[MTE] Do not allow local aliases to MTE globals

[fmayer]

With this change and appropriate linker changes (https://r.android.com/3236256)
AOSP boots with memtag-global throughout the platform.

Without this change, we would sometimes generate PC-relative references
to tagged globals, which then do not have the proper tag.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

@llvm/pr-subscribers-llvm-ir

Author: Florian Mayer (fmayer)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/106280.diff

1 Files Affected:

• (modified) llvm/lib/IR/Globals.cpp (+4)

diff --git a/llvm/lib/IR/Globals.cpp b/llvm/lib/IR/Globals.cpp
index 2bc69cdb712b0a..6b502a08b87d08 100644
--- a/llvm/lib/IR/Globals.cpp
+++ b/llvm/lib/IR/Globals.cpp
@@ -110,6 +110,10 @@ bool GlobalValue::isInterposable() const {
 }
 
 bool GlobalValue::canBenefitFromLocalAlias() const {
+  if (isTagged()) {
+    // Cannot create local aliases to MTE tagged globals.
+    return false;
+  }
   // See AsmPrinter::getSymbolPreferLocal(). For a deduplicate comdat kind,
   // references to a discarded local symbol from outside the group are not
   // allowed, so avoid the local alias.