GLIBC 2.16 Brings X32 Support, ISO C11 Compliance, Better Performance

An anonymous reader writes "The GNU C Library version 2.16 was released with many new features over the weekend. The announcement cites support for the Linux x32 ABI, ISO C11 compliance, performance improvements for math functions and some architectures, and more than 230 bug fixes."

Comments

Re:X32

[TheRaven64]

64 bit architectures give you 64-bit registers and a 64-bit address space (since pointers are, traditionally, integers that fit in registers[1]). On x86, there are a number of other advantages to using the 64-bit long mode: guaranteed SSE so you don't need slow x87 code, more registers, PC-relative addressing (useful for position-independent code) and so on. The cost of using these is that now every pointer is bigger. This has knock-on effects in terms of data cache usage.

The X32 ABI allows you to have all of the benefits of the 64-bit mode except for the larger address space. If you're using under 4GB of memory, then it can, in theory give an improvement in memory and cache usage.

There are two down sides. The first is that, in my testing of C, C++ and Objective-C code, I've found that it's very rare on a 64-bit platform for pointers to account for even 10% of the total memory usage, and usually it's a lot less. This is made worse by the fact that the X64 and X32 ABIs are incompatible, so you may need two copies of the same library in memory if you have code using both.

I was quite enthusiastic about the idea of something like X32 five or so years ago when it was very rare for programs to want more than 4GB of address space, but now it's far less clear that there's a real advantage.

[1] Not with the architecture I'm currently working with, and I'm spending a lot of time fixing compiler assumptions that this is always the case.

Re:X32

[kasperd]

If someone could explain to me why x86 architecture is suddenly named x32

It's not. X32 is the name of a programming model, not an architecture. And the X32 programming model cannot be used on the x86 architecture, X32 is a programming model for the AMD64 architecture.

The most important aspect of a programming model for C is that it defines the sizes of basic types like char, short, int, long, and pointers. Many other aspects follow more or less directly from the sizes of those types. X32 is unusual in that it is 64 bit code, but pointers are only 32 bits. When a pointer is in a CPU register the lower 32 bits is the actual value, and the upper 32 bits are always zero.