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

[Thurston Dang via llvm-commits]

================
@@ -625,14 +638,32 @@ void AArch64AsmPrinter::emitHwasanMemaccessSymbols(Module &M) {
                                      .addImm(4)
                                      .addImm(55),
                                  *STI);
-    OutStreamer->emitInstruction(
-        MCInstBuilder(AArch64::LDRBBroX)
-            .addReg(AArch64::W16)
-            .addReg(IsShort ? AArch64::X20 : AArch64::X9)
-            .addReg(AArch64::X16)
-            .addImm(0)
-            .addImm(0),
-        *STI);
+
+    if (HwasanFixedShadowBase != (unsigned long long)-1) {
+      assert(IsShort);
----------------
thurstond wrote:

Yep. Non-short is only used by Android 10 and earlier; implementing fixed shadow optimization for that is left as an exercise for the reader.

```
  // Older versions of Android do not have the required runtime support for
  // short granules, global or personality function instrumentation. On other
  // platforms we currently require using the latest version of the runtime.
  bool NewRuntime =
      !TargetTriple.isAndroid() || !TargetTriple.isAndroidVersionLT(30);

  UseShortGranules = optOr(ClUseShortGranules, NewRuntime);
  ```
  (llvm-project/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp)

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