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Re: [avr-gcc-list] Speed challenge...

From: Peter N Lewis
Subject: Re: [avr-gcc-list] Speed challenge...
Date: Sat, 27 Apr 2002 16:01:10 +0800

It occurs to me that some of the general principles for optimizing that I applied to this would be useful to some folks.

First off, you need to look at the source code. You can have avr-gcc compile to a source output with:

avr-gcc -mmcu=at90s4433 -Wall -Os -I../include -S test.c -o test.s
(ie -S = compile only, and output to a .s source file).

Then you find the section of code you're interested in, and you look at the instructions. Using an Instruction Set summary (like the one at <http://www.atmel.com/atmel/acrobat/doc0856.pdf>, or better yet, one with a table of instructions and cycles), look at the cycles used by each instruction. Generally, on the AVR, instructions that read or write to memory or IO or jumps take two cycles and most everything else takes one cycle. The time taken is directly proportional to the number of cycles, so trace through the instructions that will be executed (following each loop) and count all the cycles. Now you have a baseline to refer back to to see if you are mak9ing progress.

After that, it is mostly a matter of looking at the assembly code and looking for wasted cycles (excess memory accesses, excess jumps, more expensive than necessary instructions). With loops, pay particular attention to the "increment" and end of loop test, since they can often be rewritten to reduce the number of wasted cycles - using a loop that counts towards zero is generally preferable so that the decrement can double as the test for completion.

Of course, before you start doing any of this, you should look at the high level code and ask yourself if you really need to be doing it this way at all in the first place (for example, could your data be pre-sorted). The techniques I'm talking about are for low level optimization when you're sure you're doing the minimum that needs to be done and just want to do it a bit faster. Generally you can get about a factor of 2 improvement out of doing this - with high level code changes you can often get *much* better improvement (factors of ten or more).

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