[LLVMdev] Upstreaming x32 ABI support

[Rafael Espíndola]

Just a datapoint on why x32 is interesting.

I compiled clang on a ubuntu vm in 32 bits, 64 bits and x32.  As a
simple benchmark I then compiled gcc.c from
http://people.csail.mit.edu/smcc/projects/single-file-programs. The
results were (best of 3 runs, user time)

32:
0m19.529s

64:
0m17.044s

x32:
0m15.768s


On 2 July 2014 10:59, Pavel Chupin <pavel.v.chupin at gmail.com> wrote:
> Hi all,
>
> We (Intel) would like to revive this thread on X32 ABI support
> upstreaming. X32 ABI homepage is here:
> https://sites.google.com/site/x32abi/
>
> Currently x32 support is available in kernel/glibc/gcc/binutils and
> one can install for example Ubuntu 14 with extra x32 lib packages
> (apt-get install libc6-dev-x32) to evaluate x32-target code generated
> by GCC (gcc supports x32 on Ubuntu 14 out of box).
>
> We are planning to contribute enabling patches in LLVM/Clang trees and
> going to provide continuous support for this ABI in the future. Most
> of the changes obviously will be submitted to backend.
>
> Our current work is based on:
>
> * what’s currently done in trunk (some changes required for x32 are
> already there)
> * Michael Liao work done ~2 years ago and available here:
> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120604/058450.html
>
> What’s currently in trunk:
>
> * Triplet x86_64-*-linux-gnux32 (command line support, EnvironmentType
> enum to differentiate x32 from x86_64 by getTriple().getEnvironment()
> == llvm::Triple::GNUX32)
> https://github.com/llvm-mirror/llvm/commit/9dd2a3b1f2c253e20262535bb89b1ab6cc680ece
> * ILP32/LP64 differentiation for TargetMachine
> https://github.com/llvm-mirror/llvm/commit/a5597f0eaf1f93c6d0bc641a0cc54ecffb33955a
> * LEA tuning:
> https://github.com/llvm-mirror/llvm/commit/693c37aa86506be657dfaf8835845b0998531c3c
> * Bunch of tests (grep for gnux32):
> test/CodeGen/X86/imul64-lea.ll
> test/CodeGen/X86/stack-update-frame-opcode.ll
> test/CodeGen/X86/stack-update-frame-opcode.ll
> test/CodeGen/X86/x86-64-sret-return.ll
> test/CodeGen/X86/rip-rel-lea.ll
> test/CodeGen/X86/x86-64-sret-return-2.ll
> test/CodeGen/X86/x86-64-ptr-arg-simple.ll
> test/MC/X86/gnux32-dwarf-gen.s
>
> What’s been done by Michael
> (http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20120604/058450.html)
> but not released in trunk:
>
> * Clang Driver support (linker/libraries paths lookup, x32 specific
> flags, etc.). This change is rebased, modified and submitted here:
> http://reviews.llvm.org/D4180 . Welcome to review.
> * ELF support for x32. This change is rebased, modified and submitted
> here: http://reviews.llvm.org/D4181  . Welcome to review.
> * Use EBP/ESP as frame/stack pointer. This patch will be rebased
> modified and submitted soon.
> * Other cases which require introduction X32 specific instruction
> definitions such as JMPX32r/CALLX32r/CALLX32m.
> * X32 JIT support
> * I hope Michael correct me if I missed anything important
>
> What’s still missing or experiencing issues (just something from the top):
>
> * New tests required and several existing tests fail when extending to x32.
> * Pointer and struct return;
> * Cast int to pointer;
> * Function indirect calls;
> * TLS
> * Etc.
>
> We don’t plan to introduce new backend from scratch. Major part of the
> changes are incremental to X86 backend and extend cases such as
>
> (64bits) ? do-64 : do-32
> with
> (64bits) ? ((LP64) ? do-64 : do-x32) : do-32
>
> or some shorter form if x32 mimic x86 or x86_64. X32 ABI is very close
> to x86-64 ABI.
>
> Our plan is to start from basic stuff, focus on more tests adding and
> on stability issues, then continue with performance tuning. So the
> first 2 patches I submitted enable “Hello, world!” sample on Ubuntu
> 14. Patches are here:
>
>                 http://reviews.llvm.org/D4180
>                 http://reviews.llvm.org/D4181
>
> Next steps are to rebase and split Michael patch, cover with tests and
> submit by chunks convenient for review. Then move to other discovered
> issues.
>
> Any suggestions and patches review are welcome!
>
> --
> Pavel Chupin
> Intel Corporation
>
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