Question

Why all programs are divided into 200 basic blocks by Valgrind? And how to divided?

Answer 1

First Question

It’s been some time since I’ve worked on a Valgrind tool (even longer than this question is old), but in case anyone is still interested, here’s what I’ve dredged up from memory:

First, a distinction: a super block is a bit different from a basic block. Valgrind uses super blocks, not basic blocks. A super block may exit at any point, but a basic block will only ever exit by running off its end.

Valgrind doesn’t divide a program into 200 super blocks. I’m pretty sure that it instead breaks programs up into super blocks of no more than 200 IRStatements (which may or may not translate directly into instructions).

The reason for this I’m pretty sure is for efficiency of the translator: at least with current versions of Valgrind I’m reasonably sure it doesn’t translate your entire program up front. Translating the program into its IR format is time consuming and resource intensive, so the translator seeks to only translate as much of the program as it needs to. It does this by only translating code as it gets executed for the first time.

Second Question

Now, as to your second question… I’m not entirely sure what you’re asking. If you’re asking, “How does Valgrind decide how to divide up the program?”, then the answer is that it decides similarly to a compiler. It starts converting the program into super blocks, and starts a new super block whenever it reaches the block limit size or detects that there is an entry point into the block from elsewhere (super blocks and basic blocks can only have one entry point).

If you instead meant, “Can I change the size of an IRSB super block?”, then yes, there is an option you can pass back to Valgrind in your tools initialization code to tell it what size super blocks you want (although I don’t recall if you can increase this to an arbitrary size). None of this is documented online, and only sparsely documented in the files themselves. You can take a look at the source to the other tools to see how they pass configuration options to Valgrind during initialization. That should at least give you a good idea on which headers to look at to figure out what option you need to pass back to Valgrind.