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Porting to 64-bit Linux
An anonymous reader writes "As 64-bit architectures continue to gain popularity it is becoming more and more important to make sure that your software is ready for the shift. IBMDeveloperworks takes a look at a few of the most common pitfalls when making sure your applications are 64-bit ready. From the article: 'Major hardware vendors have recently expanded their 64-bit offerings because of the performance, value, and scalability that 64-bit platforms can provide. The constraints of 32-bit systems, particularly the 4GB virtual memory ceiling, have spurred companies to consider migrating to 64-bit platforms. Knowing how to port applications to comply with a 64-bit architecture can help you write portable and efficient code.'"
- sizeof(int) == sizeof(void*), or
- sizeof(int) == 4
If your codebase only makes the first OR the second assumption, you can tweak the compiler to like you by defines. If you also assume that sizeof(void*) == 4, you have bigger problems. Note that you can do this in rather innocent ways, like dumping a complete structure on disk, knowing that pointer values will be invalid, but just assuming that the structure will be the same size if you read it back later.In addition, and this is hellish, a 32-bit MOV is (generally) atomic on x86. You can rely on the high-order word and the low-order word staying together, without race conditions. The memory access semantics are different on x64 and many other platforms. This is not related to 64-bitness per se, you could see if you ported to multi-threaded 32-bit PPC as well, but it will still surface if you do the transition to AMD64/EM64T/x64. Or rather, it will result in an additional one-in-a-million crash in your source, that you'll blame on bad memory chips in the user's machine.
Generally architecture changes, compiler version changes, break code on large projects. Over a million lines of code, any tiny little difference in the platform that the original developers didn't think to account for will come up *somewhere*. A good example of this is if you are dumping data structures to disk or network and write a size_t variable. Suddenly, you can no longer communicate between 32 bit and 64 bit versions of your software.
As a general rule, "just a recompile" *never happens* for any architecture and compiler change on a project above a certain size. Compiler writers break compatibility with some little ol' thing they don't think anyone is using, but which everyone is actually using in *every* version, fail to implement uncommon or difficult language features, add non standard features that other compilers don't support. Then application developers do things like not swapping to network byte order and using architecture dependent data types (size_t as in the example). Between different unices, header file contents will change.
The fixes are often not that hard (usually trivial) to do between say versions of the same compiler, or endian switches... but they are still there and annoy the hell out off people trying to compile old open source software on a new platform, like say macosx was a few years ago and x86 64 is now. There's always growing pains.
If you don't make assumptions about pointer sizes in your code, always use size_t in the appropriate places, etc, it is generally just a quick recompile for x64. I find a lot of open source code (I'm sure this isn't exclusive to open source, but, well, I can't see closed source!) spits out hundreds to thousands of warnings about assigning the return of strlen() to an int and other similar and usually harmless things, but most of the time it Just Works (tm).
The only area I've ran into things being significantly harder is writing clean lock-free algorithms due to the lack of a CMPXCHG16B instruction in the original spec - only EMT64 and very recent AMD64 models have it. There are a couple ways to hack around this limitation but they aren't very pretty.
Ignoring 64Bit helps a lot to write portable code. For 99.999% of all Apps out there 64Bit is irrelevant, anyways.
I suspect what you're saying is that there is no particular need for 64-bit in most apps, which I agree with. But the point here is that the program should work correctly, which means code that makes assumptions like pointers and ints being the same size needs to be fixed. The point is that amd64 is making 64-bit platforms relevant to more users, not that everyone thinks most apps will be gee-whiz faster as a result.
As a side note, some programs may realise minor performance gains on amd64 from having more general purpose registers available. This is, of course, technically nothing to do with it being 64-bit but does mean that there is a potential benefit even if you never need more than 4GB of addressable memory.
The article doesn't appear to mention this, but there is a C99 standard header stdint.h, which defines fixed width types. I haven't seen any OSS project use it, for some reason, but it has all the types you need for portable development; int32_t, uint64_t, constant wrappers like UINT64_C, and, of course, limit constants for all of the fixed-size types. Using these is much better than all those size-based #ifdef'ed typedefs I see people use all over their code.
I did a lot of 64-bit cleaning up for the PHP project, and I can tell you that there are more subtle issues that may arise when porting from 32-bit to 64-bit.
One example:
on a 32-bit Intel machine, a double is precise enough to distinguish LONG_MAX (the highest representable long) from LONG_MAX+1 (a number that doesn't fit in a long anymore). So for instance, to determine whether a long multiplication has overflowed, you could repeat the same multiplication using doubles and compare the result to (double)LONG_MAX.
In contrast, on a 64-bit platform LONG_MAX and LONG_MAX+1 are mapped to the same double representation, so there's no way to do the comparison anymore.
As this example involves static casts, it is something the compiler will usually not warn you about.
Another thing to be careful about is passing pointers to variadic functions (eg. sscanf), because usually the compiler doesn't know the expected types, as they are buried in the format string, not in the function prototype.