[llvm] [msan] Add handleIntrinsicByApplyingToShadow; support NEON tbl/tbx (PR #114490)

[Thurston Dang via llvm-commits]

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@@ -4319,6 +4355,28 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       break;
     }
 
+    // Arm NEON vector table intrinsics have the source/table register(s) as
+    // arguments, followed by the index register. They return the output.
+    //
+    // 'TBL writes a zero if an index is out-of-range, while TBX leaves the
+    //  original value unchanged in the destination register.'
+    // Conveniently, zero denotes a clean shadow, which means out-of-range
+    // indices for TBL will initialize the user data with zero and also clean
+    // the shadow. (For TBX, neither the user data nor the shadow will be
+    // updated, which is also correct.)
+    case Intrinsic::aarch64_neon_tbl1:
+    case Intrinsic::aarch64_neon_tbl2:
+    case Intrinsic::aarch64_neon_tbl3:
+    case Intrinsic::aarch64_neon_tbl4:
+    case Intrinsic::aarch64_neon_tbx1:
+    case Intrinsic::aarch64_neon_tbx2:
+    case Intrinsic::aarch64_neon_tbx3:
+    case Intrinsic::aarch64_neon_tbx4: {
+      // The last trailing argument (index register) should be handled verbatim
+      handleIntrinsicByApplyingToShadow(I, 1);
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thurstond wrote:

`handleIntrinsicByApplyingToShadow` is a generic function that can deal with a variable number of trailing verbatim arguments. NEON TBL/TBX is a specific case where there is one trailing verbatim argument.

https://github.com/llvm/llvm-project/pull/114490