[LLVMdev] LLVM Weekly - #37, Sep 15th 2014

[Alex Bradbury]

LLVM Weekly - #37, Sep 15th 2014
================================

If you prefer, you can read a HTML version of this email at
<http://llvmweekly.org/issue/37>.

Welcome to the thirty-seventh 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.

This week's issue comes to you from sunny Tenerife. Yes, my dedication to
weekly LLVM updates is so great that I'm writing it on holiday. Enjoy! I'll
also note that I'm at PyCon UK next week where I'll be
[presenting](http://pyconuk.net/Schedule) on the results of a project we had
some interns working on over the summer creating a programming game for the
Raspberry Pi.


## News and articles from around the web

Not only does Pyston have a shiny new blog, they've also [released version
0.2](http://blog.pyston.org/2014/09/11/9/). Pyston is an implementation of
Python using LLVM, led by Dropbox. This release supports a range of language
features that weren't supported in 0.1, including support for the native C
API. The plan is to focus on performance during the development cycle for 0.3.

Sylvestre Ledru has posted a [report of progress in building Debian with
Clang](http://sylvestre.ledru.info/blog/2014/09/11/rebuild-of-debian-using-clang-3-5)
following the completion of this year's Google Summer of Code projects. Now
with Clang 3.5.0 1261 packages fail to build with Clang. Sylvestre describes
how they're attacking the problem from both sides, by submitting patches to
upstream projects as well as to Clang where appropriate (e.g. to ignore some
unsupported optimisation flags rather than erroring out).


## On the mailing lists

* Philip Reames has started a discussion on [adding optimisation hints for
'constant' loads](http://article.gmane.org/gmane.comp.compilers.llvm.devel/76702).
A
common case is where a field is initialised exactly once and then is never
modified. If this invariant could be expressed, it could improve alias
analysis as the AA pass would never consider that field to MayAlias with
something else (Philip reports that the obvious approach of using type-based
alias analysis isn't quite enough).

* Hal Finkel has posted an [RFC on attaching attributes to
values](http://article.gmane.org/gmane.comp.compilers.llvm.devel/76647).
Currently, attributes such as noalias and nonnull can be attached to function
parameters, but in cases such as C++11 lambdas these can be packed up into a
structure and the attributes are lost. Some followup discussion focused on
whether these could be represented as metadata. The problem there of course is
that metadata is intended to be droppable (i.e. is semantically unimportant).
I very much like the
[suggestion](http://article.gmane.org/gmane.comp.compilers.llvm.devel/76700)
from Philip Reames that the test suite should run with a pass that forcibly
drops metadata to verify it truly is safe to drop.

* Robin Morisset has posted a [proposal on implementing a fence elimination
algorithm](http://article.gmane.org/gmane.comp.compilers.llvm.devel/76800).
The proposed algorithm is based on partial redundancy elimination. He's
looking for feedback on the suggested implementation approach.

* There's been a little bit of discussion on the topic of [rekindling work on
VMKit](http://article.gmane.org/gmane.comp.compilers.llvm.devel/76679).


## LLVM commits

* The start of the llvm.assume infrastructure has been committed, as well as
an AlignmentFromAssumptions pass. See [the original
RFC](http://article.gmane.org/gmane.comp.compilers.llvm.devel/74941) for a
refresher on the llvm.assume intrinsic.
[r217342](http://reviews.llvm.org/rL217342),
[r217344](http://reviews.llvm.org/rL217344).

* LLVM's sample profile reader has been refactored into lib/ProfileData.
[r217437](http://reviews.llvm.org/rL217437).

* The AMD 16H Jaguar microarchitecture now has a scheduling model.
[r217457](http://reviews.llvm.org/rL217457).

* The 'bigobj' COFF variant can now be read.
[r217496](http://reviews.llvm.org/rL217496).


## Clang commits

* The `__builtin_assume` and `__builtin_assume_aligned` intrinsics have been
added. [r217349](http://reviews.llvm.org/rL217349).

* The thread safety TIL (Typed Intermediate Language) has seen a major update.
[r217556](http://reviews.llvm.org/rL217556).


## Other project commits

* LLD gained support for AArch64 Mach-O.
[r217469](http://reviews.llvm.org/rL217469).