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Mixed-type division in DSP lib

Offline Bryan Audiffred over 9 years ago

Note: This was originally posted on 30th November 2012 at http://forums.arm.com

Some of the ARM CMSIS routines employ mixed-type division without explicit casting. See below. The last line is a q31_t/uint32_t operation. I could not find an explicit behavior for this situation in a recent C standard, but I believe both operands would be converted to unsigned using the K&R rules. My compiler is indeed providing the UDIV instruction which gives erroneous results if sum is negative.

Is this a code issue or compiler implementation issue?

***********************

void arm_mean_q15(
q15_t * pSrc,
uint32_t blockSize,
q15_t * pResult)
{
q31_t sum = 0; /* Temporary result storage */
...

*pResult = (q15_t) (sum / blockSize);
}

Regards,
Bryan

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Offline Dr. Paul Beckmann over 9 years ago

Note: This was originally posted on 3rd December 2012 at http://forums.arm.com

Hi Bryan and Emilien,
Thanks for reporting this issue with the CMSIS library. This is indeed a bug and the signed sum result is converted to unsigned. We'll get this fixed in the next release of the CMSIS library.

-Paul
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Offline Dr. Paul Beckmann over 9 years ago

Note: This was originally posted on 3rd December 2012 at http://forums.arm.com

Hi Bryan and Emilien,
Thanks for reporting this issue with the CMSIS library. This is indeed a bug and the signed sum result is converted to unsigned. We'll get this fixed in the next release of the CMSIS library.

-Paul
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