmmap();
+}
+
 char **GetArgv() {
   return *_NSGetArgv();
 }
@@ -1213,10 +1280,11 @@ uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
     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",
-          (void *)space_size, (void *)largest_gap_found,
-          (void *)max_occupied_addr, (void *)new_max_vm);
+          "\tspace_size = %p\n\tlargest_gap_found = %p\n\tmax_occupied_addr "
+          "= %p\n\tnew_max_vm = %p\n",
+          (void*)space_size, (void*)largest_gap_found, (void*)max_occupied_addr,
+          (void*)new_max_vm);
+      ReportShadowAllocFail(shadow_size_bytes, alignment);
       CHECK(0 && "cannot place shadow");
     }
     RestrictMemoryToMaxAddress(new_max_vm);
@@ -1227,6 +1295,7 @@ uptr MapDynamicShadow(uptr shadow_size_bytes, uptr shadow_scale,
                                             nullptr, nullptr);
     if (shadow_start == 0) {
       Report("Unable to find a memory range after restricting VM.\n");
+      ReportShadowAllocFail(shadow_size_bytes, alignment);
       CHECK(0 && "cannot place shadow after restricting vm");
     }
   }
@@ -1242,26 +1311,19 @@ uptr MapDynamicShadowAndAliases(uptr shadow_size, uptr alias_size,
 }
 
 uptr FindAvailableMemoryRange(uptr size, uptr alignment, uptr left_padding,
-                              uptr *largest_gap_found,
-                              uptr *max_occupied_addr) {
-  typedef vm_region_submap_short_info_data_64_t RegionInfo;
-  enum { kRegionInfoSize = VM_REGION_SUBMAP_SHORT_INFO_COUNT_64 };
-  // Start searching for available memory region past PAGEZERO, which is
-  // 4KB on 32-bit and 4GB on 64-bit.
-  mach_vm_address_t start_address =
-    (SANITIZER_WORDSIZE == 32) ? 0x000000001000 : 0x000100000000;
-
+                              uptr* largest_gap_found,
+                              uptr* max_occupied_addr) {
   const mach_vm_address_t max_vm_address = GetMaxVirtualAddress() + 1;
-  mach_vm_address_t address = start_address;
-  mach_vm_address_t free_begin = start_address;
+  mach_vm_address_t address = GAP_SEARCH_START_ADDRESS;
+  mach_vm_address_t free_begin = GAP_SEARCH_START_ADDRESS;
   kern_return_t kr = KERN_SUCCESS;
   if (largest_gap_found) *largest_gap_found = 0;
   if (max_occupied_addr) *max_occupied_addr = 0;
   while (kr == KERN_SUCCESS) {
     mach_vm_size_t vmsize = 0;
     natural_t depth = 0;
-    RegionInfo vminfo;
-    mach_msg_type_number_t count = kRegionInfoSize;
+    vm_region_submap_short_info_data_64_t vminfo;
+    mach_msg_type_number_t count = VM_REGION_SUBMAP_SHORT_INFO_COUNT_64;
     kr = mach_vm_region_recurse(mach_task_self(), &address, &vmsize, &depth,
                                 (vm_region_info_t)&vminfo, &count);