[llvm] [msan] Convert target("aarch64.svcount") from compile-time crash to MSan false negatives (PR #165028)

[Thurston Dang via llvm-commits]

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@@ -1544,6 +1545,27 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     }
   }
 
+  static bool isAArch64SVCount(Type *Ty) {
+    if (!isa<TargetExtType>(Ty))
+      return false;
+
+    TargetExtType* TTy = cast<TargetExtType>(Ty);
+    return TTy->getName() == "aarch64.svcount";
+  }
+
+  // This is intended to match the "AArch64 Predicate-as-Counter Type" (aka
+  // 'target("aarch64.svcount")', but not e.g., <vscale x 4 x i32>.
+  static bool isScalableNonVectorType(Type *Ty) {
+    if (!isAArch64SVCount(Ty))
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thurstond wrote:

The not branch will happen if a similar predicate is introduced for RISC-V, Loongaarch, etc. Adding an assert would break them unnecessarily.

OTOH at the moment I am (willfully) unaware of such a predicate on other platforms; hence, the log message for debugging because it is somewhat unexpected.

https://github.com/llvm/llvm-project/pull/165028