diff --git a/examples/platforms/nrf52840/crypto/nrf52840-mbedtls-config.h b/examples/platforms/nrf52840/crypto/nrf52840-mbedtls-config.h
index 1ebb8ddc3..2713e983e 100644
--- a/examples/platforms/nrf52840/crypto/nrf52840-mbedtls-config.h
+++ b/examples/platforms/nrf52840/crypto/nrf52840-mbedtls-config.h
@@ -30,4 +30,4 @@
 
 #if defined(__ICCARM__)
     _Pragma("diag_suppress=Pe549")
-#endif
\ No newline at end of file
+#endif
diff --git a/examples/platforms/nrf52840/crypto/sha256_alt.h b/examples/platforms/nrf52840/crypto/sha256_alt.h
index e4233dda0..96327806f 100644
--- a/examples/platforms/nrf52840/crypto/sha256_alt.h
+++ b/examples/platforms/nrf52840/crypto/sha256_alt.h
@@ -35,6 +35,10 @@
 extern "C" {
 #endif
 
+#if defined(__CC_ARM)
+#pragma anon_unions
+#endif
+	
 /**
  * \brief          SHA-256 context structure
  */
@@ -51,6 +55,10 @@ typedef union
 }
 mbedtls_sha256_context;
 
+#if defined(__CC_ARM)
+#pragma no_anon_unions
+#endif
+
 /**
  * @brief Initialize SHA-256 context
  *
diff --git a/examples/platforms/nrf52840/flash.c b/examples/platforms/nrf52840/flash.c
index 0c34b05cc..d3e700ed8 100644
--- a/examples/platforms/nrf52840/flash.c
+++ b/examples/platforms/nrf52840/flash.c
@@ -40,31 +40,55 @@
 
 #include "platform-nrf5.h"
 
-extern uint32_t __flash_data_start;
-extern uint32_t __flash_data_end;
-
 #define FLASH_PAGE_ADDR_MASK 0xFFFFF000
 #define FLASH_PAGE_SIZE      4096
-#define FLASH_START_ADDR     ((uint32_t)&__flash_data_start)
-#define FLASH_END_ADDR       ((uint32_t)&__flash_data_end)
+
+static uint32_t sFlashDataStart;
+static uint32_t sFlashDataEnd;
 
 static inline uint32_t mapAddress(uint32_t aAddress)
 {
-    return aAddress + FLASH_START_ADDR;
+    return aAddress + sFlashDataStart;
 }
 
 otError utilsFlashInit(void)
 {
+#if defined(__CC_ARM)
+    // Temporary solution for Keil compiler.
+    uint32_t const bootloaderAddr = NRF_UICR->NRFFW[0];
+    uint32_t const pageSize       = NRF_FICR->CODEPAGESIZE;
+    uint32_t const codeSize       = NRF_FICR->CODESIZE;
+
+    if (bootloaderAddr != 0xFFFFFFFF)
+    {
+        sFlashDataEnd = bootloaderAddr;
+    }
+    else
+    {
+        sFlashDataEnd = pageSize * codeSize;
+    }
+
+    sFlashDataStart = sFlashDataEnd - (pageSize * SETTINGS_CONFIG_PAGE_NUM);
+
+#elif defined(__GNUC__) || defined(__ICCARM__)
+    extern uint32_t __flash_data_start;
+    extern uint32_t __flash_data_end;
+
+    sFlashDataStart = (uint32_t)&__flash_data_start;
+    sFlashDataEnd   = (uint32_t)&__flash_data_end;
+
+#endif
+
     // Just ensure that the start and end addresses are page-aligned.
-    assert((FLASH_START_ADDR % FLASH_PAGE_SIZE) == 0);
-    assert((FLASH_END_ADDR % FLASH_PAGE_SIZE) == 0);
+    assert((sFlashDataStart % FLASH_PAGE_SIZE) == 0);
+    assert((sFlashDataEnd % FLASH_PAGE_SIZE) == 0);
 
     return OT_ERROR_NONE;
 }
 
 uint32_t utilsFlashGetSize(void)
 {
-    return FLASH_END_ADDR - FLASH_START_ADDR;
+    return sFlashDataEnd - sFlashDataStart;
 }
 
 otError utilsFlashErasePage(uint32_t aAddress)
diff --git a/examples/platforms/nrf52840/openthread-core-nrf52840-config.h b/examples/platforms/nrf52840/openthread-core-nrf52840-config.h
index 60ba17854..7f7afb43a 100644
--- a/examples/platforms/nrf52840/openthread-core-nrf52840-config.h
+++ b/examples/platforms/nrf52840/openthread-core-nrf52840-config.h
@@ -156,4 +156,12 @@
  */
 #define OPENTHREAD_CONFIG_MBEDTLS_HEAP_SIZE_NO_DTLS             2048
 
+/*
+ * Suppress the ARMCC warning on unreachable statement,
+ * e.g. break after assert(false) or ExitNow() macro.
+ */
+#if defined(__CC_ARM)
+    _Pragma("diag_suppress=111")
+#endif
+
 #endif  // OPENTHREAD_CORE_NRF52840_CONFIG_H_
diff --git a/examples/platforms/nrf52840/platform.c b/examples/platforms/nrf52840/platform.c
index 931fe1cd9..4e1f1bca6 100644
--- a/examples/platforms/nrf52840/platform.c
+++ b/examples/platforms/nrf52840/platform.c
@@ -94,3 +94,14 @@ __WEAK void PlatformEventSignalPending(void)
 {
     // Intentionally empty
 }
+
+#if defined(__CC_ARM)
+__WEAK void __aeabi_assert(const char *aExpr, const char *aFile, int aLine)
+{
+    (void) aExpr;
+    (void) aFile;
+    (void) aLine;
+
+    while (1);
+}
+#endif
diff --git a/examples/platforms/nrf52840/softdevice.h b/examples/platforms/nrf52840/softdevice.h
index 34aaf556e..9a15275c4 100644
--- a/examples/platforms/nrf52840/softdevice.h
+++ b/examples/platforms/nrf52840/softdevice.h
@@ -35,7 +35,7 @@
 #ifndef SOFTDEVICE_H_
 #define SOFTDEVICE_H_
 
-#if defined ( __GNUC__ )
+#if defined(__GNUC__)
     _Pragma("GCC diagnostic push")
     _Pragma("GCC diagnostic ignored \"-Wreturn-type\"")
     _Pragma("GCC diagnostic ignored \"-Wunused-parameter\"")
@@ -47,7 +47,7 @@
 #include <nrf_soc.h>
 #include <nrf_nvic.h>
 
-#if defined ( __GNUC__ )
+#if defined(__GNUC__)
     _Pragma("GCC diagnostic pop")
 #endif
 
diff --git a/examples/platforms/nrf52840/usb-cdc-uart.c b/examples/platforms/nrf52840/usb-cdc-uart.c
index 7db9ea234..c9be43bf7 100644
--- a/examples/platforms/nrf52840/usb-cdc-uart.c
+++ b/examples/platforms/nrf52840/usb-cdc-uart.c
@@ -33,9 +33,7 @@
  */
 
 #if defined(__GNUC__)
-
 #pragma GCC diagnostic ignored "-Wpedantic"
-
 #endif
 
 #include <openthread/config.h>
diff --git a/include/openthread/icmp6.h b/include/openthread/icmp6.h
index 5ba9826c8..aed42dfc9 100644
--- a/include/openthread/icmp6.h
+++ b/include/openthread/icmp6.h
@@ -86,12 +86,12 @@ struct otIcmp6Header
     uint8_t      mType;      ///< Type
     uint8_t      mCode;      ///< Code
     uint16_t     mChecksum;  ///< Checksum
-    union
+    union OT_TOOL_PACKED_FIELD
     {
         uint8_t  m8[OT_ICMP6_HEADER_DATA_SIZE / sizeof(uint8_t)];
         uint16_t m16[OT_ICMP6_HEADER_DATA_SIZE / sizeof(uint16_t)];
         uint32_t m32[OT_ICMP6_HEADER_DATA_SIZE / sizeof(uint32_t)];
-    } mData;                 ///< Message-specific data
+    } mData;  ///< Message-specific data
 } OT_TOOL_PACKED_END;
 
 /**
diff --git a/include/openthread/platform/toolchain.h b/include/openthread/platform/toolchain.h
index 6b9e83f2f..c3971ce94 100644
--- a/include/openthread/platform/toolchain.h
+++ b/include/openthread/platform/toolchain.h
@@ -246,6 +246,18 @@ extern "C" {
     CODE                                \
     _Pragma("diag_default=Pe111")
 
+#elif defined(__CC_ARM)
+
+#include <stddef.h>
+
+#define OT_UNUSED_VARIABLE(VARIABLE)    \
+    do                                  \
+    {                                   \
+        if (&VARIABLE == NULL) {}       \
+    } while (false)
+
+#define OT_UNREACHABLE_CODE(CODE)  CODE
+
 #else
 
 #define OT_UNUSED_VARIABLE(VARIABLE)    \
diff --git a/include/openthread/types.h b/include/openthread/types.h
index 9fe96db04..27506acfc 100644
--- a/include/openthread/types.h
+++ b/include/openthread/types.h
@@ -416,7 +416,7 @@ typedef struct otExtAddress
 OT_TOOL_PACKED_BEGIN
 struct otIp6Address
 {
-    union
+    union OT_TOOL_PACKED_FIELD
     {
         uint8_t  m8[OT_IP6_ADDRESS_SIZE];                      ///< 8-bit fields
         uint16_t m16[OT_IP6_ADDRESS_SIZE / sizeof(uint16_t)];  ///< 16-bit fields
@@ -693,11 +693,17 @@ enum
 /**
  * This structure represents an IPv6 prefix.
  */
-typedef struct otIp6Prefix
+OT_TOOL_PACKED_BEGIN
+struct otIp6Prefix
 {
     otIp6Address  mPrefix;  ///< The IPv6 prefix.
     uint8_t       mLength;  ///< The IPv6 prefix length.
-} otIp6Prefix;
+} OT_TOOL_PACKED_END;
+
+/**
+ * This type represents an IPv6 prefix.
+ */
+typedef struct otIp6Prefix otIp6Prefix;
 
 #define OT_NETWORK_DATA_ITERATOR_INIT  0    ///< Initializer for otNetworkDataIterator.
 
diff --git a/src/cli/cli.cpp b/src/cli/cli.cpp
index 2a10140a2..cdc433c8d 100644
--- a/src/cli/cli.cpp
+++ b/src/cli/cli.cpp
@@ -470,11 +470,13 @@ void Interpreter::ProcessChild(int argc, char *argv[])
     otError error = OT_ERROR_NONE;
     otChildInfo childInfo;
     long value;
-    bool isTable = false;
+    bool isTable;
 
     VerifyOrExit(argc > 0, error = OT_ERROR_PARSE);
 
-    if (strcmp(argv[0], "list") == 0 || (isTable = (strcmp(argv[0], "table") == 0)))
+    isTable = (strcmp(argv[0], "table") == 0);
+
+    if (isTable || strcmp(argv[0], "list") == 0)
     {
         if (isTable)
         {
@@ -1412,12 +1414,14 @@ void Interpreter::ProcessNeighbor(int argc, char *argv[])
 {
     otError error = OT_ERROR_NONE;
     otNeighborInfo neighborInfo;
-    bool isTable = false;
+    bool isTable;
     otNeighborInfoIterator iterator = OT_NEIGHBOR_INFO_ITERATOR_INIT;
 
     VerifyOrExit(argc > 0, error = OT_ERROR_PARSE);
 
-    if (strcmp(argv[0], "list") == 0 || (isTable = (strcmp(argv[0], "table") == 0)))
+    isTable = (strcmp(argv[0], "table") == 0);
+
+    if (isTable || strcmp(argv[0], "list") == 0)
     {
         if (isTable)
         {
@@ -2224,11 +2228,13 @@ void Interpreter::ProcessRouter(int argc, char *argv[])
     otError error = OT_ERROR_NONE;
     otRouterInfo routerInfo;
     long value;
-    bool isTable = false;
+    bool isTable;
 
     VerifyOrExit(argc > 0, error = OT_ERROR_PARSE);
 
-    if (strcmp(argv[0], "list") == 0 || (isTable = (strcmp(argv[0], "table") == 0)))
+    isTable = (strcmp(argv[0], "table") == 0);
+
+    if (isTable || strcmp(argv[0], "list") == 0)
     {
         if (isTable)
         {
diff --git a/src/cli/cli_console.hpp b/src/cli/cli_console.hpp
index afdacca3e..5b81e6e77 100644
--- a/src/cli/cli_console.hpp
+++ b/src/cli/cli_console.hpp
@@ -68,7 +68,7 @@ public:
      * @returns The number of bytes placed in the output queue.
      *
      */
-    int Output(const char *aBuf, uint16_t aBufLength);
+    virtual int Output(const char *aBuf, uint16_t aBufLength);
 
     /**
      * This method delivers formatted output to the client.
@@ -79,7 +79,7 @@ public:
      * @returns The number of bytes placed in the output queue.
      *
      */
-    int OutputFormat(const char *fmt, ...);
+    virtual int OutputFormat(const char *fmt, ...);
 
     /**
      * This method sets a callback that is called when console has some output.
diff --git a/src/core/coap/coap_secure.hpp b/src/core/coap/coap_secure.hpp
index bc763415a..25360d67e 100644
--- a/src/core/coap/coap_secure.hpp
+++ b/src/core/coap/coap_secure.hpp
@@ -199,7 +199,7 @@ public:
      * @param[in]  aMessageInfo  A reference to the message info associated with @p aMessage.
      *
      */
-    void Receive(Message &aMessage, const Ip6::MessageInfo &aMessageInfo);
+    virtual void Receive(Message &aMessage, const Ip6::MessageInfo &aMessageInfo);
 
 private:
     virtual otError Send(Message &aMessage, const Ip6::MessageInfo &aMessageInfo);
diff --git a/src/core/mac/mac_frame.hpp b/src/core/mac/mac_frame.hpp
index 2b4ae5b4d..eb6628651 100644
--- a/src/core/mac/mac_frame.hpp
+++ b/src/core/mac/mac_frame.hpp
@@ -174,7 +174,6 @@ struct Address
  * This class implements IEEE 802.15.4 MAC frame generation and parsing.
  *
  */
-OT_TOOL_PACKED_BEGIN
 class Frame: public otRadioFrame
 {
 public:
@@ -827,7 +826,7 @@ private:
 
     static uint8_t GetKeySourceLength(uint8_t aKeyIdMode);
 
-} OT_TOOL_PACKED_END;
+};
 
 OT_TOOL_PACKED_BEGIN
 class Beacon
diff --git a/src/core/net/dhcp6_server.cpp b/src/core/net/dhcp6_server.cpp
index b0a12e22d..880bc1a06 100644
--- a/src/core/net/dhcp6_server.cpp
+++ b/src/core/net/dhcp6_server.cpp
@@ -428,7 +428,7 @@ otError Dhcp6Server::SendReply(otIp6Address &aDst, uint8_t *aTransactionId, Clie
     SuccessOrExit(error = AppendRapidCommit(*message));
 
     memset(&messageInfo, 0, sizeof(messageInfo));
-    memcpy(&messageInfo.GetPeerAddr().mFields.m8, &aDst, sizeof(otIp6Address));
+    memcpy(messageInfo.GetPeerAddr().mFields.m8, &aDst, sizeof(otIp6Address));
     messageInfo.mPeerPort = kDhcpClientPort;
     SuccessOrExit(error = mSocket.SendTo(*message, messageInfo));
 
diff --git a/src/core/net/ip6_headers.hpp b/src/core/net/ip6_headers.hpp
index 7e7038a32..2e1c44efa 100644
--- a/src/core/net/ip6_headers.hpp
+++ b/src/core/net/ip6_headers.hpp
@@ -115,7 +115,7 @@ enum
 OT_TOOL_PACKED_BEGIN
 struct HeaderPoD
 {
-    union
+    union OT_TOOL_PACKED_FIELD
     {
         uint8_t   m8[kVersionClassFlowSize / sizeof(uint8_t)];
         uint16_t  m16[kVersionClassFlowSize / sizeof(uint16_t)];
diff --git a/src/core/thread/lowpan.cpp b/src/core/thread/lowpan.cpp
index 09aab63cd..138738958 100644
--- a/src/core/thread/lowpan.cpp
+++ b/src/core/thread/lowpan.cpp
@@ -1178,7 +1178,7 @@ otError FragmentHeader::Init(const uint8_t *aFrame, uint8_t aFrameLength)
     otError error = OT_ERROR_PARSE;
 
     VerifyOrExit(aFrameLength >= sizeof(mDispatchSize) + sizeof(mTag));
-    memcpy(&mDispatchSize, aFrame, sizeof(mDispatchSize) + sizeof(mTag));
+    memcpy(reinterpret_cast<uint8_t *>(&mDispatchSize), aFrame, sizeof(mDispatchSize) + sizeof(mTag));
     aFrame += sizeof(mDispatchSize) + sizeof(mTag);
     aFrameLength -= sizeof(mDispatchSize) + sizeof(mTag);
 
diff --git a/src/core/thread/lowpan.hpp b/src/core/thread/lowpan.hpp
index a8428c387..629e15673 100644
--- a/src/core/thread/lowpan.hpp
+++ b/src/core/thread/lowpan.hpp
@@ -378,11 +378,11 @@ private:
 
     uint8_t  mDispatchHopsLeft;
     uint8_t  mDeepHopsLeft;
-    struct
+    struct OT_TOOL_PACKED_FIELD
     {
         uint16_t mSource;
         uint16_t mDestination;
-    } mAddress OT_TOOL_PACKED_FIELD;
+    } mAddress;
 } OT_TOOL_PACKED_END;
 
 /**
diff --git a/src/core/utils/child_supervision.cpp b/src/core/utils/child_supervision.cpp
index 6081cd151..aeed16044 100644
--- a/src/core/utils/child_supervision.cpp
+++ b/src/core/utils/child_supervision.cpp
@@ -97,7 +97,6 @@ void ChildSupervisor::SendMessage(Child &aChild)
 {
     ThreadNetif &netif = GetNetif();
     Message *message = NULL;
-    otError error = OT_ERROR_NONE;
     uint8_t childIndex;
 
     VerifyOrExit(aChild.GetIndirectMessageCount() == 0);
@@ -111,9 +110,9 @@ void ChildSupervisor::SendMessage(Child &aChild)
     // `ChildSupervisor::GetDestination(message)`.
 
     childIndex = netif.GetMle().GetChildIndex(aChild);
-    SuccessOrExit(error = message->Append(&childIndex, sizeof(childIndex)));
+    SuccessOrExit(message->Append(&childIndex, sizeof(childIndex)));
 
-    SuccessOrExit(error = netif.SendMessage(*message));
+    SuccessOrExit(netif.SendMessage(*message));
     message = NULL;
 
     otLogInfoMle(GetInstance(), "Sending supervision message to child 0x%04x", aChild.GetRloc16());
diff --git a/src/ncp/ncp_base_mtd.cpp b/src/ncp/ncp_base_mtd.cpp
index d62771aeb..34b7e3d33 100644
--- a/src/ncp/ncp_base_mtd.cpp
+++ b/src/ncp/ncp_base_mtd.cpp
@@ -756,16 +756,18 @@ otError NcpBase::InsertPropertyHandler_THREAD_ON_MESH_NETS(void)
     otBorderRouterConfig borderRouterConfig;
     bool stable = false;
     uint8_t flags = 0;
+    uint8_t prefixLength;
 
     memset(&borderRouterConfig, 0, sizeof(otBorderRouterConfig));
 
     VerifyOrExit(mAllowLocalNetworkDataChange == true, error = OT_ERROR_INVALID_STATE);
 
     SuccessOrExit(error = mDecoder.ReadIp6Address(borderRouterConfig.mPrefix.mPrefix));
-    SuccessOrExit(error = mDecoder.ReadUint8(borderRouterConfig.mPrefix.mLength));
+    SuccessOrExit(error = mDecoder.ReadUint8(prefixLength));
     SuccessOrExit(error = mDecoder.ReadBool(stable));
     SuccessOrExit(error = mDecoder.ReadUint8(flags));
 
+    borderRouterConfig.mPrefix.mLength = prefixLength;
     borderRouterConfig.mStable = stable;
     borderRouterConfig.mPreference =
         ((flags & SPINEL_NET_FLAG_PREFERENCE_MASK) >> SPINEL_NET_FLAG_PREFERENCE_OFFSET);
@@ -786,13 +788,16 @@ otError NcpBase::RemovePropertyHandler_THREAD_ON_MESH_NETS(void)
 {
     otError error = OT_ERROR_NONE;
     otIp6Prefix ip6Prefix;
+    uint8_t prefixLength;
 
     memset(&ip6Prefix, 0, sizeof(otIp6Prefix));
 
     VerifyOrExit(mAllowLocalNetworkDataChange == true, error = OT_ERROR_INVALID_STATE);
 
     SuccessOrExit(error = mDecoder.ReadIp6Address(ip6Prefix.mPrefix));
-    SuccessOrExit(error = mDecoder.ReadUint8(ip6Prefix.mLength));
+    SuccessOrExit(error = mDecoder.ReadUint8(prefixLength));
+
+    ip6Prefix.mLength = prefixLength;
 
     error = otBorderRouterRemoveOnMeshPrefix(mInstance, &ip6Prefix);
 
@@ -1131,16 +1136,18 @@ otError NcpBase::InsertPropertyHandler_THREAD_OFF_MESH_ROUTES(void)
     otExternalRouteConfig routeConfig;
     bool stable = false;
     uint8_t flags = 0;
+    uint8_t prefixLength;
 
     memset(&routeConfig, 0, sizeof(otExternalRouteConfig));
 
     VerifyOrExit(mAllowLocalNetworkDataChange == true, error = OT_ERROR_INVALID_STATE);
 
     SuccessOrExit(error = mDecoder.ReadIp6Address(routeConfig.mPrefix.mPrefix));
-    SuccessOrExit(error = mDecoder.ReadUint8(routeConfig.mPrefix.mLength));
+    SuccessOrExit(error = mDecoder.ReadUint8(prefixLength));
     SuccessOrExit(error = mDecoder.ReadBool(stable));
     SuccessOrExit(error = mDecoder.ReadUint8(flags));
 
+    routeConfig.mPrefix.mLength = prefixLength;
     routeConfig.mStable = stable;
     routeConfig.mPreference = FlagByteToExternalRoutePreference(flags);
 
@@ -1154,13 +1161,16 @@ otError NcpBase::RemovePropertyHandler_THREAD_OFF_MESH_ROUTES(void)
 {
     otError error = OT_ERROR_NONE;
     otIp6Prefix ip6Prefix;
+    uint8_t prefixLength;
 
     memset(&ip6Prefix, 0, sizeof(otIp6Prefix));
 
     VerifyOrExit(mAllowLocalNetworkDataChange == true, error = OT_ERROR_INVALID_STATE);
 
     SuccessOrExit(error = mDecoder.ReadIp6Address(ip6Prefix.mPrefix));
-    SuccessOrExit(error = mDecoder.ReadUint8(ip6Prefix.mLength));
+    SuccessOrExit(error = mDecoder.ReadUint8(prefixLength));
+
+    ip6Prefix.mLength = prefixLength;
 
     error = otBorderRouterRemoveRoute(mInstance, &ip6Prefix);
 
diff --git a/third_party/NordicSemiconductor/dependencies/sdk_config.h b/third_party/NordicSemiconductor/dependencies/sdk_config.h
index 064a75dc2..4b30c5eea 100644
--- a/third_party/NordicSemiconductor/dependencies/sdk_config.h
+++ b/third_party/NordicSemiconductor/dependencies/sdk_config.h
@@ -5,10 +5,8 @@
 #undef PACKAGE
 
 #if defined(__GNUC__)
-
 #pragma GCC diagnostic ignored "-Wpedantic"
 #pragma GCC diagnostic ignored "-Wunused-parameter"
-
 #endif
 
 //==========================================================
diff --git a/third_party/NordicSemiconductor/device/nrf52840.h b/third_party/NordicSemiconductor/device/nrf52840.h
index 53f5e97cc..b9543bfca 100644
--- a/third_party/NordicSemiconductor/device/nrf52840.h
+++ b/third_party/NordicSemiconductor/device/nrf52840.h
@@ -149,12 +149,12 @@ typedef enum {
 #define __FPU_PRESENT                  1            /*!< FPU present or not                                                    */
 /** @} */ /* End of group Configuration_of_CMSIS */
 
-#if defined ( __GNUC__ )
+#if defined(__GNUC__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wpedantic"
 #endif
 #include "core_cm4.h"                               /*!< Cortex-M4 processor and core peripherals                              */
-#if defined ( __GNUC__ )
+#if defined(__GNUC__)
 #pragma GCC diagnostic pop
 #endif