[LLVMdev] LLVM Weekly - #16, Apr 21st 2014

[Alex Bradbury]

LLVM Weekly - #16, Apr 21st 2014
================================

If you prefer, you can read a HTML version of this email at
<http://llvmweekly.org/issue/16>

Welcome to the 16th 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.

Apologies that last week's LLVM Weekly went out twice via email. Mailgun have
the useful ability to schedule an email for the future, but when this is done
incorrectly have no ability to cancel it via the API. Possibly there is no way
for them to cancel it either, I have no way to know as my support ticket on
the issue was never answered.

Seeing as it's Easter, does anybody know why GCC has a [GNU breaking
out of an egg as a
logo](http://commons.wikimedia.org/wiki/File:GNU_Compiler_Collection_logo.svg)?

## News and articles from around the web

The new backend to [Emscripten](https://github.com/kripken/emscripten) which
is implemented as an LLVM backend has [now been merged to Emscripten's master
branch](https://groups.google.com/forum/#!topic/emscripten-discuss/FACubOKw2TI).
This should result in a noticeable speedup in compile times.

Phoronix have published a [small set of benchmarks comparing GCC
4.9RC1 and Clang 3.5
HEAD](http://www.phoronix.com/scan.php?page=article&item=gcc49_compiler_llvm35&num=1).

Diego Novillo has [announced
AutoFDO](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72199), a
tool which will convert profile data generated with [Linux
Perf](https://perf.wiki.kernel.org) to a format compatible with LLVM's
sample-based profiler.

The Polly project have [minutes from another phone
call](https://docs.google.com/document/d/14d3ehkH2MsvBdqsEOSYjysH0Ztyzb75Lp843hnxh2IA/edit),
this time focusing on delinearization.

## On the mailing lists

* Jingyue Wu [shares an initial patch for common sub-expression elimination
(CSE) for
getelementptr](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72366).
He and his colleagues had found that with the NVPTX backend, loop unrolling
often lead to groups of getelementptr instructions which added to register
pressure, but that these could often to optimised with some simple CSE.
Feedback so far is positive on including something like this is a
target-independent pass, though there's interest in seeing whether it helps on
any other backends with open source benchmarks.

* Austin Seipp has [revived the issue of named register
variables](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72374). He
adds perspective on the the utility of this feature for the Glasgow Haskell
Compiler (GHC). They are currently suffering a large performance loss on OS X
with clang due to the inability to dedicate a register for fast access to
thread-local storage. Renato Golin [followed
up](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72378) with how
he thinks it is best to move forwards on the issue, while Mark Seaborn [shared
a neat trick the NaCl team
use](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72375) where
they inline `pthread_getspecific` after verifying it contains the code they
expect.

* Kostya Serebryany [shares his concerns about contention between threads on
access to profile
counters](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72247). For
a multi-threaded VP9 encoder, he's seeing 7x overhead when running with
`-fprofile-instr-generate` vs 15% when single-threaded. There's too much for
me to summarise, but there's a lot of discussion on potential solutions which
the interested reader should certainly take a look at.

* Dan Gohman answers a question about LLVM IR access to struct fields with a
[concise summary of how alias analysis information is currently used in
LLVM](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72331).

* Richard Carback is [seeking feedback on his TableGen DAGISel
documentation](http://article.gmane.org/gmane.comp.compilers.llvm.devel/72201).
His
[notes](https://github.com/draperlaboratory/fracture/wiki/How-TableGen%27s-DAGISel-Backend-Works)
seem like a great start at documenting what has been up till now one of the
more mysterious parts of LLVM. Particularly those who have experience with
this code are encouraged to check the document for accuracy.



## LLVM commits

* LLVM's internal BumpPtrAllocator has been switched to using a vector of
pointers to slabs rather than a single linked list and the underlying
allocator is now a template parameter.
[r206147](http://llvm-reviews.chandlerc.com/rL206147),
[r206149](http://llvm-reviews.chandlerc.com/rL206149). The allocator can now
also pass the size to the deallocation function, which improves performance
with some libraries (e.g. tcmalloc).
[r206265](http://llvm-reviews.chandlerc.com/rL20265).

* Support for building persistent strongly connected components has been added
to the LazyCallGraph. There are detailed comments on the reasoning of this
approach and some details on implementation in the commit message.
[r206581](http://llvm-reviews.chandlerc.com/rL206581).

* Constant hoisting has been enabled on PowerPC.
[r206141](http://llvm-reviews.chandlerc.com/rL206141).

* PseudoSourceValue is no longer a subclass of Value.
[r206255](http://llvm-reviews.chandlerc.com/rL206255).

* A DebugInfoVerifier has been implemented.
[r206300](http://llvm-reviews.chandlerc.com/rL206300).

* MIPS gained initial support for the IEEE 754-2008 NaN encoding.
[r206396](http://llvm-reviews.chandlerc.com/rL202812).

* OnDiskHashTable has been moved from Clang to LLVM.
[r206438](http://llvm-reviews.chandlerc.com/rL206438).

* ARM's IR-based atomics pass has been moved from Target to CodeGen, which
allows it to be used by ARM64.
[r206485](http://llvm-reviews.chandlerc.com/rL206485),
[r206490](http://llvm-reviews.chandlerc.com/rL206490).

* Module verification is now off by default in release builds for the JIT, but
this can be overridden. [r206561](http://llvm-reviews.chandlerc.com/rL206561).

* The Cortex-A53 machine model description has been ported from AArch64 to
ARM64. [r206652](http://llvm-reviews.chandlerc.com/rL206652).

## Clang commits

* There is now a new hash algorithm for calculating the function hash for
instruction profiling, rewritten to help ensure the hash changes when control
flow does. [r206397](http://reviews.llvm.org/rL206397).

* The thread safety analysis SSA pass has been rewritten.
[r206338](http://reviews.llvm.org/rL206338).

* Support for big endian ARM64 was added to Targets.cpp.
[r206390](http://reviews.llvm.org/rL206390). It is also now possible to
disable NEON and crypto support for ARM64.
[r206394](http://reviews.llvm.org/rL206394).

## Other project commits

* LLD now supports `--defsym=<symbol>=<symbol>`, as supported by GNU LD.
[r206417](http://reviews.llvm.org/rL206417).