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
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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
Use an anonymous union to avoid c type punning.
Change the definition of
CRC_TypeDefin the header file to use an anonymous union.Use this new union in the
CRC_Handle_8andCRC_Handle_16functions.Obligatory ST Micro License and disclaimer.
Credit to Olaf who hinted to this answer in comments.