Compilers

Directed graph traversal, orderings and applications to data-flow analysis

On the graph traversal and their efficiency.

• Pre-order depth-first-search
• Reverse post-order depth-first-search

This could be a very helpful reference when designing data-flow analysis on programs.

Programming

Semantic Version

How to use version numbers in a sensible way: the MAJOR.MINOR.PATCH format. And I quote:

1. MAJOR version when you make incompatible API changes.
2. MINOR version when you add functionality in a backwards-compatible manner.
3. PATCH version when you make backwards-compatible bug fixes.

Internet

I’m a Privacy Advocate, but I Still Use Windows 10 and Google Now

Your personal data is the fuel to power the Internet today, and how should we face it.

Compilers

Troubles with GCC signed integer overflow optimization

BUG 30475 - assert(int+100>int) optimized away

An interesting ‘bug’ in some versions of GCC (and Clang as well) implementation. Since it’s ‘undefined’ behavior after all, compiler is not obliged to implement it as a defined behavior. Use -fwrapv flag in GCC to inform the compiler that integer value wraps.

Python

Profiling Python in Production

Signal timeout for every small amount of time (say, 1ms in this case) and record the current stack, and we can infer time spent in each function precisely enough. A smart way of profiling large Python programs.

Note: python signal callback passes signal type and signal handler, and signal handler takes signal number and current stack frame.

Hitchhiker’s Guide to Python

Great book to Python, covering code style, best practices and scenario guide. Just started reading it.

C language is an old-school programming language, learned by almost all professional programmers. Still, it never failed to surprise me each time I dig in a little deeper, as it’s full of small details, some hardly noticed, such as this one I recently discovered by accident.

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I’ve recently come across a very interesting article on BitHacks – the low level magics for bit level operations. Some of the tricks introduced here are really excellently clever, some of them may even make you exclaim for their genius!

I had a lot of fun reading through some of the BitHacks. It’s also worth noticing these BitHacks are not only for intellectual pleasures, they provide actual boosts to algorithm performance as well. When an operations is used often enough, the overall performance benefits to the whole program might be significant.

I couldn’t help but keep wondering how on earth did these clever CS guys ever come up with such algorithms. I tried very hard to find some answers and the following are some patterns I noticed in this attempt. Still, honestly, I highly doubt if I can come up with same solutions myself if I ever run into these problems again. Some of them are just to clever.

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Finally! Just finished all the lecture videos of “Algorithm I” from Coursera, and I believe now it’s a good time to review and summarize.

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This is a holiday reading summary. I recently came across two interesting blogs on fundamental concepts in computer science, both with the title “10 Papers Every Programmer Should Read (At Least Twice)”. One could be found in here, and another one in Fogus’ blog. Topics of these papers range from Programming Language theories, functional programming, to Lamport’s distributed system theories. I will read and summarize some of them in my blog. It’ll be 20 papers, and 40 paper-readings to do if I do read each one twice. So, it might be a long time before all is finished.

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This is a quite boring post on programming language trivia, which doesn’t dig into anything deep.

GCC and Clang

The GCC is mostly implemented in C, and used to use Bison for parser generation, according to its Wikipedia page. By default, it generates right recursive table driven LALR parser.

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A little bit more interesting discoveries while digging into the passwd code file.

The ‘s’ flag in file permission

First, the file permission of passwd executable is -rwsr-xr-x. There’s an ‘s’ flag which don’t usually appear in common Unix files. The usage of the ‘s’ field is explained here:

http://en.wikipedia.org/wiki/Setuid

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Passwd

The passwd is program on Unix systems to manage users’ passwords. The user and password information on most Unix systems is stored in two separate files: /etc/passwd for user information, and /etc/shadow for password information, including encrypted password value, expiration data, UID, GID, and etc… The rationale behind storing information in separate files is discussed in Why shadow your passwd file?

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I’ve been auditing a course in computer language implementation and particularly interested in parser generator. Just spent an afternoon reading about the Python parser generator PLY. It’s a pure Python Implementation of Lex and Yacc. And here is the PLY documentation I’ve been reading the whole afternoon.

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