diff --git a/src/coreclr/jit/codegenxarch.cpp b/src/coreclr/jit/codegenxarch.cpp
index 597c5009f369a..ff3229921dd86 100644
--- a/src/coreclr/jit/codegenxarch.cpp
+++ b/src/coreclr/jit/codegenxarch.cpp
@@ -2782,7 +2782,7 @@ void CodeGen::genCodeForInitBlkUnroll(GenTreeBlk* node)
         assert(src->IsIntegralConst(0));
         assert(willUseSimdMov);
 #ifdef TARGET_AMD64
-        assert(size % 16 == 0);
+        assert(size >= XMM_REGSIZE_BYTES);
 #else
         assert(size % 8 == 0);
 #endif
@@ -2797,16 +2797,26 @@ void CodeGen::genCodeForInitBlkUnroll(GenTreeBlk* node)
     {
         regNumber srcXmmReg = node->GetSingleTempReg(RBM_ALLFLOAT);
 
+        unsigned regSize = (size >= YMM_REGSIZE_BYTES) && compiler->compOpportunisticallyDependsOn(InstructionSet_AVX)
+                               ? YMM_REGSIZE_BYTES
+                               : XMM_REGSIZE_BYTES;
+
         if (src->gtSkipReloadOrCopy()->IsIntegralConst(0))
         {
             // If the source is constant 0 then always use xorps, it's faster
             // than copying the constant from a GPR to a XMM register.
-            emit->emitIns_R_R(INS_xorps, EA_16BYTE, srcXmmReg, srcXmmReg);
+            emit->emitIns_R_R(INS_xorps, EA_ATTR(regSize), srcXmmReg, srcXmmReg);
         }
         else
         {
             emit->emitIns_Mov(INS_movd, EA_PTRSIZE, srcXmmReg, srcIntReg, /* canSkip */ false);
             emit->emitIns_R_R(INS_punpckldq, EA_16BYTE, srcXmmReg, srcXmmReg);
+
+            if (regSize == YMM_REGSIZE_BYTES)
+            {
+                // Extend the bytes in the lower lanes to the upper lanes
+                emit->emitIns_R_R_R_I(INS_vinsertf128, EA_32BYTE, srcXmmReg, srcXmmReg, srcXmmReg, 1);
+            }
 #ifdef TARGET_X86
             // For x86, we need one more to convert it from 8 bytes to 16 bytes.
             emit->emitIns_R_R(INS_punpckldq, EA_16BYTE, srcXmmReg, srcXmmReg);
@@ -2814,7 +2824,6 @@ void CodeGen::genCodeForInitBlkUnroll(GenTreeBlk* node)
         }
 
         instruction simdMov      = simdUnalignedMovIns();
-        unsigned    regSize      = XMM_REGSIZE_BYTES;
         unsigned    bytesWritten = 0;
 
         while (bytesWritten < size)
@@ -2828,8 +2837,21 @@ void CodeGen::genCodeForInitBlkUnroll(GenTreeBlk* node)
 #endif
             if (bytesWritten + regSize > size)
             {
+#ifdef TARGET_AMD64
+                if (size - bytesWritten <= XMM_REGSIZE_BYTES)
+                {
+                    regSize = XMM_REGSIZE_BYTES;
+                }
+
+                // Shift dstOffset back to use full SIMD move
+                unsigned shiftBack = regSize - (size - bytesWritten);
+                assert(shiftBack <= regSize);
+                bytesWritten -= shiftBack;
+                dstOffset -= shiftBack;
+#else
                 assert(srcIntReg != REG_NA);
                 break;
+#endif
             }
 
             if (dstLclNum != BAD_VAR_NUM)
@@ -2849,14 +2871,51 @@ void CodeGen::genCodeForInitBlkUnroll(GenTreeBlk* node)
         size -= bytesWritten;
     }
 
-    // Fill the remainder using normal stores.
+// Fill the remainder using normal stores.
+#ifdef TARGET_AMD64
+    unsigned regSize = REGSIZE_BYTES;
+
+    while (regSize > size)
+    {
+        regSize /= 2;
+    }
+
+    for (; size > regSize; size -= regSize, dstOffset += regSize)
+    {
+        if (dstLclNum != BAD_VAR_NUM)
+        {
+            emit->emitIns_S_R(INS_mov, EA_ATTR(regSize), srcIntReg, dstLclNum, dstOffset);
+        }
+        else
+        {
+            emit->emitIns_ARX_R(INS_mov, EA_ATTR(regSize), srcIntReg, dstAddrBaseReg, dstAddrIndexReg,
+                                dstAddrIndexScale, dstOffset);
+        }
+    }
+
+    if (size > 0)
+    {
+        unsigned shiftBack = regSize - size;
+        assert(shiftBack <= regSize);
+        dstOffset -= shiftBack;
+
+        if (dstLclNum != BAD_VAR_NUM)
+        {
+            emit->emitIns_S_R(INS_mov, EA_ATTR(regSize), srcIntReg, dstLclNum, dstOffset);
+        }
+        else
+        {
+            emit->emitIns_ARX_R(INS_mov, EA_ATTR(regSize), srcIntReg, dstAddrBaseReg, dstAddrIndexReg,
+                                dstAddrIndexScale, dstOffset);
+        }
+    }
+#else // TARGET_X86
     for (unsigned regSize = REGSIZE_BYTES; size > 0; size -= regSize, dstOffset += regSize)
     {
         while (regSize > size)
         {
             regSize /= 2;
         }
-
         if (dstLclNum != BAD_VAR_NUM)
         {
             emit->emitIns_S_R(INS_mov, EA_ATTR(regSize), srcIntReg, dstLclNum, dstOffset);
@@ -2867,6 +2926,7 @@ void CodeGen::genCodeForInitBlkUnroll(GenTreeBlk* node)
                                 dstAddrIndexScale, dstOffset);
         }
     }
+#endif
 }
 
 #ifdef TARGET_AMD64
@@ -3017,8 +3077,13 @@ void CodeGen::genCodeForCpBlkUnroll(GenTreeBlk* node)
         regNumber tempReg = node->GetSingleTempReg(RBM_ALLFLOAT);
 
         instruction simdMov = simdUnalignedMovIns();
-        for (unsigned regSize = XMM_REGSIZE_BYTES; size >= regSize;
-             size -= regSize, srcOffset += regSize, dstOffset += regSize)
+
+        // Get the largest SIMD register available if the size is large enough
+        unsigned regSize = (size >= YMM_REGSIZE_BYTES) && compiler->compOpportunisticallyDependsOn(InstructionSet_AVX)
+                               ? YMM_REGSIZE_BYTES
+                               : XMM_REGSIZE_BYTES;
+
+        for (; size >= regSize; size -= regSize, srcOffset += regSize, dstOffset += regSize)
         {
             if (srcLclNum != BAD_VAR_NUM)
             {
@@ -3041,15 +3106,109 @@ void CodeGen::genCodeForCpBlkUnroll(GenTreeBlk* node)
             }
         }
 
-        // TODO-CQ-XArch: On x86 we could copy 8 byte at once by using MOVQ instead of four 4 byte MOV stores.
-        // On x64 it may also be worth copying a 4/8 byte remainder using MOVD/MOVQ, that avoids the need to
-        // allocate a GPR just for the remainder.
+        if (size > 0)
+        {
+            if (size <= XMM_REGSIZE_BYTES)
+            {
+                regSize = XMM_REGSIZE_BYTES;
+            }
+
+            // Copy the remainder by moving the last regSize bytes of the buffer
+            unsigned shiftBack = regSize - size;
+            assert(shiftBack <= regSize);
+
+            srcOffset -= shiftBack;
+            dstOffset -= shiftBack;
+
+            if (srcLclNum != BAD_VAR_NUM)
+            {
+                emit->emitIns_R_S(simdMov, EA_ATTR(regSize), tempReg, srcLclNum, srcOffset);
+            }
+            else
+            {
+                emit->emitIns_R_ARX(simdMov, EA_ATTR(regSize), tempReg, srcAddrBaseReg, srcAddrIndexReg,
+                                    srcAddrIndexScale, srcOffset);
+            }
+
+            if (dstLclNum != BAD_VAR_NUM)
+            {
+                emit->emitIns_S_R(simdMov, EA_ATTR(regSize), tempReg, dstLclNum, dstOffset);
+            }
+            else
+            {
+                emit->emitIns_ARX_R(simdMov, EA_ATTR(regSize), tempReg, dstAddrBaseReg, dstAddrIndexReg,
+                                    dstAddrIndexScale, dstOffset);
+            }
+        }
+
+        return;
     }
 
+    // Fill the remainder with normal loads/stores
     if (size > 0)
     {
         regNumber tempReg = node->GetSingleTempReg(RBM_ALLINT);
 
+#ifdef TARGET_AMD64
+        unsigned regSize = REGSIZE_BYTES;
+
+        while (regSize > size)
+        {
+            regSize /= 2;
+        }
+
+        for (; size > regSize; size -= regSize, srcOffset += regSize, dstOffset += regSize)
+        {
+            if (srcLclNum != BAD_VAR_NUM)
+            {
+                emit->emitIns_R_S(INS_mov, EA_ATTR(regSize), tempReg, srcLclNum, srcOffset);
+            }
+            else
+            {
+                emit->emitIns_R_ARX(INS_mov, EA_ATTR(regSize), tempReg, srcAddrBaseReg, srcAddrIndexReg,
+                                    srcAddrIndexScale, srcOffset);
+            }
+
+            if (dstLclNum != BAD_VAR_NUM)
+            {
+                emit->emitIns_S_R(INS_mov, EA_ATTR(regSize), tempReg, dstLclNum, dstOffset);
+            }
+            else
+            {
+                emit->emitIns_ARX_R(INS_mov, EA_ATTR(regSize), tempReg, dstAddrBaseReg, dstAddrIndexReg,
+                                    dstAddrIndexScale, dstOffset);
+            }
+        }
+
+        if (size > 0)
+        {
+            unsigned shiftBack = regSize - size;
+            assert(shiftBack <= regSize);
+
+            srcOffset -= shiftBack;
+            dstOffset -= shiftBack;
+
+            if (srcLclNum != BAD_VAR_NUM)
+            {
+                emit->emitIns_R_S(INS_mov, EA_ATTR(regSize), tempReg, srcLclNum, srcOffset);
+            }
+            else
+            {
+                emit->emitIns_R_ARX(INS_mov, EA_ATTR(regSize), tempReg, srcAddrBaseReg, srcAddrIndexReg,
+                                    srcAddrIndexScale, srcOffset);
+            }
+
+            if (dstLclNum != BAD_VAR_NUM)
+            {
+                emit->emitIns_S_R(INS_mov, EA_ATTR(regSize), tempReg, dstLclNum, dstOffset);
+            }
+            else
+            {
+                emit->emitIns_ARX_R(INS_mov, EA_ATTR(regSize), tempReg, dstAddrBaseReg, dstAddrIndexReg,
+                                    dstAddrIndexScale, dstOffset);
+            }
+        }
+#else // TARGET_X86
         for (unsigned regSize = REGSIZE_BYTES; size > 0; size -= regSize, srcOffset += regSize, dstOffset += regSize)
         {
             while (regSize > size)
@@ -3077,6 +3236,7 @@ void CodeGen::genCodeForCpBlkUnroll(GenTreeBlk* node)
                                     dstAddrIndexScale, dstOffset);
             }
         }
+#endif
     }
 }
 
diff --git a/src/coreclr/jit/lowerxarch.cpp b/src/coreclr/jit/lowerxarch.cpp
index c4d56ed23780b..f76340a136098 100644
--- a/src/coreclr/jit/lowerxarch.cpp
+++ b/src/coreclr/jit/lowerxarch.cpp
@@ -225,7 +225,7 @@ void Lowering::LowerBlockStore(GenTreeBlk* blkNode)
                     {
                         const bool canUse16BytesSimdMov = !blkNode->IsOnHeapAndContainsReferences();
 #ifdef TARGET_AMD64
-                        const bool willUseOnlySimdMov = canUse16BytesSimdMov && (size % 16 == 0);
+                        const bool willUseOnlySimdMov = canUse16BytesSimdMov && (size >= XMM_REGSIZE_BYTES);
 #else
                         const bool willUseOnlySimdMov = (size % 8 == 0);
 #endif
diff --git a/src/coreclr/jit/lsraxarch.cpp b/src/coreclr/jit/lsraxarch.cpp
index 356ad74c1702d..6f5892d1712fe 100644
--- a/src/coreclr/jit/lsraxarch.cpp
+++ b/src/coreclr/jit/lsraxarch.cpp
@@ -1364,7 +1364,7 @@ int LinearScan::BuildBlockStore(GenTreeBlk* blkNode)
             switch (blkNode->gtBlkOpKind)
             {
                 case GenTreeBlk::BlkOpKindUnroll:
-                    if ((size % XMM_REGSIZE_BYTES) != 0)
+                    if (size < XMM_REGSIZE_BYTES)
                     {
                         regMaskTP regMask = allRegs(TYP_INT);
 #ifdef TARGET_X86
@@ -1381,7 +1381,7 @@ int LinearScan::BuildBlockStore(GenTreeBlk* blkNode)
                     if (size >= XMM_REGSIZE_BYTES)
                     {
                         buildInternalFloatRegisterDefForNode(blkNode, internalFloatRegCandidates());
-                        SetContainsAVXFlags();
+                        SetContainsAVXFlags(size);
                     }
                     break;