There's the mailing list post with the adding of x32 support to LLVM and the C/C++ Clang compiler. "The patch to LLVM adds X32 psABI support and introduce a new 'X32' environment for x86-64 target to tell X32 psABI from LP64 psABI (i.e. the original x86-64 ABI). The patch to clang adds '-mx32' option to generate x32 binaries as well as support calling convention changes due to pointer size reduction."
For those who have forgotten about x32, "This new osABI, X32 psABI, allows programs to run in the 64-bit mode but only using 32-bit pointers. Though, programs will be limited to 4GB virtual address space (not all application requires more than 4GB), X32 psABI helps reduce memory footprint and file size and allow them run faster."
In terms of performance figures for x32 LLVM/Clang, as sent by Michael Liao, "X32 psABI could boost ~10% performance from legacy 32-bit and has roughly the same performance from 64-bit app. It also reduces file size by +5% from the original x86-64 psABI and has roughly the same file size from legacy 32-bit app. Several workloads do have much better performance due to the reduction of memory footprint, e.g. 181.mcf in CINT2000, up to 100+% and 429.mcf in CINT2006, up to 30+%)."
It was with the Linux 3.4 kernel that x32 ABI support landed while the necessary changes to binutils, glibc, and GCC have already landed / merged to master. Ubuntu is one of the distributions planning for a Linux x32 flavor.
More information on Linux x32 is available from the Google Site.