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
@@ -279,10 +279,34 @@
 #    define kLowShadowBeg ASAN_SHADOW_OFFSET
 #    define kLowShadowEnd MEM_TO_SHADOW(kLowMemEnd)
 
+#if !SANITIZER_IOS
 #    define kHighMemBeg (MEM_TO_SHADOW(kHighMemEnd) + 1)
 
 #    define kHighShadowBeg MEM_TO_SHADOW(kHighMemBeg)
 #    define kHighShadowEnd MEM_TO_SHADOW(kHighMemEnd)
+#else // !SANITIZER_IOS
+#    define WOULD_SHADOWS_OVERLAP MEM_TO_SHADOW(kHighMemEnd) + 1 <= kLowShadowEnd + (kHighMemEnd >> ASAN_SHADOW_SCALE)
+// The above expression says, if true, that you can't really fit low shadow, high shadow, 
+// and the shadowed high memory in the current available address space while using the old algorithm 
+// where kHighMemBeg == (MEM_TO_SHADOW(kHighMemEnd) + 1) and 
+// highShadowSize == (kHighMemEnd >> ASAN_SHADOW_SCALE)
+#    define kHighMemBeg \
+        ( WOULD_SHADOWS_OVERLAP ? \
+          RoundUpTo(kLowShadowEnd + ((kHighMemEnd - kLowShadowEnd) >> ASAN_SHADOW_SCALE) , GetMmapGranularity()) : \
+          MEM_TO_SHADOW(kHighMemEnd) + 1 \
+        )
+
+#    define kHighShadowBeg \
+        ( WOULD_SHADOWS_OVERLAP ? \
+          kLowShadowEnd + 1 + GetMmapGranularity() : \
+          MEM_TO_SHADOW(kHighMemBeg) \
+        )
+#    define kHighShadowEnd \
+        ( WOULD_SHADOWS_OVERLAP ? \
+          RoundUpTo(kLowShadowEnd + ((kHighMemEnd - kLowShadowEnd) >> ASAN_SHADOW_SCALE) , GetMmapGranularity()) - 1  : \
+          MEM_TO_SHADOW(kHighMemEnd) \
+        )
+#endif // !SANITIZER_IOS
 
 #    define kMidShadowBeg MEM_TO_SHADOW(kMidMemBeg)
 #    define kMidShadowEnd MEM_TO_SHADOW(kMidMemEnd)