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F28335 compiler doubles address offset when parens are used?

lslonim
Expert 1700 points
Other Parts Discussed in Thread: TMS320F28335

Is this a compiler bug? Adding a set of parentheses around an address changes the way it is computed. This caused a failure in the code that

took quite a while to track down. No parens, the offset is 0xAA. Use parens, the offset is 0x55. See C code and generated assembly below.

Running CCS 6.1.3.00033, compiler TI v15.12.3.LTS, chip TMS320F28335

    *((volatile unsigned long*) FLASH_BASE + 0x55 )    = 0x00980098;

     795 000000f7 9A98          MOV       AL,#152               ; [CPU_] |126|
     796 000000f8 9B98          MOV       AH,#152               ; [CPU_] |126|
     797 000000f9 8F20          MOVL      XAR4,#2097322         ; [CPU_U] |126|
         000000fa 00AA
     798 000000fb 1EC4          MOVL      *+XAR4[0],ACC         ; [CPU_] |126|

     -------------------------------------------------------
    
     *((volatile unsigned long*) (FLASH_BASE + 0x55) )    = 0x00980098;

     795 000000f7 9A98          MOV       AL,#152               ; [CPU_] |126|
     796 000000f8 9B98          MOV       AH,#152               ; [CPU_] |126|
     797 000000f9 8F20          MOVL      XAR4,#2097237         ; [CPU_U] |126|
         000000fa 0055
     798 000000fb 1EC4          MOVL      *+XAR4[0],ACC         ; [CPU_] |126|

Thanks,
Lloyd

  • 0
    TI__Guru**** 232840 points

    The compiler handles this expression correctly.  The parenthesis does change the computation in a meaningful way.  

    The linchpin idea is, for some pointer expression "ptr + int-type-expression", that int-type-expression is scaled, or multiplied, by the size of whatever is pointed to by ptr.  So this expression ...

    (volatile unsigned long*) FLASH_BASE + 0x55

    Means to change FLASH_BASE to type "volatile unsigned long *", then added a scaled 0x55 to it.  In this specific case, that means multiply 0x55 by sizeof(unsigned long) == 2, to get 0xAA.  

    By contrast, this expression ...

    (volatile unsigned long*) (FLASH_BASE + 0x55)

    Means to add 0x55 to FLASH_BASE before changing the type.  Because the scaling for this pointer addition is 1, evidenced by adding only 0x55, then the type of FLASH_BASE must be "int *", or something equivalent.

    Thanks and regards,

    -George

  • 0 in reply to George Mock
    Expert 1700 points
    I've done 30 years of C programming and there's still something to learn. Thanks very much for the education!