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Forum: ARM programming with GCC/GNU tools parentheses around logic operands


parentheses around logic operands
von Roman M. (romez777)

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Hello,

I'm implementing I2C interface. I took Atmel's TWI code for reference.
It gets compiled perfectly except one spot and warning accompaning it:

/* Set the TWI Master Mode Register */
pTwi->TWI_MMR =  SlaveAddr | IntAddrSize & ~AT91C_TWI_MREAD;

warning: suggest parentheses around arithmetic in operand of |

I understand the priority of '&' operator is higher then others. So how
should I place parentheses correctly (it may give various results)?

Datasheet is not very eloquent about this.

Thanks in advance.

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Re: parentheses around logic operands
von Clifford S. (clifford)

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It is merely suggesting that you are explicit about your intent so write
either:

pTwi->TWI_MMR =  SlaveAddr | (IntAddrSize & ~AT91C_TWI_MREAD);

or

pTwi->TWI_MMR =  (SlaveAddr | IntAddrSize) & ~AT91C_TWI_MREAD;

depending upon what you intended.

You are right that if you intended the first form then that is identical
to your original code, but it is as well to be explicit because bitwise
operator precedence often catches the unwary.

This warning is enabled by the -Wparentheses option, and in later
versions of GCC by -Wall.
http://gcc.gnu.org/onlinedocs/gcc-4.2.2/gcc/Warning-Options.html#index-Wparentheses-228


Clifford

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