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Cortex M4 FPU against fixed point math

michele corradin
Note: This was originally posted on 1st March 2012 at http://forums.arm.com

Hi,
I'm working on 3 cortex, a STM32F027, cortex M3, a TI Cortex M4 and an Infineon Cortex M4.
I would like to move from a TI C2000 TMS320F2810 (fixed point 32bit core) to an M4 to control a 3 phase power bridge.
My algorithms nowadays work in fixed point math, IQ22, and are based for 98% on simple multiplications and some sine/cosine calculations: PI, PID, Pll, low pass filter, notch, ..
I ported the algorithm in the cortex mainly redifining the IQmpy, moltiplication, and the IQsin, sine calculation first in fixed point then in floating point.
I was expencitng to have a speed improvment running in floating because every multiplication in fixed math requires a shift while in floating I don't need the shift but I'm exeriencing a dramatic slow down of the algorithm running in floating point.
I'm doing my test in IAR.
I checked the assembler and I verified the compiler is using the floating point.
My only explenation is that the FPU doesn't have, as far as I know, direct access to the CPU registers so every multiplication in FPU requires 2 loads to the FPU registers and another load to move the result to the CPU register.
Is there anybody that can confirm me that?
Thank you very much
michele
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