LLVM Weekly - #7, Feb 17th 2014
==============================
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Welcome to the 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
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<asb at asbradbury.org>, or @llvmweekly or @asbradbury on Twitter.
As always, apologies if I didn't pick up your favourite mailing list
discussion or commits. Do drop me a line if you think I'm missing something
interesting. There haven't been that many external news stories or blog
posts
(that I've found) in the last week, but it's been a particularly busy
week on
the mailing lists with a whole bunch of interesting discussions or RFCs.
## News and articles from around the web
The [ELLCC Embedded Compiler Collection](http://ellcc.org/) now [has
precompiled builds
available](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70441) for
ARM, i386, Mips, PowerPC and x86-64. Handily, these are all statically linked.
The aim of ELLCC is to provide an easy to use multi-target cross compilation
environment for embedded systems, and is of course based on LLVM and clang.
The next Waterloo Region LLVM Social [has been announced for Feb
20th](http://article.gmane.org/gmane.comp.debugging.lldb.devel/3237/).
The next Paris LLVM Social [will take place on Feb
24th](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70330), hosted
by Mozilla.
## On the mailing lists
* Phlip Reames contributed an RFC on [using GetElementPtr as the canonical
form for pointer addressing in LLVM IR before
CodeGenPrepare](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70430).
The motivation behind this proposal will be interesting to many, particularly
the Rust community and other language implementors looking to target LLVM.
Philip and his colleagues (at Azul Systems I believe) are looking to support
precise garbage collection in LLVM, and excitingly a complete upstreamable
implementation should be coming "relatively shortly. (i.e. months, not
weeks,
not years)"
* As a followup to last week's discussions on the GCC mailing list, Renato
Golin has written a [detailed proposal on how LLVM and GCC developers can
co-operate](http://article.gmane.org/gmane.comp.gcc.devel/134419/). He is
welcoming responses [on
Slashdot](http://developers.slashdot.org/comments.pl?sid=4769281&cid=46209457).
Chris Lattner is very much in favour of the effort, though points out that
there are some lists already used for this purpose (such as the 'Itanium ABI
list', which I assume refers to
[cxx-abi-dev](http://sourcerytools.com/cgi-bin/mailman/listinfo/cxx-abi-dev))
and perhaps other similar vendor-neutral mailing lists could be created.
* Tom Roeder has posted an [RFC on the integration of control-flow integrity
techniques in to
LLVM](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70274).
Control-flow integrity (CFI) helps to protect against a variety of attacks by
ensuring control flow at run time matches a path determined ahead of time, and
is described in [this 2005
paper](http://research.microsoft.com/pubs/69217/ccs05-cfi.pdf). Tom has an
implementation that has been tested on Chromium, producing an overhead of ~4%
for the dromaeo benchmark. JF Bastien responded indicating the PNaCl team
would be interested in this work, while other respondents discussed
representing a jump table in LLVM IR as opposed to inline assembly.
* Jordan Rose kicked off a discussion on [the future of the clang static
analyzer](http://article.gmane.org/gmane.comp.compilers.clang.devel/35023).
One issue he raises is how to mark issues as false positives, hopefully in a
more sophisiticated way than disabling checkers. He also mentions long term
plans for adding a full-program analysis mode that would work across
translation units. Ted Kremenek [follows
up](http://article.gmane.org/gmane.comp.compilers.clang.devel/35025) with more
details about treating reported diagnostics as 'issues' which might be
suppressed individually.
* Andrew Bokhanko from Intel initiated a [discussion on adding some sort of
SIMD pragma to
Clang](http://article.gmane.org/gmane.comp.compilers.clang.devel/34995).
Options include upstreaming the implementation of OpenMP 4's `#pragma omp
simd` from [clang-omp](http://clang-omp.github.com).
* Mahdi Hamzeh writes to the mailing list with a [query about if-converting
before register
allocation](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70348).
Andrew Trick points to the EarlyIfConverter, which can be run before register
allocation but only generates conditional moves rather than predicating
instructions. He [suggests an
approach](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70354) for
implementing if-conversion as a target-specific pass.
* Vadim Chugunov [kicks off a discussion on a compiler-rt
roadmap](article.gmane.org/gmane.comp.compilers.llvm.devel/70280). Issues
include a potential rename, whether libunwind should be removed, and ensuring
that it builds for all platforms that LLVM can target.
* Matt Arsenault is [seeking ideas on solving a problem where loop induction
variables are larger than they need to
be](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70390).
* Howard Hinnant is stepping down as code owner of libc++ and it [looks like
Marshall Clow](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70419)
is going to take over. Many LLVM devs also drop by to thank Howard for his
hard work on libc++.
* Ted Kremenek gives a [great summary of the status of ArrayBoundChecker and
ArrayBoundCheckerV2](http://article.gmane.org/gmane.comp.compilers.clang.devel/35049).
## LLVM commits
* AsmPrinter::EmitInlineAsm() will no longer use the EmitRawText() call for
targets with mature MC (MachineCode) support. The MCASMInfo::UseIntegratedAS
field was added. When true, the integrated assembler will parse inline
assembly even when emitting assembly output.
[r201237](http://llvm-reviews.chandlerc.com/rL201333).
* The RTDyld API learned to precompute the amount of memory required for all
sections in a module, and reserveAllocationSpace to reserve the given amount
of space. [r201259](http://llvm-reviews.chandlerc.com/rL201259).
* The LTO API gained `lto_module_create_from_memory_with_path`, which is
mainly useful for emitting warning messages from the linker.
[r201114](http://llvm-reviews.chandlerc.com/rL201114).
* ScalarEvolution will now analyze the trip count of loops where there is a
switch guarding the exit.
[r201159](http://llvm-reviews.chandlerc.com/rL201159).
* The notes on Phabricator now include a suggestion to provide a link to the
Differential revision (the code review) on commits that have been reviewed in
this way. Please do this!
[r201160](http://llvm-reviews.chandlerc.com/rL201160).
* The AArch64 backend now recognises Cortex-A53 and Cortex-A57.
[r201305](http://llvm-reviews.chandlerc.com/rL201305).
* The ARM integrated assembler gained partial support for the
`.arch_extension` directive.
[r201471](http://llvm-reviews.chandlerc.com/rL201471).
## Clang commits
* There have been yet more updates to Clang's Microsoft ABI compatibility.
Clang now understands Microsoft's `pointers_to_members` pragma.
[r201105](http://llvm-reviews.chandlerc.com/rL201105) as well as the
`vtordisp` pragma [r201274](http://llvm-reviews.chandlerc.com/rL201274). In
other MS ABI developments, the `-vm{b,g,s,m,v}` are now supported too.
[r201175](http://llvm-reviews.chandlerc.com/rL201175).
* The command line option `-fstack-protector-strong` was added.
[r201120](http://llvm-reviews.chandlerc.com/rL2011120). See also
[r200601](http://llvm-reviews.chandlerc.com/rL200601) for info on the
sspstrong function attribute.
* Frontend support for the OpenMP `if` clause was committed.
[r201297](http://llvm-reviews.chandlerc.com/rL201297).
* You can use the `--rtlib=compiler-rt` flag to attempt to link against
compiler-rt rather than libgcc.
[r201307](http://llvm-reviews.chandlerc.com/rL201307).
* The `-Wignored-pragmas` diagnostic was added, that will warn in any case
where a pragma would have a side effect but is ignored.
[r201102](http://llvm-reviews.chandlerc.com/rL201102).
* The name of the checker producing each diagnostic message is now available
through `getCheckName()`. This can be used to enable/disable a specific
checker. [r201186](http://llvm-reviews.chandlerc.com/rL201186).
* Clang now understands the `-fbuild-session-timestamp=` and
`-fmodules-validate-once-per-build-session` flags which allows you to make it
verify source files for a module only once during a build.
[r201224](http://llvm-reviews.chandlerc.com/rL201224),
[r201225](http://llvm-reviews.chandlerc.com/rL201225).
## Other project commits
* The sanitizer projects in compiler-rt gained the beginnings for a deadlock
detector. [r201302](http://llvm-reviews.chandlerc.com/rL201302),
[r201407](http://llvm-reviews.chandlerc.com/rL201407)..
* The original compiler-rt functions (i.e. those that act as a libgcc
replacement) now live in the lib/builtins directory.
[r201393](http://llvm-reviews.chandlerc.com/rL201393).
* In lldb, the user can now specify trap-handler-names, a list of functions
which should be treated as trap handlers.
[r201386](http://llvm-reviews.chandlerc.com/rL201386).