[llvm] [hwasan] Optimize outlined memaccess for fixed shadow on Aarch64 (PR #88544)

[Thurston Dang via llvm-commits]

================
@@ -929,11 +929,33 @@ void HWAddressSanitizer::instrumentMemAccessOutline(Value *Ptr, bool IsWrite,
 
   IRBuilder<> IRB(InsertBefore);
   Module *M = IRB.GetInsertBlock()->getParent()->getParent();
-  IRB.CreateCall(Intrinsic::getDeclaration(
-                     M, UseShortGranules
-                            ? Intrinsic::hwasan_check_memaccess_shortgranules
-                            : Intrinsic::hwasan_check_memaccess),
-                 {ShadowBase, Ptr, ConstantInt::get(Int32Ty, AccessInfo)});
+
+  // Aarch64 makes it difficult to embed large constants (such as the shadow
+  // offset) in the code. Our intrinsic supports a 16-bit constant (to be left
+  // shifted by 32 bits) - this is not an onerous constraint, since
+  // 1) kShadowBaseAlignment == 32 2) an offset of 4TB (1024 << 32) is
+  // representable, and ought to be good enough for anybody.
+  bool useFixedShadowIntrinsic = true;
+  if (TargetTriple.isAArch64() && ClMappingOffset.getNumOccurrences() > 0 &&
----------------
thurstond wrote:

Mapping.Offset could be zero because of a manually specified ClMappingOffset option, or because it is Fuchsia or Khwasan. I want to make sure that this optimization is only applied for a manually specified ClMappingOffset option.

```
  if (TargetTriple.isOSFuchsia()) {
    ...
    Offset = 0;
  } else if (ClMappingOffset.getNumOccurrences() > 0) {
    ...  
    Offset = ClMappingOffset;
  } else if (ClEnableKhwasan || InstrumentWithCalls) {
    ...
    Offset = 0;
```


https://github.com/llvm/llvm-project/pull/88544