Definition of variables, an operation of variables with different data types and casting

Dear all,

I have an experience to develop C code for different microcontrollers and DSPs. However, this is first time to develop code for Arm Core microcontrollers. I am using STM32F334 and STM32F745 for different projects. Last couple of weeks, I have learnt to control peripherals and I hadn't calculated any mathematical expression.  I started to write mathematical calculations and I realized they took a lot of time.

For example:

uint16_t Middle, Low, High;

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Middle = ((Low + High) * 0.5);

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This code takes ~6.25us. Actually, it adds two unsigned integer and one floating point multiplication. It should be very fast because STM32F334 has FPU. I used this code for Texas Instrument TMS320F335. (Texas Complier understand 0.5 is floating and no need to declared again.) I searched on internet and I revised some sample codes and I realized that I should write (float)0.5 or 0.5f. Then I changed the code as like below:

Middle = ((Low + High) * 0.5f);

This new code takes 259ns. Which is ~24 times faster than previous code. (I have developed many codes for TMS320F28335 and they worked well. I thought it was in C and this is also in C languages and it should work.) One letter makes the execution time much faster. I also tried to understand if I need to declared integer number or casting floating point number into integer number but I observed no need to do it. For example:

Middle = (uint16_t) ((Low + High) * 0.5f);   is exactly consumes same time with Middle = ((Low + High) * 0.5f);

or Middle = 34; takes same time with Middle = (uint16_t)34;...

Another example:

#define Offset 5.0f

#define Correction 1.002f

volatile float Voltage;

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Voltage = ((ADC1->DR) - Offset)*Correction;   

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ADC1->DR is 32-bit unsigned integer and all others are floating numbers. I was not sure if I have to write the code as below:

Voltage = ((float)(ADC1->DR) - Offset)*Correction;   

I tried and I didn't see any difference. So, time is very crucial in my code. I have to write an optimum code as much as I can. That is why I don't use HAL function and I wrote my own functions.  I searched documents to learn how to define variables or how to handle different data type in one equation. I think it is about Arm Complier and I had a look to Arm complier user guide. However, I didn't find any enough explanations.

I would like to ask you if you have a document/sample codes and etc, could you please share me.

Last thing that: if Offset is greater than 31.0f, the execution time is more than if the offset is lower than 32.0f. What is the relation?

#define Offset 32.0f   It is slower... 2^5 = 32?  It took few hundreds nanosecond more than if Offset is 31.0f.

Thank you very much in advance.

Regards