diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index ceeece41782f4..d04cae018a79d 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -2720,34 +2720,55 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   // of elements.
   //
   // For example, suppose we have:
-  //   VectorA: <a1, a2, a3, a4, a5, a6>
-  //   VectorB: <b1, b2, b3, b4, b5, b6>
-  //   ReductionFactor: 3.
+  //   VectorA: <a0, a1, a2, a3, a4, a5>
+  //   VectorB: <b0, b1, b2, b3, b4, b5>
+  //   ReductionFactor: 3
+  //   Shards: 1
   // The output would be:
-  //   <a1|a2|a3, a4|a5|a6, b1|b2|b3, b4|b5|b6>
+  //   <a0|a1|a2, a3|a4|a5, b0|b1|b2, b3|b4|b5>
+  //
+  // If we have:
+  //   VectorA: <a0, a1, a2, a3, a4, a5, a6, a7>
+  //   VectorB: <b0, b1, b2, b3, b4, b5, b6, b7>
+  //   ReductionFactor: 2
+  //   Shards: 2
+  // then a and be each have 2 "shards", resulting in the output being
+  // interleaved:
+  //   <a0|a1, a2|a3, b0|b1, b2|b3, a4|a5, a6|a7, b4|b5, b6|b7>
   //
   // This is convenient for instrumenting horizontal add/sub.
   // For bitwise OR on "vertical" pairs, see maybeHandleSimpleNomemIntrinsic().
   Value *horizontalReduce(IntrinsicInst &I, unsigned ReductionFactor,
-                          Value *VectorA, Value *VectorB) {
+                          unsigned Shards, Value *VectorA, Value *VectorB) {
     assert(isa<FixedVectorType>(VectorA->getType()));
-    unsigned TotalNumElems =
+    unsigned NumElems =
         cast<FixedVectorType>(VectorA->getType())->getNumElements();
 
+    [[maybe_unused]] unsigned TotalNumElems = NumElems;
     if (VectorB) {
       assert(VectorA->getType() == VectorB->getType());
-      TotalNumElems = TotalNumElems * 2;
+      TotalNumElems *= 2;
     }
 
-    assert(TotalNumElems % ReductionFactor == 0);
+    assert(NumElems % (ReductionFactor * Shards) == 0);
 
     Value *Or = nullptr;
 
     IRBuilder<> IRB(&I);
     for (unsigned i = 0; i < ReductionFactor; i++) {
       SmallVector<int, 16> Mask;
-      for (unsigned X = 0; X < TotalNumElems; X += ReductionFactor)
-        Mask.push_back(X + i);
+
+      for (unsigned j = 0; j < Shards; j++) {
+        unsigned Offset = NumElems / Shards * j;
+
+        for (unsigned X = 0; X < NumElems / Shards; X += ReductionFactor)
+          Mask.push_back(Offset + X + i);
+
+        if (VectorB) {
+          for (unsigned X = 0; X < NumElems / Shards; X += ReductionFactor)
+            Mask.push_back(NumElems + Offset + X + i);
+        }
+      }
 
       Value *Masked;
       if (VectorB)
@@ -2769,7 +2790,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   ///
   /// e.g., <2 x i32> @llvm.aarch64.neon.saddlp.v2i32.v4i16(<4 x i16>)
   ///       <16 x i8> @llvm.aarch64.neon.addp.v16i8(<16 x i8>, <16 x i8>)
-  void handlePairwiseShadowOrIntrinsic(IntrinsicInst &I) {
+  void handlePairwiseShadowOrIntrinsic(IntrinsicInst &I, unsigned Shards) {
     assert(I.arg_size() == 1 || I.arg_size() == 2);
 
     assert(I.getType()->isVectorTy());
@@ -2792,8 +2813,8 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     if (I.arg_size() == 2)
       SecondArgShadow = getShadow(&I, 1);
 
-    Value *OrShadow = horizontalReduce(I, /*ReductionFactor=*/2, FirstArgShadow,
-                                       SecondArgShadow);
+    Value *OrShadow = horizontalReduce(I, /*ReductionFactor=*/2, Shards,
+                                       FirstArgShadow, SecondArgShadow);
 
     OrShadow = CreateShadowCast(IRB, OrShadow, getShadowTy(&I));
 
@@ -2808,7 +2829,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   /// conceptually operates on
   ///     (<4 x i16> [[VAR1]], <4 x i16> [[VAR2]])
   /// and can be handled with ReinterpretElemWidth == 16.
-  void handlePairwiseShadowOrIntrinsic(IntrinsicInst &I,
+  void handlePairwiseShadowOrIntrinsic(IntrinsicInst &I, unsigned Shards,
                                        int ReinterpretElemWidth) {
     assert(I.arg_size() == 1 || I.arg_size() == 2);
 
@@ -2852,8 +2873,8 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       SecondArgShadow = IRB.CreateBitCast(SecondArgShadow, ReinterpretShadowTy);
     }
 
-    Value *OrShadow = horizontalReduce(I, /*ReductionFactor=*/2, FirstArgShadow,
-                                       SecondArgShadow);
+    Value *OrShadow = horizontalReduce(I, /*ReductionFactor=*/2, Shards,
+                                       FirstArgShadow, SecondArgShadow);
 
     OrShadow = CreateShadowCast(IRB, OrShadow, getShadowTy(&I));
 
@@ -6031,48 +6052,66 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     // Packed Horizontal Add/Subtract
     case Intrinsic::x86_ssse3_phadd_w:
     case Intrinsic::x86_ssse3_phadd_w_128:
-    case Intrinsic::x86_avx2_phadd_w:
     case Intrinsic::x86_ssse3_phsub_w:
     case Intrinsic::x86_ssse3_phsub_w_128:
-    case Intrinsic::x86_avx2_phsub_w: {
-      handlePairwiseShadowOrIntrinsic(I, /*ReinterpretElemWidth=*/16);
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1,
+                                      /*ReinterpretElemWidth=*/16);
+      break;
+
+    case Intrinsic::x86_avx2_phadd_w:
+    case Intrinsic::x86_avx2_phsub_w:
+      // TODO: Shards = 2
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1,
+                                      /*ReinterpretElemWidth=*/16);
       break;
-    }
 
     // Packed Horizontal Add/Subtract
     case Intrinsic::x86_ssse3_phadd_d:
     case Intrinsic::x86_ssse3_phadd_d_128:
-    case Intrinsic::x86_avx2_phadd_d:
     case Intrinsic::x86_ssse3_phsub_d:
     case Intrinsic::x86_ssse3_phsub_d_128:
-    case Intrinsic::x86_avx2_phsub_d: {
-      handlePairwiseShadowOrIntrinsic(I, /*ReinterpretElemWidth=*/32);
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1,
+                                      /*ReinterpretElemWidth=*/32);
+      break;
+
+    case Intrinsic::x86_avx2_phadd_d:
+    case Intrinsic::x86_avx2_phsub_d:
+      // TODO: Shards = 2
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1,
+                                      /*ReinterpretElemWidth=*/32);
       break;
-    }
 
     // Packed Horizontal Add/Subtract and Saturate
     case Intrinsic::x86_ssse3_phadd_sw:
     case Intrinsic::x86_ssse3_phadd_sw_128:
-    case Intrinsic::x86_avx2_phadd_sw:
     case Intrinsic::x86_ssse3_phsub_sw:
     case Intrinsic::x86_ssse3_phsub_sw_128:
-    case Intrinsic::x86_avx2_phsub_sw: {
-      handlePairwiseShadowOrIntrinsic(I, /*ReinterpretElemWidth=*/16);
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1,
+                                      /*ReinterpretElemWidth=*/16);
+      break;
+
+    case Intrinsic::x86_avx2_phadd_sw:
+    case Intrinsic::x86_avx2_phsub_sw:
+      // TODO: Shards = 2
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1,
+                                      /*ReinterpretElemWidth=*/16);
       break;
-    }
 
     // Packed Single/Double Precision Floating-Point Horizontal Add
     case Intrinsic::x86_sse3_hadd_ps:
     case Intrinsic::x86_sse3_hadd_pd:
-    case Intrinsic::x86_avx_hadd_pd_256:
-    case Intrinsic::x86_avx_hadd_ps_256:
     case Intrinsic::x86_sse3_hsub_ps:
     case Intrinsic::x86_sse3_hsub_pd:
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1);
+      break;
+
+    case Intrinsic::x86_avx_hadd_pd_256:
+    case Intrinsic::x86_avx_hadd_ps_256:
     case Intrinsic::x86_avx_hsub_pd_256:
-    case Intrinsic::x86_avx_hsub_ps_256: {
-      handlePairwiseShadowOrIntrinsic(I);
+    case Intrinsic::x86_avx_hsub_ps_256:
+      // TODO: Shards = 2
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1);
       break;
-    }
 
     case Intrinsic::x86_avx_maskstore_ps:
     case Intrinsic::x86_avx_maskstore_pd:
@@ -6455,7 +6494,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     // Add Long Pairwise
     case Intrinsic::aarch64_neon_saddlp:
     case Intrinsic::aarch64_neon_uaddlp: {
-      handlePairwiseShadowOrIntrinsic(I);
+      handlePairwiseShadowOrIntrinsic(I, /*Shards=*/1);
       break;
     }