[asan] LLVM: Switch to using dynamic shadow offset on iOS

[zaks.anna]

Summary

The VM layout is not stable between iOS version releases, so switch to dynamic shadow offset.

This is the LLVM counterpart of https://reviews.llvm.org/D25218

[kcc]

LGTM
You know better, but it's just very sad that we can't have zero offset here.
It would have saved you 5+% in CPU and code size.

lib/Transforms/Instrumentation/AddressSanitizer.cpp

 
 #define DEBUG_TYPE "asan"
 
 static const uint64_t kDefaultShadowScale = 3;
 static const uint64_t kDefaultShadowOffset32 = 1ULL << 29;
 static const uint64_t kDefaultShadowOffset64 = 1ULL << 44;
 static const uint64_t kDynamicShadowSentinel = ~(uint64_t)0;
 static const uint64_t kIOSShadowOffset32 = 1ULL << 30;
-static const uint64_t kIOSShadowOffset64 = 0x120200000;
 static const uint64_t kIOSSimShadowOffset32 = 1ULL << 30;
 static const uint64_t kIOSSimShadowOffset64 = kDefaultShadowOffset64;
 static const uint64_t kSmallX86_64ShadowOffset = 0x7FFF8000;  // < 2G.
 static const uint64_t kLinuxKasan_ShadowOffset64 = 0xdffffc0000000000;
 static const uint64_t kPPC64_ShadowOffset64 = 1ULL << 41;
 static const uint64_t kSystemZ_ShadowOffset64 = 1ULL << 52;
 static const uint64_t kMIPS32_ShadowOffset32 = 0x0aaa0000;
 static const uint64_t kMIPS64_ShadowOffset64 = 1ULL << 37;
else if (IsLinux && IsX86_64) {
       else
         Mapping.Offset = kSmallX86_64ShadowOffset;
     } else if (IsWindows && IsX86_64) {
       Mapping.Offset = kWindowsShadowOffset64;
     } else if (IsMIPS64)
       Mapping.Offset = kMIPS64_ShadowOffset64;
     else if (IsIOS)
       // If we're targeting iOS and x86, the binary is built for iOS simulator.
-      Mapping.Offset = IsX86_64 ? kIOSSimShadowOffset64 : kIOSShadowOffset64;
+      // We are using dynamic shadow offset on the 64-bit devices.
+      Mapping.Offset =
+        IsX86_64 ? kIOSSimShadowOffset64 : kDynamicShadowSentinel;
     else if (IsAArch64)
       Mapping.Offset = kAArch64_ShadowOffset64;
     else
       Mapping.Offset = kDefaultShadowOffset64;
   }
 
   if (ClForceDynamicShadow) {
     Mapping.Offset = kDynamicShadowSentinel;