[LLVMdev] LLVM Weekly - #19, May 12th 2014

[Alex Bradbury]

LLVM Weekly - #19, May 12th 2014
================================

If you prefer, you can read a HTML version of this email at
<http://llvmweekly.org/issue/19>

Welcome to the ninteenth issue of LLVM Weekly, a weekly newsletter (published
every Monday) covering developments in LLVM, Clang, and related projects.
LLVM Weekly is brought to you by [Alex Bradbury](http://asbradbury.org).
Subscribe to future issues at <http://llvmweekly.org> and pass it on to anyone
else you think may be interested. Please send any tips or feedback to
<asb at asbradbury.org>, or @llvmweekly or @asbradbury on Twitter.

I'm flying out to San Francisco tomorrow and will be there for the Bay Area
Maker Faire at the weekend with some other Raspberry Pi Foundation people. If
you're around, be sure to say hi.

## News and articles from around the web

LLVM 3.4.1 [has been
released](http://blog.llvm.org/2014/05/llvm-341-release.html). This is a
bug-fix release so offers API and ABI compatibility with LLVM 3.4. Thanks to
everyone who contributed to the release by suggesting or backporting patches,
and for testing.

John Regehr has [shared some early results and
discussion](http://blog.regehr.org/archives/1146) on using
[Souper](https://github.com/google/souper/) (a new superoptimizer for LLVM IR)
in combination with Csmith and C-reduce in order to find missed optimisations
and then produce minimal test cases. This has already resulted in a
[new performance bug being
filed](http://llvm.org/bugs/show_bug.cgi?id=19711) with
I'm sure many more to come.

[Crange](https://github.com/crange/crange), a tool to index and
cross-reference C/C++ source code built on top of Clang has [been
released](http://article.gmane.org/gmane.comp.compilers.clang.devel/36677). It
aims to offer a more complete database than e.g. ctags, though the running
time on a large codebase like the Linux kernel is currently very high.

[llgo](https://github.com/go-llvm/llgo), the LLVM-based compiler for Go is
[now
self-hosting](https://groups.google.com/forum/#!topic/llgo-dev/8uBfmIkGM88/discussion).

Last week I asked for benchmarks of the new [JavascriptCore Fourth Tier LLVM
JIT](https://trac.webkit.org/wiki/FTLJIT). Arewefastyet from Mozilla now
includes such results. FTLJIT [does particularly well on asm.js
examples](http://arewefastyet.com/#machine=12&view=breakdown&suite=asmjs-apps).


## On the mailing lists

* The merge of AArch64 in to ARM64 has [made impressive
progress](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72832).
It's likely the final switchover will take place this week, with AArch64 being
deleted and ARM64 renamed to AArch64. Ana Pazos [shared a list of bugs
currently waiting to be
squashed](http://article.gmane.org/gmane.comp.compilers.llvm.cvs/187546).

* Andy Lutomirski [requests that the llvm integer arithmetic with overflow
intrinsics be exposed as C
intrinsics](http://article.gmane.org/gmane.comp.compilers.clang.devel/36685).
It was pointed out that they [actually are
exposed](http://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins)
though Google is doing a poor job of indexing this fact, and in response a
[bug has been filed with
GCC](http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61129) to expose
similar
intrinsics.

* Suminda Dharmasena [kicked off a conversation about reducing or removing
platform specific LLVM
instructions](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72850).
The thread morphed into an interesting discussion about [the issues arising
from gaps in LLVM's coverage for atomic memory
operations](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72866).

* Chandler Carruth [asks for input on the problem of safely suspending JITting
threads through the C LLVM
API](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72847).


## LLVM commits

* A new algorithm has been implemented for tail call marking. A build of clang
now ends up with 470k cals in the IR marked as tail vs 375k before. The total
tail call to loop conversions remains the same though.
[r208017](http://reviews.llvm.org/rL208017).

* `llvm::function_ref` has been introduced and described in the LLVM
programmers manual. It is a type-erased reference to a callable object.
[r208025](http://reviews.llvm.org/rL208025),
[r208067](http://reviews.llvm.org/rL208067).

* Initial support for named register intrinsics (as previously discussed [on
the mailing
list](http://article.gmane.org/gmane.comp.compilers.llvm.devel/71567) has
landed. Right now, only the stack pointer is supported. Other non-allocatable
registers could be supported with not too much difficulty, allocatable
registers are much harder. [r208104](http://reviews.llvm.org/rL208104).

* The `-disable-cfi` option has been removed. LLVM now requires assemblers to
support cfi (control-flow integrity) directives in order to generate stack
unwinding information. [r207979](http://reviews.llvm.org/rL207979).

* The superword-level parallelism (SLP) pass is now enabled by default for
link time optimisation. [r208013](http://reviews.llvm.org/rL208013).

* The llvm-cov documentation has been expanded
[r208098](http://reviews.llvm.org/rL208098).

* The second and third patch of a series to improve MergeFunctions performance
to `O(n*log(n))` has been merged. [r208173](http://reviews.llvm.org/rL208173),
[r208189](http://reviews.llvm.org/rL208189).

* The standard 'x86-64' CPU used as the default architecture now uses the
Sandy Bridge scheduling model in the hope this provides a reasonable default
over a wide range of modern x86-64 CPUs.
[r208230](http://reviews.llvm.org/rL208230).

* Custom lowering for the `llvm.{u|s}add.with.otherflow.i32` intrinsics as
been added for ARM. [r208435](http://reviews.llvm.org/rL208435).


## Clang commits

* MSVC ABI compatibility has again been improved. Clang now understands that
the 'sret' (a structure return pointer) is passed after 'this' for MSVC.
[r208458](http://reviews.llvm.org/rL208458).

* Initial codegen from OpenMP's `#pragma omp parallel` has landed.
[r208077](http://reviews.llvm.org/rL208077).

* Field references to struct names and C++11 aliases are now supported from
inline asm. [r208053](http://reviews.llvm.org/rL208053).

* Parsing and semantic analysis has been implemented for the OpenMP
`proc_bind` clause. [r208060](http://reviews.llvm.org/rL208060).

* clang-format gained initial support for JavaScript regex literals (yes,
clang-format can reformat your JavaScript!).
[r208281](http://reviews.llvm.org/rL208281).


## Other project commits

* libcxxabi gained support for ARM zero-cost exception handling.
[r208466](http://reviews.llvm.org/rL208466).

* In libcxx, std::vector gained Address Sanitizer support.
[r208319](http://reviews.llvm.org/rL208319).

* The test suite from [OpenUH](http://web.cs.uh.edu/~openuh/) has been added
to the openmp repository. [208472](http://reviews.llvm.org/rL208472).