ST Micro HAL, stm32f7xx_hal_crc.c How to solve [Warning] dereferencing type-punned pointer will break strict-aliasing rules

Question

In STM32Cube_FW_F7_V1.3.0 and still in STM32Cube_FW_F7_V1.11.0

I get the following warning when the compiling the stm32f7 HAL library with -Os or -O2.
Note: -O1 compiles without warnings.

[Warning] dereferencing type-punned pointer will break strict-aliasing rules 

The warning is justified. The code appears to work correctly. However, I would like to treat warnings as errors.

Question: How can the code be changed to remove the warning without breaking the crc module?

Note: I could not find a solution to this anywhere online.

This is one of the lines that causes the warning

 *(__IO uint16_t*) (&hcrc->Instance->DR) = (uint16_t)(((uint16_t)(pBuffer[4*i])<<8) | (uint16_t)(pBuffer[4*i+1]));

I believe this is a little tricky and intimidating to change because it is dealing with a hardware crc register.

This is ST Micro's licence, which states I'm required to include when distributing source code, it is not relevant to the question

  /******************************************************************************
  * @attention
  *
  * <h2><center>&copy; COPYRIGHT(c) 2015 STMicroelectronics</center></h2>
  *
  * Redistribution and use in source and binary forms, with or without modification,
  * are permitted provided that the following conditions are met:
  *   1. Redistributions of source code must retain the above copyright notice,
  *      this list of conditions and the following disclaimer.
  *   2. Redistributions in binary form must reproduce the above copyright notice,
  *      this list of conditions and the following disclaimer in the documentation
  *      and/or other materials provided with the distribution.
  *   3. Neither the name of STMicroelectronics nor the names of its contributors
  *      may be used to endorse or promote products derived from this software
  *      without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
  */

Relevant Code:

#define     __IO    volatile             /*!< Defines 'read / write' permissions */

typedef struct
{
  __IO uint32_t  DR;          /*!< CRC Data register,                           Address offset: 0x00 */
  __IO uint8_t   IDR;         /*!< CRC Independent data register,               Address offset: 0x04 */
  uint8_t        RESERVED0;   /*!< Reserved, 0x05                                                    */
  uint16_t       RESERVED1;   /*!< Reserved, 0x06                                                    */
  __IO uint32_t  CR;          /*!< CRC Control register,                        Address offset: 0x08 */
  uint32_t       RESERVED2;   /*!< Reserved,                                                    0x0C */
  __IO uint32_t  INIT;        /*!< Initial CRC value register,                  Address offset: 0x10 */
  __IO uint32_t  POL;         /*!< CRC polynomial register,                     Address offset: 0x14 */
} CRC_TypeDef;


/**             
  * @brief  Enter 8-bit input data to the CRC calculator.
  *         Specific data handling to optimize processing time.  
  * @param  hcrc: CRC handle
  * @param  pBuffer: pointer to the input data buffer
  * @param  BufferLength: input data buffer length
  * @retval uint32_t CRC (returned value LSBs for CRC shorter than 32 bits)
  */
static uint32_t CRC_Handle_8(CRC_HandleTypeDef *hcrc, uint8_t pBuffer[], uint32_t BufferLength)
{
  uint32_t i = 0; /* input data buffer index */

   /* Processing time optimization: 4 bytes are entered in a row with a single word write,
    * last bytes must be carefully fed to the CRC calculator to ensure a correct type
    * handling by the IP */
   for(i = 0; i < (BufferLength/4); i++)
   {
     hcrc->Instance->DR = (uint32_t)(((uint32_t)(pBuffer[4*i])<<24) | ((uint32_t)(pBuffer[4*i+1])<<16) | ((uint32_t)(pBuffer[4*i+2])<<8) | (uint32_t)(pBuffer[4*i+3]));
   }
   /* last bytes specific handling */
   if((BufferLength%4) != 0)
   {
     if(BufferLength%4 == 1)
     {
       *(__IO uint8_t*) (&hcrc->Instance->DR) = pBuffer[4*i];
     }
     if(BufferLength%4 == 2)
     {
       //the following line gives the warning (rightfully) '[Warning] dereferencing type-punned pointer will break strict-aliasing rules'
       *(__IO uint16_t*) (&hcrc->Instance->DR) = (uint16_t)(((uint16_t)(pBuffer[4*i])<<8) | (uint16_t)(pBuffer[4*i+1]));
     }
     if(BufferLength%4 == 3)
     {
       //the following line gives the warning (rightfully) '[Warning] dereferencing type-punned pointer will break strict-aliasing rules'
       *(__IO uint16_t*) (&hcrc->Instance->DR) = (uint16_t)(((uint16_t)(pBuffer[4*i])<<8) | (uint16_t)(pBuffer[4*i+1]));
       *(__IO uint8_t*) (&hcrc->Instance->DR) = pBuffer[4*i+2];       
     }
   }

  /* Return the CRC computed value */ 
  return hcrc->Instance->DR;
}

The warning also appears in the 16 bit CRC function as well

Answer 1

Use an anonymous union to avoid c type punning.

Change the definition of CRC_TypeDef in the header file to use an anonymous union.

typedef struct
{
  union
  {
    __IO uint32_t  DR;          /*!< CRC Data register 32 bit access,            Address offset: 0x00 */
    __IO uint16_t  DR16;        /*!< CRC Data register 16 bit access,            Address offset: 0x00 */
    __IO uint8_t   DR8;         /*!< CRC Data register 8 bit access,             Address offset: 0x00 */
  };
  __IO uint8_t   IDR;         /*!< CRC Independent data register,               Address offset: 0x04 */
  uint8_t        RESERVED0;   /*!< Reserved, 0x05                                                    */
  uint16_t       RESERVED1;   /*!< Reserved, 0x06                                                    */
  __IO uint32_t  CR;          /*!< CRC Control register,                        Address offset: 0x08 */
  uint32_t       RESERVED2;   /*!< Reserved,                                                    0x0C */
  __IO uint32_t  INIT;        /*!< Initial CRC value register,                  Address offset: 0x10 */
  __IO uint32_t  POL;         /*!< CRC polynomial register,                     Address offset: 0x14 */
} CRC_TypeDef;

Use this new union in the CRC_Handle_8 and CRC_Handle_16 functions.

/**             
  * @brief  Enter 8-bit input data to the CRC calculator.
  *         Specific data handling to optimize processing time.  
  * @param  hcrc: CRC handle
  * @param  pBuffer: pointer to the input data buffer
  * @param  BufferLength: input data buffer length
  * @retval uint32_t CRC (returned value LSBs for CRC shorter than 32 bits)
  */
static uint32_t CRC_Handle_8(CRC_HandleTypeDef *hcrc, uint8_t pBuffer[], uint32_t BufferLength)
{
  uint32_t i = 0; /* input data buffer index */

   /* Processing time optimization: 4 bytes are entered in a row with a single word write,
    * last bytes must be carefully fed to the CRC calculator to ensure a correct type
    * handling by the IP */
   for(i = 0; i < (BufferLength/4); i++)
   {
     hcrc->Instance->DR = (uint32_t)(((uint32_t)(pBuffer[4*i])<<24) | ((uint32_t)(pBuffer[4*i+1])<<16) | ((uint32_t)(pBuffer[4*i+2])<<8) | (uint32_t)(pBuffer[4*i+3]));
   }
   /* last bytes specific handling */
   if((BufferLength%4) != 0)
   {
     if(BufferLength%4 == 1)
     {
       hcrc->Instance->DR8 = pBuffer[4*i];
     }
     if(BufferLength%4 == 2)
     {
       hcrc->Instance->DR16 = (uint16_t)(((uint16_t)(pBuffer[4*i])<<8) | (uint16_t)(pBuffer[4*i+1]));
     }
     if(BufferLength%4 == 3)
     {
       hcrc->Instance->DR16 = (uint16_t)(((uint16_t)(pBuffer[4*i])<<8) | (uint16_t)(pBuffer[4*i+1]));
       hcrc->Instance->DR8 = pBuffer[4*i+2];       
     }
   }

  /* Return the CRC computed value */ 
  return hcrc->Instance->DR;
}

/**             
  * @brief  Enter 16-bit input data to the CRC calculator.
  *         Specific data handling to optimize processing time.  
  * @param  hcrc: CRC handle
  * @param  pBuffer: pointer to the input data buffer
  * @param  BufferLength: input data buffer length
  * @retval uint32_t CRC (returned value LSBs for CRC shorter than 32 bits)
  */  
static uint32_t CRC_Handle_16(CRC_HandleTypeDef *hcrc, uint16_t pBuffer[], uint32_t BufferLength)
{
  uint32_t i = 0;  /* input data buffer index */

  /* Processing time optimization: 2 HalfWords are entered in a row with a single word write,
   * in case of odd length, last HalfWord must be carefully fed to the CRC calculator to ensure 
   * a correct type handling by the IP */
  for(i = 0; i < (BufferLength/2); i++)
  {
    hcrc->Instance->DR = (((uint32_t)(pBuffer[2*i])<<16) | (uint32_t)(pBuffer[2*i+1]));
  }
  if((BufferLength%2) != 0)
  {
    hcrc->Instance->DR16 = pBuffer[2*i];
  }

  /* Return the CRC computed value */ 
  return hcrc->Instance->DR;
}

Obligatory ST Micro License and disclaimer.

  ******************************************************************************
  * @attention
  *
  * <h2><center>&copy; COPYRIGHT(c) 2015 STMicroelectronics</center></h2>
  *
  * Redistribution and use in source and binary forms, with or without modification,
  * are permitted provided that the following conditions are met:
  *   1. Redistributions of source code must retain the above copyright notice,
  *      this list of conditions and the following disclaimer.
  *   2. Redistributions in binary form must reproduce the above copyright notice,
  *      this list of conditions and the following disclaimer in the documentation
  *      and/or other materials provided with the distribution.
  *   3. Neither the name of STMicroelectronics nor the names of its contributors
  *      may be used to endorse or promote products derived from this software
  *      without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
  */

Credit to Olaf who hinted to this answer in comments.