[msan][NFCI] Generalize handleIntrinsicByApplyingToShadow to allow alternative intrinsic for shadows (llvm#124831)

llvm#124159 uses
handleIntrinsicByApplyingToShadow for horizontal add/sub, but Vitaly
recommends always using the add version to avoid false negatives for
fully uninitialized data
(llvm#124662).

This patch lays the groundwork by generalizing
handleIntrinsicByApplyingToShadow to allow using a different intrinsic
(of the same type as the original intrinsic) for the shadow. Planned
work will apply it to horizontal sub.

Author: thurstond

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -4049,7 +4049,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     //   consider this an acceptable tradeoff for performance.
     // To make shadow propagation precise, we want the equivalent of
     // "horizontal OR", but this is not available.
-    return handleIntrinsicByApplyingToShadow(I, /* trailingVerbatimArgs */ 0);
+    return handleIntrinsicByApplyingToShadow(
+        I, /*shadowIntrinsicID=*/I.getIntrinsicID(),
+        /*trailingVerbatimArgs*/ 0);
   }
 
   /// Handle Arm NEON vector store intrinsics (vst{2,3,4}, vst1x_{2,3,4},
@@ -4156,6 +4158,10 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   ///     shadow[out] =
   ///         intrinsic(shadow[var1], shadow[var2], opType) | shadow[opType]
   ///
+  /// Typically, shadowIntrinsicID will be specified by the caller to be
+  /// I.getIntrinsicID(), but the caller can choose to replace it with another
+  /// intrinsic of the same type.
+  ///
   /// CAUTION: this assumes that the intrinsic will handle arbitrary
   ///          bit-patterns (for example, if the intrinsic accepts floats for
   ///          var1, we require that it doesn't care if inputs are NaNs).
@@ -4165,6 +4171,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   ///
   /// The origin is approximated using setOriginForNaryOp.
   void handleIntrinsicByApplyingToShadow(IntrinsicInst &I,
+                                         Intrinsic::ID shadowIntrinsicID,
                                          unsigned int trailingVerbatimArgs) {
     IRBuilder<> IRB(&I);
 
@@ -4188,7 +4195,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     }
 
     CallInst *CI =
-        IRB.CreateIntrinsic(I.getType(), I.getIntrinsicID(), ShadowArgs);
+        IRB.CreateIntrinsic(I.getType(), shadowIntrinsicID, ShadowArgs);
     Value *CombinedShadow = CI;
 
     // Combine the computed shadow with the shadow of trailing args
@@ -4664,7 +4671,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     case Intrinsic::aarch64_neon_tbx3:
     case Intrinsic::aarch64_neon_tbx4: {
       // The last trailing argument (index register) should be handled verbatim
-      handleIntrinsicByApplyingToShadow(I, 1);
+      handleIntrinsicByApplyingToShadow(
+          I, /*shadowIntrinsicID=*/I.getIntrinsicID(),
+          /*trailingVerbatimArgs*/ 1);
       break;
     }