[LLVMdev] LLVM Weekly - #8, Feb 24th 2014

[Alex Bradbury]

LLVM Weekly - #8, Feb 24th 2014
===============================

If you prefer, you can read a HTML version of this email at
<http://llvmweekly.org/issue/8>

Welcome to the eighth 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'll be at the [Raspberry Jamboree](http://www.raspberrypi.org/archives/6074)
at the end of the week, so if you're going as well be sure to say hi.

## News and articles from around the web

* As a followup to the proposal that Philip Reames posted last week, where he
described plans for contributing precise garbage collection support to LLVM,
he has [written a blog post about why the llvm.gcroot intrinsic is
insufficient for this
purpose](http://www.philipreames.com/Blog/2014/02/21/why-not-use-gcroot/). A
followup post is promised describing the approach they've decided to take. The
article is is written so as to be accessible to those who may not be familiar
with GC implementation. He also raises some interesting issues with gcroot,
even when using it for a non-relocating collector.

* Some may remember the [Dagger project](http://dagger.repzret.org/) for
decompilation of programs to LLVM IR from EuroLLVM 2013
([slides](http://llvm.org/devmtg/2013-04/bougacha-slides.pdf)). The promised
source code release didn't happen, but the developers have [posted an
update](http://dagger.repzret.org/update-feb-2014/) detailing what they've
been up to. There have been a lot of design changes, and some of the work has
been submitted upstream as patches to LLVM MC. "At this point we don't really
have a schedule; whenever we feel a patch is ready to go, we submit it to the
community. The goal being, once we're done, our work becomes a full part of
LLVM, where we and all contributors can continue to advance it!"

* Tamás Szelei has written up a [useful guide to implementing a code generator
with libclang and Python](http://szelei.me/code-generator/).

* The Emscripten project is [getting ready to use its 'fastcomp' LLVM backend
by
default](https://groups.google.com/forum/#!topic/emscripten-discuss/mZMkX3uBG4s).
Previously they had a series of passes written in Javascript to convert from
LLVM IR to Javascript, but this is now implemented as a C++ LLVM backend. See
[their wiki](https://github.com/kripken/emscripten/wiki/LLVM-Backend) for more
info.

* Agner has [updated his popular optimisation
manuals](http://agner.org/optimize/blog/read.php?i=311#311) to include test
results for AMD Steamroller processors, as well as adding some more AVX-512
information.

* The [DWARF Debugging Information Format Committee](http://dwarfstd.org) are
welcoming comments, suggestions or proposals for changes to DWARF [until March
31st](http://article.gmane.org/gmane.comp.compilers.clang.devel/35175/).
Although DWARF Version 5 is 'nearing completion', it seems that no drafts have
been published so you'll have to base your comments on DWARF 4. Do drop me a
note if you know otherwise.

* There have been [several
updates](http://sylvestre.ledru.info/blog/2014/02/22/some-updates-on-llvm-org)
to [http://llvm.org/apt/](http://llvm.org/apt/). The upcoming Ubuntu 14.04 is
now a supported distribution, and additionally both the stable and development
versions of LLVM/Clang are built and can be installed side-by-side.

## On the mailing lists

* Renato Golin [reminds
us](http://article.gmane.org/gmane.comp.compilers.clang.devel/35229) that
although work is underway to update all buildbots to support C++11, the switch
to use `-std=c++11` has not yet been flipped, so you'll have to hold off on
using C++11 features in LLVM/Clang patches for just a little longer.

* Saleem Abulrasool [points to an issue running recent Clang on the Linux
kernel](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70584)
related to the integrated assembler. As regular readers will know, the
behaviour was recently changed so that for backends which have an integrated
assembler, any inline assembly will be validated by it during compilation,
even when compiling with `-S` (i.e. outputting assembly). The problem is that
the Linux kernel is purposely including invalid assembly in some cases when
outputting assembler files. Early responses are in favour of keeping current
behaviour, people who are doing weird and wacky things can just use the
`-no-integrated-as` switch.

* In response to the earlier discussion about unwind behaviour in LLVM/Clang,
Logan Chien has [posted a detailed description of the problems he
sees](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70461).

* Kevin Qin writes [asking about adding register allocation
constraints](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70511).
Often the mailing list threads which get highlighted in LLVM Weekly are about
particularly hairy problems that don't currently have a good solution. I'm
happy to saw this problem has a simple solution though, as Tim Northover and
Quentin Colombet point out the `@earlyclobber` constraint can be used to
ensure the output register allocated is different to the input registers.

* While working on changes to CodeGenPrepare, Quentin Combet noted his patch
would introduce a dependency from libLLVMScalarOpts (where CodeGenPrepare
currently lives) to libLLVMCodeGen. He [writes to the
list](http://article.gmane.org/gmane.comp.compilers.llvm.devel/70528) asking
for views on how to solve this problem. The forming consensus seems to be that
it should just be moved to CodeGen. Potentially, any IR pass that depends
directly on TargetLowering should be moved also. The move to CodeGenPrepare to
lib/CodeGen has now [been
committed](http://llvm-reviews.chandlerc.com/rL201912).

* Per Viberg is [soliciting
comments](article.gmane.org/gmane.comp.compilers.clang.devel/35185) on his
design draft for improving the detection of uninitialized arguments.


## LLVM commits

* The llvm-profdata tool was introduced. This tool will merge profile data
generated by PGO instrumentation in Clang, though may later pick up more
functionality. [r201535](http://llvm-reviews.chandlerc.com/rL201535).

* In a long overdue cleanup, various member variables were renamed from TD to
DL to match the renaming of TargetData to DataLayout.
[r201581](http://llvm-reviews.chandlerc.com/rL201581),
[r201827](http://llvm-reviews.chandlerc.com/rL201827),
[r201833](http://llvm-reviews.chandlerc.com/rL201833).Additionally, DebugLoc
variables which were named DL have now been renamed to DbgLoc so as not to be
confused with DataLayout.
[r201606](http://llvm-reviews.chandlerc.com/rL201606).

* MCAsmParser now supports required parameters in macros, increasing GNU
assembler compatibility.
[r201630](http://llvm-reviews.chandlerc.com/rL201630).

* A new TargetLowering hook, `isVectorShiftByScalarCheap` was added to
indicate whether it's significantly cheaper to shift a vector by a scalar
rather than by another vector with different values for different lanes. This
is used by the new `OptimizeShuffleVectorInst` in CodeGenPrepare which tries
to sink shufflevector instructions down to the basic block they're used so
CodeGen can determine if the right hand of a shift is really a scalar.
[r201655](http://llvm-reviews.chandlerc.com/rL201655).

* Private linkage is now properly supported for MachO.
[r201700](http://llvm-reviews.chandlerc.com/rL201700).

* getNameWithPrefix and getSymbol were moved from TargetLowering to
TargetMachine, which removes the dependency from Target to CodeGen.
[r201711](http://llvm-reviews.chandlerc.com/rL201711).

## Clang commits

* The PGO instrumentation will now compute counts in a separate AST traversal.
The reasons for and advantages of this change are documented in detail in the
commit message. [r201528](http://llvm-reviews.chandlerc.com/rL201528).

* Some initial work was committed on documenting available attributes in
Clang. Attribute authors are encouraged to submit missing documentation (the
method of documentation is described in the addition to the
InternalManual.rst). [r201515](http://llvm-reviews.chandlerc.com/rL201515).

* The IdenticalExprChecker has been extended to check the two branches of an
if as well as logical and bitwise expressions. For those not familiar, this
checker tries to warn about the unintended use of identical expressions.
[r201701](http://llvm-reviews.chandlerc.com/rL201701),
[r201702](http://llvm-reviews.chandlerc.com/rL201702).

* CGRecordLayoutBuilder has been completely rewritten to remove cruft,
simplify the implementation, and to work in one pass.
[r201907](http://llvm-reviews.chandlerc.com/rL201907).

* The CastSizeChecker was taught how to correctly deal with flexible array
members. [r201583](http://llvm-reviews.chandlerc.com/rL201583).

* A number of thread-safety attributes have been renamed (with their old name
silently deprecated).e.g. `lockable` is now `capability`,
`exclusive_locks_required` is now `requires_capability`.
[r201585](http://llvm-reviews.chandlerc.com/rL201585). Additionally, the
documentation was updated and greatly expanded.
[r201598](http://llvm-reviews.chandlerc.com/rL201598).

* Initial virtual file system support [discussed previously on the mailing
list](http://article.gmane.org/gmane.comp.compilers.clang.devel/34941) has
landed. [r201618](http://llvm-reviews.chandlerc.com/rL201618),
[r201635](http://llvm-reviews.chandlerc.com/rL201635).

* The vcvtX intrinsics were added for v8 ARM as opposed to only being
recognised when targeting AArch64.
[r201661](http://llvm-reviews.chandlerc.com/rL201661).

* The hard-float ARM EABI (often known as gnueabihf) is now supported for
FreeBSD. [r201662](http://llvm-reviews.chandlerc.com/rL201662).

* Clang will now provide `max_align_t` in C11 and C++11 modes. Note the
complaint in the commit message though that `max_align_t` as defined is not
'good' or 'useful'. [r201729](http://llvm-reviews.chandlerc.com/rL201729).

* Again, there were a number of commits related to increasing compatibility
with the MS ABI. None of them immediately leaped out at me as worth
highlighting individually, so I recommend you have a flick through last weeks
commits if you're particularly interested.

## Other project commits

* Rudimentary support for a standalone compiler-rt build system was added,
which will allow the compiler-rt libraries to be built seperately from
LLVM/Clang. [r201647](http://llvm-reviews.chandlerc.com/rL201647),
[r201656](http://llvm-reviews.chandlerc.com/rL201656).

* Assembly functions for AddressSanitizer on x86/amd64 were added.
[r201650](http://llvm-reviews.chandlerc.com/rL201650).

* LLDB gained a hardware watchpoint implementation for FreeBSD
[r201706](http://llvm-reviews.chandlerc.com/rL201706).

* Polly gained support for polyhedral dead code elimination.
[r201817](http://llvm-reviews.chandlerc.com/rL201817).

* A patch was added to lldb to provide initial support for the Hexagon DSP.
[r201665](http://llvm-reviews.chandlerc.com/rL201655).