X32 ABI

The x32 ABI is an application binary interface (ABI) and one of the interfaces of the Linux kernel. The x32 ABI provides 32-bit integers, long and pointers (ILP32) on Intel and AMD 64-bit hardware. The ABI allows programs to take advantage of the benefits of x86-64 instruction set (larger number of CPU registers, better floating-point performance, faster position-independent code, shared libraries, function parameters passed via registers, faster syscall instruction) while using 32-bit pointers and thus avoiding the overhead of 64-bit pointers.^[1][2][3]

Though the x32 ABI limits the program to a virtual address space of 4 GiB, it also decreases the memory footprint of the program by making pointers smaller. This can allow it to run faster by fitting more code and more data into cache.^[1][2][3] The best results during testing were with the 181.mcf SPEC CPU 2000 benchmark, in which the x32 ABI version was 40% faster than the x86-64 version.^[3][4] On average, x32 is 5–8% faster on the SPEC CPU integer benchmarks compared to x86-64. There is no speed advantage over x86-64 in the SPEC CPU floating-point benchmarks.^[5] There are also some application benchmarks that demonstrate the advantages of the x32 ABI.^{[6] [7]}

Running a userspace that consists mostly of programs compiled in ILP32 mode and which also have principal access to 64-bit CPU instructions has not been uncommon, especially in the field of "classic RISC" chips. For example, the Solaris operating system does so for both SPARC and x86-64. On the Linux side, Debian also ships an ILP32 userspace. The underlying reason is the somewhat "more expensive" nature of LP64 code,^[8] just like it has been shown for x86-64. In that regard, the x32 ABI extends the ILP32-on-64bit concept to the x86-64 platform.

Several people had discussed the benefits of an x86-64 ABI with 32-bit pointers in the years since the Athlon 64's release in 2003, notably Donald Knuth in 2008.^[9] There was little publicly visible progress towards implementing such a mode until August 27, 2011, when Hans Peter Anvin announced to the Linux kernel mailing list that he and H. J. Lu had been working on the x32 ABI.^[10]

That same day, Linus Torvalds replied with a concern that the use of 32-bit time values in the x32 ABI could cause problems in the future.^[11][12] This is because the use of 32-bit time values would cause the time values to overflow in the year 2038.^[11][12] Following this request, the developers of the x32 ABI changed the time values to 64-bit.^[13]

A presentation at the Linux Plumbers Conference on September 7, 2011, covered the x32 ABI.^[2]

The x32 ABI was merged into the Linux kernel for the 3.4 release with support being added to the GNU C Library in version 2.16.^[14]

In December 2018 there was discussion as to whether to deprecate the x32 ABI, which has not happened as of April 2023.^[15] Neither it has happened as of May 2024, and even more, it got some new development.^[16]