diff --git a/autotest/cpp/test_gdal.cpp b/autotest/cpp/test_gdal.cpp
index 9be606d82c6b..88a2e9d4a9e3 100644
--- a/autotest/cpp/test_gdal.cpp
+++ b/autotest/cpp/test_gdal.cpp
@@ -4454,6 +4454,41 @@ TEST_F(test_gdal, gdal_gcp_class)
     }
 }
 
+TEST_F(test_gdal, RasterIO_gdt_unknown)
+{
+    GDALDatasetUniquePtr poDS(GDALDriver::FromHandle(GDALGetDriverByName("MEM"))
+                                  ->Create("", 1, 1, 1, GDT_Float64, nullptr));
+    CPLErrorHandlerPusher oErrorHandler(CPLQuietErrorHandler);
+    GByte b = 0;
+    GDALRasterIOExtraArg sExtraArg;
+    INIT_RASTERIO_EXTRA_ARG(sExtraArg);
+    EXPECT_EQ(poDS->RasterIO(GF_Read, 0, 0, 1, 1, &b, 1, 1, GDT_Unknown, 1,
+                             nullptr, 0, 0, 0, &sExtraArg),
+              CE_Failure);
+    EXPECT_EQ(poDS->RasterIO(GF_Read, 0, 0, 1, 1, &b, 1, 1, GDT_TypeCount, 1,
+                             nullptr, 0, 0, 0, &sExtraArg),
+              CE_Failure);
+    EXPECT_EQ(poDS->GetRasterBand(1)->RasterIO(GF_Read, 0, 0, 1, 1, &b, 1, 1,
+                                               GDT_Unknown, 0, 0, &sExtraArg),
+              CE_Failure);
+    EXPECT_EQ(poDS->GetRasterBand(1)->RasterIO(GF_Read, 0, 0, 1, 1, &b, 1, 1,
+                                               GDT_TypeCount, 0, 0, &sExtraArg),
+              CE_Failure);
+}
+
+TEST_F(test_gdal, CopyWords_gdt_unknown)
+{
+    CPLErrorHandlerPusher oErrorHandler(CPLQuietErrorHandler);
+    GByte b = 0;
+    GByte b2 = 0;
+    CPLErrorReset();
+    GDALCopyWords(&b, GDT_Byte, 0, &b2, GDT_Unknown, 0, 1);
+    EXPECT_EQ(CPLGetLastErrorType(), CE_Failure);
+    CPLErrorReset();
+    GDALCopyWords(&b, GDT_Unknown, 0, &b2, GDT_Byte, 0, 1);
+    EXPECT_EQ(CPLGetLastErrorType(), CE_Failure);
+}
+
 // Test GDALRasterBand::ReadRaster
 TEST_F(test_gdal, ReadRaster)
 {
diff --git a/autotest/gcore/rasterio.py b/autotest/gcore/rasterio.py
index 54848bd48d72..c0e8d3ee95ed 100755
--- a/autotest/gcore/rasterio.py
+++ b/autotest/gcore/rasterio.py
@@ -22,14 +22,6 @@
 
 from osgeo import gdal
 
-
-###############################################################################
-@pytest.fixture(autouse=True, scope="module")
-def module_disable_exceptions():
-    with gdaltest.disable_exceptions():
-        yield
-
-
 ###############################################################################
 # Test writing a 1x1 buffer to a 10x6 raster and read it back
 
@@ -217,6 +209,7 @@ def test_rasterio_4():
 # Test error cases of ReadRaster()
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_5():
 
     ds = gdal.Open("data/byte.tif")
@@ -284,6 +277,7 @@ def test_rasterio_5():
 # Test error cases of WriteRaster()
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_6():
 
     ds = gdal.GetDriverByName("MEM").Create("", 2, 2)
@@ -757,6 +751,7 @@ def test_rasterio_overview_subpixel_resampling():
 # Test error when getting a block
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_10():
     ds = gdal.Open("data/byte_truncated.tif")
 
@@ -1431,6 +1426,7 @@ def test_rasterio_dataset_readarray_cint16():
     assert got[1] == numpy.array([[3 + 4j]])
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_rasterband_write_on_readonly():
 
     ds = gdal.Open("data/byte.tif")
@@ -1440,6 +1436,7 @@ def test_rasterio_rasterband_write_on_readonly():
     assert err != 0
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_dataset_write_on_readonly():
 
     ds = gdal.Open("data/byte.tif")
@@ -3012,6 +3009,7 @@ def test_rasterio_writeraster_from_memoryview():
 # Test ReadRaster() in an existing buffer
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_readraster_in_existing_buffer():
 
     ds = gdal.GetDriverByName("MEM").Create("", 2, 1)
@@ -3048,6 +3046,7 @@ def test_rasterio_readraster_in_existing_buffer():
 # Test ReadBlock() in an existing buffer
 
 
+@gdaltest.disable_exceptions()
 def test_rasterio_readblock_in_existing_buffer():
 
     ds = gdal.GetDriverByName("MEM").Create("", 2, 1)
@@ -3074,6 +3073,7 @@ def test_rasterio_readblock_in_existing_buffer():
 # Test ReadRaster() in an existing buffer and alignment issues
 
 
+@gdaltest.disable_exceptions()
 @pytest.mark.parametrize(
     "datatype",
     [
@@ -3390,3 +3390,18 @@ def test_rasterio_overview_selection():
         assert ds.GetRasterBand(1).ReadRaster(0, 0, 100, 100, 29, 29)[0] == 2
         assert ds.GetRasterBand(1).ReadRaster(0, 0, 100, 100, 25, 25)[0] == 3
         assert ds.GetRasterBand(1).ReadRaster(0, 0, 100, 100, 24, 24)[0] == 3
+
+
+###############################################################################
+# Check robustness to GDT_Unknown
+
+
+def test_rasterio_gdt_unknown():
+
+    with gdal.GetDriverByName("MEM").Create("", 1, 1) as ds:
+        # Caught at the SWIG level
+        with pytest.raises(Exception, match="Illegal value for data type"):
+            ds.ReadRaster(buf_type=gdal.GDT_Unknown)
+        # Caught at the SWIG level
+        with pytest.raises(Exception, match="Illegal value for data type"):
+            ds.GetRasterBand(1).ReadRaster(buf_type=gdal.GDT_Unknown)
diff --git a/autotest/gcore/vrtmisc.py b/autotest/gcore/vrtmisc.py
index 01d0a06d7d0c..52bd757867d0 100755
--- a/autotest/gcore/vrtmisc.py
+++ b/autotest/gcore/vrtmisc.py
@@ -1088,3 +1088,13 @@ def test_vrt_read_netcdf():
             xml_data = xml.read()
             # print(xml_data)
             assert 'NETCDF:"alldatatypes.nc":ubyte_var' in xml_data
+
+
+###############################################################################
+
+
+def test_vrtmisc_add_band_gdt_unknown():
+    vrt_ds = gdal.GetDriverByName("VRT").Create("", 1, 1, 0)
+    options = ["subClass=VRTSourcedRasterBand", "blockXSize=32", "blockYSize=48"]
+    with pytest.raises(Exception, match="Illegal GDT_Unknown/GDT_TypeCount argument"):
+        vrt_ds.AddBand(gdal.GDT_Unknown, options)
diff --git a/autotest/gdrivers/mem.py b/autotest/gdrivers/mem.py
index 17fd0537d97d..878155dda27b 100755
--- a/autotest/gdrivers/mem.py
+++ b/autotest/gdrivers/mem.py
@@ -67,8 +67,7 @@ def test_mem_1():
     #######################################################
     # Setup dataset
     drv = gdal.GetDriverByName("MEM")
-    gdaltest.mem_ds = drv.Create("mem_1.mem", 50, 3)
-    ds = gdaltest.mem_ds
+    ds = drv.Create("mem_1.mem", 50, 3)
 
     assert ds.GetProjection() == "", "projection wrong"
 
@@ -777,8 +776,16 @@ def test_mem_alpha_ismaskband():
 
 
 ###############################################################################
-# cleanup
+# Check robustness to GDT_Unknown
 
 
-def test_mem_cleanup():
-    gdaltest.mem_ds = None
+def test_mem_gdt_unknown():
+
+    with pytest.raises(Exception, match="Illegal GDT_Unknown/GDT_TypeCount argument"):
+        gdal.GetDriverByName("MEM").Create("", 1, 1, 1, gdal.GDT_Unknown)
+
+    with gdal.GetDriverByName("MEM").Create("", 1, 1, 0, gdal.GDT_Unknown) as ds:
+        with pytest.raises(
+            Exception, match="Illegal GDT_Unknown/GDT_TypeCount argument"
+        ):
+            ds.AddBand(gdal.GDT_Unknown)
diff --git a/autotest/gdrivers/vrtwarp.py b/autotest/gdrivers/vrtwarp.py
index 41f5b1c26cc0..5532084e1fbd 100755
--- a/autotest/gdrivers/vrtwarp.py
+++ b/autotest/gdrivers/vrtwarp.py
@@ -771,3 +771,15 @@ def test_vrtwarp_autocreatewarpedvrt_invalid_nodata():
     ds.GetRasterBand(1).SetNoDataValue(-9999)
     vrt_ds = gdal.AutoCreateWarpedVRT(ds)
     assert vrt_ds.GetRasterBand(1).DataType == gdal.GDT_Byte
+
+
+###############################################################################
+
+
+def test_vrtwarp_add_band_gdt_unknown():
+
+    ds = gdal.GetDriverByName("MEM").Create("", 1, 1, 1, gdal.GDT_Byte)
+    ds.SetGeoTransform([0, 1, 0, 0, 0, -1])
+    vrt_ds = gdal.AutoCreateWarpedVRT(ds)
+    with pytest.raises(Exception, match="Illegal GDT_Unknown/GDT_TypeCount argument"):
+        vrt_ds.AddBand(gdal.GDT_Unknown)
diff --git a/frmts/mem/memdataset.cpp b/frmts/mem/memdataset.cpp
index d265626f2e35..57b64d9486fd 100644
--- a/frmts/mem/memdataset.cpp
+++ b/frmts/mem/memdataset.cpp
@@ -710,6 +710,12 @@ CPLErr MEMDataset::AddBand(GDALDataType eType, char **papszOptions)
 {
     const int nBandId = GetRasterCount() + 1;
     const GSpacing nPixelSize = GDALGetDataTypeSizeBytes(eType);
+    if (nPixelSize == 0)
+    {
+        ReportError(CE_Failure, CPLE_IllegalArg,
+                    "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return CE_Failure;
+    }
 
     /* -------------------------------------------------------------------- */
     /*      Do we need to allocate the memory ourselves?  This is the       */
@@ -1333,7 +1339,7 @@ MEMDataset *MEMDataset::Create(const char * /* pszFilename */, int nXSize,
     /*      First allocate band data, verifying that we can get enough      */
     /*      memory.                                                         */
     /* -------------------------------------------------------------------- */
-    const int nWordSize = GDALGetDataTypeSize(eType) / 8;
+    const int nWordSize = GDALGetDataTypeSizeBytes(eType);
     if (nBandsIn > 0 && nWordSize > 0 &&
         (nBandsIn > INT_MAX / nWordSize ||
          (GIntBig)nXSize * nYSize > GINTBIG_MAX / (nWordSize * nBandsIn)))
diff --git a/frmts/vrt/vrtdataset.cpp b/frmts/vrt/vrtdataset.cpp
index 58e142714d48..149ba251c83b 100644
--- a/frmts/vrt/vrtdataset.cpp
+++ b/frmts/vrt/vrtdataset.cpp
@@ -1601,6 +1601,13 @@ VRTDataset *VRTDataset::OpenXML(const char *pszXML, const char *pszVRTPath,
 CPLErr VRTDataset::AddBand(GDALDataType eType, char **papszOptions)
 
 {
+    if (eType == GDT_Unknown || eType == GDT_TypeCount)
+    {
+        ReportError(CE_Failure, CPLE_IllegalArg,
+                    "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return CE_Failure;
+    }
+
     SetNeedsFlush();
 
     /* ==================================================================== */
diff --git a/frmts/vrt/vrtwarped.cpp b/frmts/vrt/vrtwarped.cpp
index 432758767e53..229cdc749c55 100644
--- a/frmts/vrt/vrtwarped.cpp
+++ b/frmts/vrt/vrtwarped.cpp
@@ -2165,6 +2165,13 @@ CPLErr VRTWarpedDataset::IRasterIO(
 CPLErr VRTWarpedDataset::AddBand(GDALDataType eType, char ** /* papszOptions */)
 
 {
+    if (eType == GDT_Unknown || eType == GDT_TypeCount)
+    {
+        ReportError(CE_Failure, CPLE_IllegalArg,
+                    "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return CE_Failure;
+    }
+
     SetBand(GetRasterCount() + 1,
             new VRTWarpedRasterBand(this, GetRasterCount() + 1, eType));
 
diff --git a/gcore/gdaldataset.cpp b/gcore/gdaldataset.cpp
index f52217e1efd1..ea56ac9038c5 100644
--- a/gcore/gdaldataset.cpp
+++ b/gcore/gdaldataset.cpp
@@ -2724,7 +2724,8 @@ CPLErr GDALDataset::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
 
         psExtraArg = &sExtraArg;
     }
-    else if (psExtraArg->nVersion != RASTERIO_EXTRA_ARG_CURRENT_VERSION)
+    else if (CPL_UNLIKELY(psExtraArg->nVersion !=
+                          RASTERIO_EXTRA_ARG_CURRENT_VERSION))
     {
         ReportError(CE_Failure, CPLE_AppDefined,
                     "Unhandled version of GDALRasterIOExtraArg");
@@ -2734,7 +2735,7 @@ CPLErr GDALDataset::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
     GDALRasterIOExtraArgSetResampleAlg(psExtraArg, nXSize, nYSize, nBufXSize,
                                        nBufYSize);
 
-    if (nullptr == pData)
+    if (CPL_UNLIKELY(nullptr == pData))
     {
         ReportError(CE_Failure, CPLE_AppDefined,
                     "The buffer into which the data should be read is null");
@@ -2745,7 +2746,7 @@ CPLErr GDALDataset::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
     /*      Do some validation of parameters.                               */
     /* -------------------------------------------------------------------- */
 
-    if (eRWFlag != GF_Read && eRWFlag != GF_Write)
+    if (CPL_UNLIKELY(eRWFlag != GF_Read && eRWFlag != GF_Write))
     {
         ReportError(
             CE_Failure, CPLE_IllegalArg,
@@ -2756,7 +2757,7 @@ CPLErr GDALDataset::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
 
     if (eRWFlag == GF_Write)
     {
-        if (eAccess != GA_Update)
+        if (CPL_UNLIKELY(eAccess != GA_Update))
         {
             ReportError(CE_Failure, CPLE_AppDefined,
                         "Write operation not permitted on dataset opened "
@@ -2771,6 +2772,12 @@ CPLErr GDALDataset::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
         nBufYSize, nBandCount, panBandMap);
     if (eErr != CE_None || bStopProcessing)
         return eErr;
+    if (CPL_UNLIKELY(eBufType == GDT_Unknown || eBufType == GDT_TypeCount))
+    {
+        ReportError(CE_Failure, CPLE_AppDefined,
+                    "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return CE_Failure;
+    }
 
     /* -------------------------------------------------------------------- */
     /*      If pixel and line spacing are defaulted assign reasonable      */
diff --git a/gcore/gdaldriver.cpp b/gcore/gdaldriver.cpp
index 728ded9e90e1..cc7f96c17f93 100644
--- a/gcore/gdaldriver.cpp
+++ b/gcore/gdaldriver.cpp
@@ -195,8 +195,8 @@ GDALDataset *GDALDriver::Create(const char *pszFilename, int nXSize, int nYSize,
     /*      Does this format support creation.                              */
     /* -------------------------------------------------------------------- */
     pfnCreate = GetCreateCallback();
-    if (pfnCreate == nullptr && pfnCreateEx == nullptr &&
-        pfnCreateVectorOnly == nullptr)
+    if (CPL_UNLIKELY(pfnCreate == nullptr && pfnCreateEx == nullptr &&
+                     pfnCreateVectorOnly == nullptr))
     {
         CPLError(CE_Failure, CPLE_NotSupported,
                  "GDALDriver::Create() ... no create method implemented"
@@ -207,7 +207,7 @@ GDALDataset *GDALDriver::Create(const char *pszFilename, int nXSize, int nYSize,
     /* -------------------------------------------------------------------- */
     /*      Do some rudimentary argument checking.                          */
     /* -------------------------------------------------------------------- */
-    if (nBands < 0)
+    if (CPL_UNLIKELY(nBands < 0))
     {
         CPLError(CE_Failure, CPLE_AppDefined,
                  "Attempt to create dataset with %d bands is illegal,"
@@ -216,9 +216,9 @@ GDALDataset *GDALDriver::Create(const char *pszFilename, int nXSize, int nYSize,
         return nullptr;
     }
 
-    if (GetMetadataItem(GDAL_DCAP_RASTER) != nullptr &&
-        GetMetadataItem(GDAL_DCAP_VECTOR) == nullptr &&
-        (nXSize < 1 || nYSize < 1))
+    if (CPL_UNLIKELY(GetMetadataItem(GDAL_DCAP_RASTER) != nullptr &&
+                     GetMetadataItem(GDAL_DCAP_VECTOR) == nullptr &&
+                     (nXSize < 1 || nYSize < 1)))
     {
         CPLError(CE_Failure, CPLE_AppDefined,
                  "Attempt to create %dx%d dataset is illegal,"
@@ -227,6 +227,14 @@ GDALDataset *GDALDriver::Create(const char *pszFilename, int nXSize, int nYSize,
         return nullptr;
     }
 
+    if (CPL_UNLIKELY(nBands != 0 &&
+                     (eType == GDT_Unknown || eType == GDT_TypeCount)))
+    {
+        CPLError(CE_Failure, CPLE_IllegalArg,
+                 "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return nullptr;
+    }
+
     /* -------------------------------------------------------------------- */
     /*      Make sure we cleanup if there is an existing dataset of this    */
     /*      name.  But even if that seems to fail we will continue since    */
diff --git a/gcore/gdalmultidim.cpp b/gcore/gdalmultidim.cpp
index 5e14ba03939b..ad0365692780 100644
--- a/gcore/gdalmultidim.cpp
+++ b/gcore/gdalmultidim.cpp
@@ -10043,7 +10043,8 @@ GDALExtendedDataType::operator=(GDALExtendedDataType &&other)
  *
  * This is the same as the C function GDALExtendedDataTypeCreate()
  *
- * @param eType Numeric data type.
+ * @param eType Numeric data type. Must be different from GDT_Unknown and
+ * GDT_TypeCount
  */
 GDALExtendedDataType GDALExtendedDataType::Create(GDALDataType eType)
 {
@@ -10471,12 +10472,19 @@ void GDALDimension::ParentDeleted()
  *
  * The returned handle should be freed with GDALExtendedDataTypeRelease().
  *
- * @param eType Numeric data type.
+ * @param eType Numeric data type. Must be different from GDT_Unknown and
+ * GDT_TypeCount
  *
  * @return a new GDALExtendedDataTypeH handle, or nullptr.
  */
 GDALExtendedDataTypeH GDALExtendedDataTypeCreate(GDALDataType eType)
 {
+    if (CPL_UNLIKELY(eType == GDT_Unknown || eType == GDT_TypeCount))
+    {
+        CPLError(CE_Failure, CPLE_IllegalArg,
+                 "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return nullptr;
+    }
     return new GDALExtendedDataTypeHS(
         new GDALExtendedDataType(GDALExtendedDataType::Create(eType)));
 }
diff --git a/gcore/gdalrasterband.cpp b/gcore/gdalrasterband.cpp
index 87c814efdedf..91ff7d416f2c 100644
--- a/gcore/gdalrasterband.cpp
+++ b/gcore/gdalrasterband.cpp
@@ -331,7 +331,8 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
         INIT_RASTERIO_EXTRA_ARG(sExtraArg);
         psExtraArg = &sExtraArg;
     }
-    else if (psExtraArg->nVersion != RASTERIO_EXTRA_ARG_CURRENT_VERSION)
+    else if (CPL_UNLIKELY(psExtraArg->nVersion !=
+                          RASTERIO_EXTRA_ARG_CURRENT_VERSION))
     {
         ReportError(CE_Failure, CPLE_AppDefined,
                     "Unhandled version of GDALRasterIOExtraArg");
@@ -341,7 +342,7 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
     GDALRasterIOExtraArgSetResampleAlg(psExtraArg, nXSize, nYSize, nBufXSize,
                                        nBufYSize);
 
-    if (nullptr == pData)
+    if (CPL_UNLIKELY(nullptr == pData))
     {
         ReportError(CE_Failure, CPLE_AppDefined,
                     "The buffer into which the data should be read is null");
@@ -352,7 +353,8 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
     /*      Some size values are "noop".  Lets just return to avoid         */
     /*      stressing lower level functions.                                */
     /* -------------------------------------------------------------------- */
-    if (nXSize < 1 || nYSize < 1 || nBufXSize < 1 || nBufYSize < 1)
+    if (CPL_UNLIKELY(nXSize < 1 || nYSize < 1 || nBufXSize < 1 ||
+                     nBufYSize < 1))
     {
         CPLDebug("GDAL",
                  "RasterIO() skipped for odd window or buffer size.\n"
@@ -365,7 +367,7 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
 
     if (eRWFlag == GF_Write)
     {
-        if (eFlushBlockErr != CE_None)
+        if (CPL_UNLIKELY(eFlushBlockErr != CE_None))
         {
             ReportError(eFlushBlockErr, CPLE_AppDefined,
                         "An error occurred while writing a dirty block "
@@ -374,7 +376,7 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
             eFlushBlockErr = CE_None;
             return eErr;
         }
-        if (eAccess != GA_Update)
+        if (CPL_UNLIKELY(eAccess != GA_Update))
         {
             ReportError(CE_Failure, CPLE_AppDefined,
                         "Write operation not permitted on dataset opened "
@@ -400,9 +402,9 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
     /* -------------------------------------------------------------------- */
     /*      Do some validation of parameters.                               */
     /* -------------------------------------------------------------------- */
-    if (nXOff < 0 || nXOff > INT_MAX - nXSize ||
-        nXOff + nXSize > nRasterXSize || nYOff < 0 ||
-        nYOff > INT_MAX - nYSize || nYOff + nYSize > nRasterYSize)
+    if (CPL_UNLIKELY(nXOff < 0 || nXOff > INT_MAX - nXSize ||
+                     nXOff + nXSize > nRasterXSize || nYOff < 0 ||
+                     nYOff > INT_MAX - nYSize || nYOff + nYSize > nRasterYSize))
     {
         ReportError(CE_Failure, CPLE_IllegalArg,
                     "Access window out of range in RasterIO().  Requested\n"
@@ -411,7 +413,7 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
         return CE_Failure;
     }
 
-    if (eRWFlag != GF_Read && eRWFlag != GF_Write)
+    if (CPL_UNLIKELY(eRWFlag != GF_Read && eRWFlag != GF_Write))
     {
         ReportError(
             CE_Failure, CPLE_IllegalArg,
@@ -419,6 +421,12 @@ CPLErr GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff, int nYOff,
             eRWFlag);
         return CE_Failure;
     }
+    if (CPL_UNLIKELY(eBufType == GDT_Unknown || eBufType == GDT_TypeCount))
+    {
+        ReportError(CE_Failure, CPLE_IllegalArg,
+                    "Illegal GDT_Unknown/GDT_TypeCount argument");
+        return CE_Failure;
+    }
 
     /* -------------------------------------------------------------------- */
     /*      Call the format specific function.                              */
diff --git a/gcore/rasterio.cpp b/gcore/rasterio.cpp
index 2bc36f23f9f7..f95d2018c4de 100644
--- a/gcore/rasterio.cpp
+++ b/gcore/rasterio.cpp
@@ -3357,8 +3357,13 @@ void CPL_STDCALL GDALCopyWords64(const void *CPL_RESTRICT pSrcData,
     // On platforms where alignment matters, be careful
     const int nSrcDataTypeSize = GDALGetDataTypeSizeBytes(eSrcType);
     const int nDstDataTypeSize = GDALGetDataTypeSizeBytes(eDstType);
-    assert(nSrcDataTypeSize != 0);
-    assert(nDstDataTypeSize != 0);
+    if (CPL_UNLIKELY(nSrcDataTypeSize == 0 || nDstDataTypeSize == 0))
+    {
+        CPLError(CE_Failure, CPLE_NotSupported,
+                 "GDALCopyWords64(): unsupported GDT_Unknown/GDT_TypeCount "
+                 "argument");
+        return;
+    }
     if (!(eSrcType == eDstType && nSrcPixelStride == nDstPixelStride) &&
         ((reinterpret_cast<uintptr_t>(pSrcData) % nSrcDataTypeSize) != 0 ||
          (reinterpret_cast<uintptr_t>(pDstData) % nDstDataTypeSize) != 0 ||
diff --git a/port/cpl_port.h b/port/cpl_port.h
index 1622d5f14751..b00476e9d7b6 100644
--- a/port/cpl_port.h
+++ b/port/cpl_port.h
@@ -1179,4 +1179,15 @@ typedef const char *const *CSLConstList;
 typedef char **CSLConstList;
 #endif
 
+#if defined(__cplusplus) && defined(GDAL_COMPILATION)
+#if defined(__GNUC__) && !defined(DOXYGEN_SKIP)
+/** Macro that evaluates to (cond), and possibly gives a hint to the compiler
+ * than (cond) is unlikely to be true.
+ */
+#define CPL_UNLIKELY(cond) __builtin_expect(static_cast<bool>(cond), 0)
+#else
+#define CPL_UNLIKELY(cond) (cond)
+#endif
+#endif
+
 #endif /* ndef CPL_BASE_H_INCLUDED */