diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index e80ec210e..9c7e023a7 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -153,12 +153,32 @@ jobs:
         script/check-simulation-build
         script/check-posix-build
 
+  clang:
+    name: clang-${{ matrix.clang_ver }}
+    runs-on: ubuntu-18.04
+    strategy:
+      matrix:
+        clang_ver: ["6.0", "7", "8", "9"]
+    env:
+      CC: clang-${{ matrix.clang_ver }}
+      CXX: clang++-${{ matrix.clang_ver }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Bootstrap
+        run: |
+          sudo apt-get update
+          sudo apt-get --no-install-recommends install -y clang-${{ matrix.clang_ver }} clang++-${{ matrix.clang_ver }} ninja-build libreadline-dev libncurses-dev
+      - name: Build
+        run: |
+          script/check-simulation-build
+          script/check-posix-build
+
   clang-m32:
     name: clang-m32-${{ matrix.clang_ver }}
     runs-on: ubuntu-18.04
     strategy:
       matrix:
-        clang_ver: [9]
+        clang_ver: ["6.0", "7", "8", "9"]
     env:
       CC: clang-${{ matrix.clang_ver }}
       CXX: clang++-${{ matrix.clang_ver }}
@@ -175,6 +195,7 @@ jobs:
           sudo apt-get --no-install-recommends install -y libreadline-dev:i386 libncurses-dev:i386
       - name: Build
         run: |
+          script/check-simulation-build
           script/check-posix-build
 
   gn:
diff --git a/script/check-simulation-build-autotools b/script/check-simulation-build-autotools
index 84a1439f8..ed6811113 100755
--- a/script/check-simulation-build-autotools
+++ b/script/check-simulation-build-autotools
@@ -72,6 +72,7 @@ build_all_features()
         "-DOPENTHREAD_CONFIG_MLE_ATTACH_BACKOFF_ENABLE=1"
         "-DOPENTHREAD_CONFIG_MLE_STEERING_DATA_SET_OOB_ENABLE=1"
         "-DOPENTHREAD_CONFIG_MPL_DYNAMIC_INTERVAL_ENABLE"
+        "-DOPENTHREAD_CONFIG_NCP_SPI_ENABLE=1"
         "-DOPENTHREAD_CONFIG_NCP_UART_ENABLE=1"
         "-DOPENTHREAD_CONFIG_PLATFORM_FLASH_API_ENABLE=1"
         "-DOPENTHREAD_CONFIG_PLATFORM_RADIO_COEX_ENABLE=1"
diff --git a/src/core/common/debug.hpp b/src/core/common/debug.hpp
index ba15fde76..50dd2b2b2 100644
--- a/src/core/common/debug.hpp
+++ b/src/core/common/debug.hpp
@@ -71,7 +71,7 @@
         }                                          \
     } while (0)
 
-#else
+#else // OPENTHREAD_CONFIG_PLATFORM_ASSERT_MANAGEMENT
 
 #define OT_ASSERT(cond) \
     do                  \
@@ -86,7 +86,7 @@
 
 #endif // OPENTHREAD_CONFIG_PLATFORM_ASSERT_MANAGEMENT
 
-#else
+#else // OPENTHREAD_CONFIG_ASSERT_ENABLE
 
 #define OT_ASSERT(cond)
 
diff --git a/src/ncp/ncp_spi.cpp b/src/ncp/ncp_spi.cpp
index b8a931b02..f4dc788c1 100644
--- a/src/ncp/ncp_spi.cpp
+++ b/src/ncp/ncp_spi.cpp
@@ -35,6 +35,7 @@
 #include <openthread/ncp.h>
 #include <openthread/platform/misc.h>
 #include <openthread/platform/spi-slave.h>
+#include <openthread/platform/toolchain.h>
 
 #include "openthread-core-config.h"
 #include "common/code_utils.hpp"
@@ -81,7 +82,7 @@ NcpSpi::NcpSpi(Instance *aInstance)
     , mTxState(kTxStateIdle)
     , mHandlingRxFrame(false)
     , mResetFlag(true)
-    , mPrepareTxFrameTask(*aInstance, &NcpSpi::PrepareTxFrame, this)
+    , mPrepareTxFrameTask(*aInstance, NcpSpi::PrepareTxFrame, this)
     , mSendFrameLength(0)
 {
     SpiFrame sendFrame(mSendFrame);
@@ -102,14 +103,15 @@ NcpSpi::NcpSpi(Instance *aInstance)
 
     mTxFrameBuffer.SetFrameAddedCallback(HandleFrameAddedToTxBuffer, this);
 
-    otPlatSpiSlaveEnable(&NcpSpi::SpiTransactionComplete, &NcpSpi::SpiTransactionProcess, this);
+    IgnoreError(otPlatSpiSlaveEnable(&NcpSpi::SpiTransactionComplete, &NcpSpi::SpiTransactionProcess, this));
 
     // We signal an interrupt on this first transaction to
     // make sure that the host processor knows that our
     // reset flag was set.
 
-    otPlatSpiSlavePrepareTransaction(mEmptySendFrameZeroAccept, kSpiHeaderSize, mEmptyReceiveFrame, kSpiHeaderSize,
-                                     /* aRequestTransactionFlag */ true);
+    IgnoreError(otPlatSpiSlavePrepareTransaction(mEmptySendFrameZeroAccept, kSpiHeaderSize, mEmptyReceiveFrame,
+                                                 kSpiHeaderSize,
+                                                 /* aRequestTransactionFlag */ true));
 }
 
 bool NcpSpi::SpiTransactionComplete(void *   aContext,
@@ -217,7 +219,8 @@ exit:
 
     sendFrame.SetHeaderAcceptLen(aInputLen - kSpiHeaderSize);
 
-    otPlatSpiSlavePrepareTransaction(aOutputBuf, aOutputLen, aInputBuf, aInputLen, (mTxState == kTxStateSending));
+    IgnoreError(
+        otPlatSpiSlavePrepareTransaction(aOutputBuf, aOutputLen, aInputBuf, aInputLen, (mTxState == kTxStateSending)));
 
     return shouldProcess;
 }
@@ -278,6 +281,9 @@ void NcpSpi::PrepareNextSpiSendFrame(void)
     readLength = mTxFrameBuffer.OutFrameRead(frameLength, sendFrame.GetData());
     OT_ASSERT(readLength == frameLength);
 
+    // Suppress the warning when assertions are disabled
+    OT_UNUSED_VARIABLE(readLength);
+
     sendFrame.SetHeaderDataLen(frameLength);
     mSendFrameLength = frameLength + kSpiHeaderSize;
 
@@ -302,7 +308,7 @@ void NcpSpi::PrepareNextSpiSendFrame(void)
         ExitNow();
     }
 
-    mTxFrameBuffer.OutFrameRemove();
+    IgnoreError(mTxFrameBuffer.OutFrameRemove());
 
 exit:
     return;
@@ -367,8 +373,9 @@ void NcpSpi::HandleRxFrame(void)
     {
         sendFrame.SetHeaderAcceptLen(kSpiBufferSize - kSpiHeaderSize);
 
-        otPlatSpiSlavePrepareTransaction(mEmptySendFrameFullAccept, kSpiHeaderSize, mReceiveFrame, kSpiBufferSize,
-                                         /* aRequestTrans */ false);
+        IgnoreError(otPlatSpiSlavePrepareTransaction(mEmptySendFrameFullAccept, kSpiHeaderSize, mReceiveFrame,
+                                                     kSpiBufferSize,
+                                                     /* aRequestTrans */ false));
 
         // No need to check the error status. Getting `OT_ERROR_BUSY`
         // is OK as everything will be set up properly from callback when