Monday Feb 09, 2015

New Studio blogger

One of my colleagues, Raj Prakash, has started a blog. If you saw the Studio videos from last year, you'll recall that Raj and I discussed memory error dectection, and appropriately enough his first post is on how you can use Studio's memory error detection capabilities on server-type applications.

Thursday Nov 13, 2014

Software in Silicon Cloud

I missed this press release about Software in Silicon Cloud. It's the announcement for a service where you can try out a SPARC M7 processor. There's an accompanying website which has the sign up plus some more information about the service.

What's particularly exciting is that it talks a bit more about Application Data Integrity (ADI). Larry Ellison called this "the most important piece of engineering we’ve done in a long, long time.".

Incorrect handling of pointers is a large contributor to bugs in software. ADI tackles this by making the hardware check that the pointer being used is valid for the region of memory it is pointing to. If it's not valid the hardware flags it as an error. Since it's done by hardware, there's minimal performance impact - it's at hardware speed, so developers can check their application in realtime.

There's a nice demo of how ADI protects against exploits like HeartBleed.

Tuesday Apr 29, 2014

Unsigned integers considered annoying

Let's talk about unsigned integers. These can be tricky because they wrap-around from big to small. Signed integers wrap-around from positive to negative. Let's look at an example. Suppose I want to do something for all iterations of a loop except for the last OFFSET of them. I could write something like:

  if (i < length - OFFSET) {}

If I assume OFFSET is 8 then for length 10, I'll do something for the first 2 iterations. The problem occurs when the length is less than OFFSET. If length is 2, then I'd expect not to do anything for any of the iterations. For a signed integer 2 minus 8 is -6 which is less than i, so I don't do anything. For an unsigned integer 2 minus 8 is 0xFFFFFFFA which is still greater than i. Hence we'll continue to do whatever it is we shouldn't be doing in this instance.

So the obvious fix for this is that for unsigned integers we do:

  if (i + OFFSET < length) {}

This works over the range that we might expect it to work. Of course we have a problem with signed integers if length happens to be close to INT_MAX, at this point adding OFFSET to a large value of i may cause it to overflow and become a large negative number - which will continue to be less than length.

With unsigned ints we encounter this same problem at UINT_MAX where adding OFFSET to i could generate a small value, which is less than the boundary.

So in these cases we might want to write:

  if (i < length - OFFSET) {}

Oh....

So basically to cover all the situations we might want to write something like:

  if ( (length > OFFSET) && (i < length - OFFSET) ) {}

If this looks rather complex, then it's important to realise that we're handling a range check - and a range has upper and lower bounds. For signed integers zero - OFFSET is representable, so we can write:

  if (i < length - OFFSET) {}

without worrying about wrap-around. However for unsigned integers we need to define both the left and right ends of the range. Hence the more complex expression.

Thursday Apr 03, 2014

Discovering the Code Analyzer

We're doing something different with the Studio 12.4 Beta programme. We're also putting together some material about the compiler and features: videos, whitepapers, etc.

One of the first videos is now officially available. You might have seen the preproduction "leak" if you happen to follow Studio on either facebook or twitter.

This first video is an interview with Raj Prakash, the project lead for the Code Analyzer.

The Code Analyzer is our suite for checking the correctness of code. Something that you would run before you deliver an application to customers.

Wednesday May 11, 2011

Best practices for linking libraries (part 1)

A while ago I was looking into some application start up problems. The problem turned out to be an issue relating to the order in which the libraries were loaded and initialised. It seemed to me that this was a rather tricky area, and it would be very helpful to document the best practices around it. I thought this would be a quick couple of pages, but it turned out to be a rather high page count, and I ended up working on the document with Steve Clamage (with Rod Evans helping out).

The first part of the document is available. This section covers basic linker good practices. Using -L and -R rather than LD_LIBRARY_PATH, generating relocatable code etc. The key take aways are:

  • Use -L to specify the path to where the libraries can be found at compile time.
  • Use -R to specify the location of the libraries at run time.
  • Use the token $ORIGIN to specify a relative path for the libraries' location. This avoids the need to have a hard-coded location where the libraries can be found.

Thursday Mar 26, 2009

When to add a membar (an example)

I was recently having a discussion on one of the OpenSolaris lists on the topic of when to use the volatile keyword, and when it was necessary to use membars.

So volatile is a clue to the compiler to load the variable from memory and immediately store it back to memory. What it does not do is to tell the hardware anything. So the application can perform the store, but that store may not be immediately visible to the rest of the system. Most of the time this is not a problem - so long as the store is visible to the processor on which the thread is executing it's fine. Variability of when the store is visible to other processors may also be fine. There is one clear situation where the ordering of store operations could be a problem - and that's unlocking mutexes.

The problem here is best illustrated by the following scenario. I lock some data structure, then store new values into it, then unlock the structure. Immediately another thread comes along and uses the values in that structure. Not an uncommon situation. Unlocking a mutex is often just a case of storing a value (of zero) into the mutex structure. And here's the potential problem. In some weaker ordering architectures there is no guarantee that other processors see the stores in the same order that they are performed. So if you have Store A followed by Store B it may be possible for other processors to observe the change in the value of B before they see the change in the value of A. In the case of mutex unlock, the store of B would be the action that unlocked the mutex, enabling other threads to access the variable A... and there could be problems if they see the old value of A.

The solution to this is to put a membar in before the store that unlocks the mutex. You can see this happening in the OpenSolaris code:

     41 /\*
     42  \* lock_clear(lp)
     43  \*	- clear lock.
     44  \*/
     45 	ENTRY(_lock_clear)
     46 	membar	#LoadStore|#StoreStore
     47 	retl
     48 	  clrb	[%o0]
     49 	SET_SIZE(_lock_clear)

The membar ensures that all the pending stores are visible to other processors before the store that releases the lock becomes visible to them.

Wednesday Sep 12, 2007

Article on discover (tool for detecting memory access errors)

My (brief) article on the Sun Memory Error Discovery Tool (aka discover) has gone live on the developer portal. The tool is designed to find memory access errors. It instruments the code and checks each memory access to see if it is valid. It can detect things like access to uninitialised memory, or accesses past the end of arrays. These can be really hard to find by inspecting the source, and even from core dumps - the symptoms of the problem often occur long after the memory access error has occurred. The one constraint on the tool is that it is currently only for single threaded apps.

About

Darryl Gove is a senior engineer in the Solaris Studio team, working on optimising applications and benchmarks for current and future processors. He is also the author of the books:
Multicore Application Programming
Solaris Application Programming
The Developer's Edge
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