Optimizing Software in C++ [pdf] | Hacker News Reader

Optimizing Software in C++ [pdf]

agner.org

82 pointssigna1111 comments

Anyone who is interested in maximizing the output of computer hardware should read this. 100% of times I've been able to significantly speed up a piece of C++ code, I did it by using a technique from this document. Cannot recommend highly enough.

This is great overall. One nitpick:

> How compilers optimize

This didn't mention scalar replacement of aggregates [1]! This is easily one of the most important optimizations for programmers in C++ and similar languages to know about.

[1]: https://books.google.com/books?id=Pq7pHwG1_OkC&pg=PA331&lpg=... is a reasonable explanation.

Lots of good stuff there. Caches(including i-cache and d-cache), volatile(and how it gets mis-construed), pointer-aliasing, float to int overhead, alloca. I don't think I've seen so much good performance stuff in a single place.

Does this have second part ? (I saw "1" at first page in headline). I am asking because I am in love with this . Thank you so much

http://www.agner.org/optimize/

for anyone who wants complete series.

Chapter 14 is twenty-four carat gold. Would love to see optimize.com as a dedicated site with Rosetta Code-style ports of optimization recipes across languages, platforms ;)

Quick link to grab all five manuals:

http://www.agner.org/optimize/optimization_manuals.zip

> Chapter 14 is twenty-four carat gold

Maybe, but the bounds-checking optimisation [14.2] just looks dangerous. If you end up with an erroneous 'int' index of -1 then the original code will error out, whereas the new code will interpret it as 2^32 - 1. This is a perfectly valid index in an array of 2^32 values. This optimisation just isn't equivalent if INT_MAX < size <= UINT_MAX.

Am I missing something?

size is a const int that holds a positive value so you won't ever have INT_MAX < size.

Yes, and the compiler knows that as well. GCC even merges the two versions because they're equivalent[0]

My point is, if size is dynamic, this is a dangerous optimisation.

[0] http://goo.gl/4Ev8as

This optimization guide is a very interesting book.

there is another article "What Every Programmer Should Know About Memory" by Ulrich Drepper http://www.akkadia.org/drepper/cpumemory.pdf ; it has a section on how to optimize memory access / tools that help in the process.

I put up my notes/summary on this article (more like a book with a 100+ pages) http://mosermichael.github.io/cstuff/all/blog/2015/12/11/wep...

Thank you for sharing this.

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