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

[Vitaly Buka via llvm-commits]

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@@ -930,11 +930,32 @@ 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)});
+  bool useFixedShadowIntrinsic = false;
+  // The memaccess fixed shadow intrinsic is only supported on AArch64,
+  // which allows a 16-bit immediate to be left-shifted by 32.
+  // Since kShadowBaseAlignment == 32, and Linux by default will not
+  // mmap above 48-bits, practically any valid shadow offset is
+  // representable.
+  // In particular, an offset of 4TB (1024 << 32) is representable, and
+  // ought to be good enough for anybody.
+  if (TargetTriple.isAArch64() && ClMappingOffset.getNumOccurrences() > 0) {
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vitalybuka wrote:

Can we do `&& Mapping.Offset != kDynamicShadowSentinel` ?
so we can make Mapping as source of truth after `ShadowMapping::init`

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