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Why does the assembly code sometimes store the register values into RAM and sometimes not?

I have two pieces of code that are very similar. Both of the code are compiled with the `-O0` flag in AC5. When I look at the assembly of the first code, I found that some variables loaded into registers would not be stored into RAM back. As show in the figure below for pOut2, pBias, pB, pB2, and pA2.

But in the assembly of the other code, all variables loaded into registers would be stored in RAM. As show in the figure below.

I don't want the assembly code store the register values into RAM, because the `STR` instruction accupies clock cycle. I want the compiler to maximize the use of registers. How can I achieve this?

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  • Well, you could start by using a higher optimization level than -O0...
    The ARM only has so many registers, and if your function has too many local variables, it won't have any choice other than to put some of them on the stack. (your pictures don't show enough of the code to see if this is what is happening, though.)

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  • Well, you could start by using a higher optimization level than -O0...
    The ARM only has so many registers, and if your function has too many local variables, it won't have any choice other than to put some of them on the stack. (your pictures don't show enough of the code to see if this is what is happening, though.)

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