Part 1 After given an Inline assembler version of the volume program I made in the last lab, I got some results that shocked me. After running it with the same 500,000,000 sample size It took only 1.2 seconds of computing time, which is better than even the best variant (bit-shifting) of the program I had made by over 50%. I answered some questions below to further my understanding: 1. What is another way of defining variables instead of the (type name register) format? This can be done using normal type variables as the compiler will automatically put values into registers. 2. For the line vol_int = (int16_t) (0.5 * 32767.0); should 32767 or 32768 be used? 32767 should be used because the int will round the value and 32768 is not in the int16_t range. 3. What does __asm__("dup v1.8h,w22"); do? The duplicate simply means copy the int value into a new vector register . This is for SIMD instructions. 4. What happens if we remove : "=r"(in_cursor)
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In conclusion, the -O3 flag was the most important discovery with trying to optimize clib. It offered a significant speed up with no interference, and provided the chance to uniform a many times used function, strdup. Overall the function is built extremely well with very advanced logic. Attempting to alter said logic sprouted many errors and warnings and left only simple compiler optimizations such as loop unrolling which made small differences in speed up. Clib as a whole is a great idea, offering many compartmentalized features for the C programming language that programmers could definitely find useful when developing. I hope in the future I can get more involved in writing code for open source projects such as clib whether that be doing optimization work or building from the ground up. This project not only gave me an insight on how these open source projects work but also at real world coding and improvement opportunities. I can honestly say now that I have had some experience
To try to squeeze out a bit more performance I attempted to some compiler optimizations. Unfortunately, due to the sheer complexity of the algorithm, I was unable to find other logic complexities to simplify. I tried some loop unrolling to make the compiler have to work a little less, some examples are here below: I made a graph to demonstrate the minute differences this makes in the test vectors below: At most a few millisecond difference is all that can be acquired, and this is only from the finalcount[] array as the digest array produces errors if not compiled in a loop along with other for loops in the code. To test this I simply altered the sha1.c code and ran the make file to see if the vectors passed or failed. As mentioned this is a compiler optimzation, in other words it is completed already, especially at the -O3 level where the benchmarking was done. I would not recommend this change to be pushed upstream normally due to the insignificant time change
As mentioned in Part 01 of the project a very noticeable and easy to apply optimization was the -O3 compiler flag as before there was no optimization done at all. This seemed like the -O3 flag was simply forgotten or overlooked, however as I attempted to apply the flag I came across some issues that were disguised by only applying minimal optimization. When I first added the flag to both the STATIC and #else macros in the Makefile I got this issue in the strdup.h file in deps/strdup: fatal error: expected identifier or '(' before '__extension__' After some research and code manipulation I have discovered there are many work arounds to this problem. For one, I could use a different Makefile for the Sha1 function such as in the development version, however this seems impractical and is more like a temporary band-aid solution. Secondly I could alter the strdup function is strdup.h so it does not share the same name as the same function present in string.h which I piece