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Optimizing code for II/e vs II/f

Altera_Forum
Honored Contributor II
‎02-19-2010 11:56 PM
1,251 Views

Hi, 

 

I noticed when I compiled for II/e that it's putting a bunch of multiply instructions in the assembler, where I would not normally want them. Example: Subtracting two pointer ended up using a multiply. Obviously, I want not multiplies generated in the assembler for II/e. 

 

Does the compiler not receive a hint as to whether it's compiling for II/e vs II/f? Can I tell to that I want to compile for II/e? Can I tell it not to use multiplies? 

 

I know they're basically using GCC, which has a -b and -m option, but I'm not sure they're supported for the Nios II cross-compiled tools. 

 

Thanks, Peter
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Altera_Forum
Honored Contributor II
‎02-23-2010 03:04 PM
344 Views

Subtracting 2 pointers is more likely to lead to a divide than a multiply! 

Adding an integral value to a pointer is likely to generate a multiply. 

If the compiler doesn't generate a multiply, it can only generate a shift instead, and fast shifts require the 'mul' instruction. 

Certainly a sequence of shifts and adds (which would likely be faster than a multiply on a system with hardware shift but not multiply) is likely to always be slower on a Nios2. 

Try -fno-hw-mul
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