These linkages were introduced some time ago, but it was never very
clear what exactly their semantics were or what they should be used
for. Some investigation found these uses:
* utf-16 strings in clang.
* non-unnamed_addr strings produced by the sanitizers.
It turns out they were just working around a more fundamental problem.
For some sections a MachO linker needs a symbol in order to split the
section into atoms, and llvm had no idea that was the case. I fixed
that in r201700 and it is now safe to use the private linkage. When
the object ends up in a section that requires symbols, llvm will use a
'l' prefix instead of a 'L' prefix and things just work.
With that, these linkages were already dead, but there was a potential
future user in the objc metadata information. I am still looking at
CGObjcMac.cpp, but at this point I am convinced that linker_private
and linker_private_weak are not what they need.
The objc uses are currently split in
* Regular symbols (no '\01' prefix). LLVM already directly provides
whatever semantics they need.
* Uses of a private name (start with "\01L" or "\01l") and private
linkage. We can drop the "\01L" and "\01l" prefixes as soon as llvm
agrees with clang on L being ok or not for a given section. I have two
patches in code review for this.
* Uses of private name and weak linkage.
The last case is the one that one could think would fit one of these
linkages. That is not the case. The semantics are
* the linker will merge these symbol by *name*.
* the linker will hide them in the final DSO.
Given that the merging is done by name, any of the private (or
internal) linkages would be a bad match. They allow llvm to rename the
symbols, and that is really not what we want. From the llvm point of
view, these objects should really be (linkonce|weak)(_odr)?.
For now, just keeping the "\01l" prefix is probably the best for these
symbols. If we one day want to have a more direct support in llvm,
IMHO what we should add is not a linkage, it is just a hidden_symbol
attribute. It would be applicable to multiple linkages. For example,
on weak it would produce the current behavior we have for objc
metadata. On internal, it would be equivalent to private (and we
should then remove private).
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As an example that was not actually being used, it suffered from a slow bitrot.
The two main issues with it were that it had no cmake support and
included a copy of the autoconf directory. The reality is that
autoconf is not easily composable. The lack of composabilty is why we
have clang options in llvm's configure. Suggesting that users include
a copy of autoconf/ in their projects seems a bad idea.
We are also in the process of switching to cmake, so pushing autoconf
to new project is probably not what we want.
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On ELF and COFF an alias is just another name for a position in the file.
There is no way to refer to a position in another file, so an alias to
undefined is meaningless.
MachO currently doesn't support aliases. The spec has a N_INDR, which when
implemented will have a different set of restrictions. Adding support for
it shouldn't be harder than any other IR extension.
For now, having the IR represent what is actually possible with current
tools makes it easier to fix the design of GlobalAlias.
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The syntax for "cmpxchg" should now look something like:
cmpxchg i32* %addr, i32 42, i32 3 acquire monotonic
where the second ordering argument gives the required semantics in the case
that no exchange takes place. It should be no stronger than the first ordering
constraint and cannot be either "release" or "acq_rel" (since no store will
have taken place).
rdar://problem/15996804
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The official specifications state the name to be ARMNT (as per the Microsoft
Portable Executable and Common Object Format Specification v8.3).
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The grammar for LLVM IR is not well specified in any document but seems
to obey the following rules:
- Attributes which have parenthesized arguments are never preceded by
commas. This form of attribute is the only one which ever has
optional arguments. However, not all of these attributes support
optional arguments: 'thread_local' supports an optional argument but
'addrspace' does not. Interestingly, 'addrspace' is documented as
being a "qualifier". What constitutes a qualifier? I cannot find a
definition.
- Some attributes use a space between the keyword and the value.
Examples of this form are 'align' and 'section'. These are always
preceded by a comma.
- Otherwise, the attribute has no argument. These attributes do not
have a preceding comma.
Sometimes an attribute goes before the instruction, between the
instruction and it's type, or after it's type. 'atomicrmw' has
'volatile' between the instruction and the type while 'call' has 'tail'
preceding the instruction.
With all this in mind, it seems most consistent for 'inalloca' on an
'inalloca' instruction to occur before between the instruction and the
type. Unlike the current formulation, there would be no preceding
comma. The combination 'alloca inalloca' doesn't look particularly
appetizing, perhaps a better spelling of 'inalloca' is down the road.
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The following changes have been applied:
- Removed 'align 4'. We can simplify this away, as it does not provide useful
information in the example.
- Use named instructions instead of '%0'. This is nicer, but more importantly
this makes the IR valid. Before we had two assignments to %0 in a single
example.
- Add a missing branch instruction to make the loop structure clear.
- Move one access into outer.for.body to make it not look that empty.
- The statments that are only in the outer loop body should not reference the
inner loop metadata, but only the outer loop. Only statements in both loops
should reference both surrounding loops.
- Rename the array indexes to make them all independent. Before there were
identical array indexes in the inner and the outer loop. We want to
avoid this special case as it may lead to confusion.
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The distinction between "identified" and "literal" struct types is fully
documented in a later section.
Patch by Philip Reames!
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directly care about the Value class (it is templated so that the key can
be any arbitrary Value subclass), it is in fact concretely tied to the
Value class through the ValueHandle's CallbackVH interface which relies
on the key type being some Value subclass to establish the value handle
chain.
Ironically, the unittest is already in the right library.
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It's easy to copy unintentionally when using 'auto', particularly inside
range-based for loops. Best practise is to use 'const&' unless there's
a good reason not to.
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The current coding standards restrict the use of struct to PODs, but no
one has been following them. This patch updates the standards to
clarify when structs are dangerous and describe common practice in LLVM.
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facilitate the nice formatting of lambdas passed there. Suggested by
Chris during review of my lambda additions, and something I strongly
agree with.
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about a few constructs in C++11 that are worth starting off in
a consistent manner within the codebase.
This will be matched with a change to clang-format's LLVM style which
will switch the options to support C++11 and use these conventions.
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The docs now build cleanly. Yay!
The following warnings were fixed:
/home/sean/pg/llvm/llvm/docs/HowToReleaseLLVM.rst:364: WARNING: Enumerated list ends without a blank line; unexpected unindent.
/home/sean/pg/llvm/llvm/docs/InAlloca.rst:: WARNING: document isn't included in any toctree
/home/sean/pg/llvm/llvm/docs/CodingStandards.rst:85: WARNING: Title underline too short.
Supported C++11 Language and Library Features
-------------------------------------------
/home/sean/pg/llvm/llvm/docs/CodingStandards.rst:85: WARNING: Title underline too short.
Supported C++11 Language and Library Features
-------------------------------------------
/home/sean/pg/llvm/llvm/docs/GettingStarted.rst:185: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/sean/pg/llvm/llvm/docs/GettingStarted.rst:565: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/sean/pg/llvm/llvm/docs/GettingStarted.rst:567: WARNING: Block quote ends without a blank line; unexpected unindent.
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The switch has been thrown. While I'm still watching for any failures or
problems with this, the documentation can go ahead and move forward.
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bots when using the standard library facilities. The missing pieces here
aren't always in useful discreet chunks.
Fortunately, the missing pieces are few and far between, and we can
emulate most of them in our headers as needed.
Based on feedback from Lang and Dave.
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A lot of this is writing down common knowledge and things often
communicated on mailing lists and in discussions. It could live in the
Programmer's Manual alternatively, but that felt slightly less
well-fitting.
It also includes (and was motivated by) the section on the relevant
language standards for LLVM and the specific features that will be
enabled with the switch to C++11.
With this, all of the documentation for the C++11 switch is, I think, in
place. I plan to flip the switch RSN. =]
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standards.
It claims the document intentionally doesn't give fixed standards for
brace placement or spacing, and then the document goes on to do
precisely that in several places. Instead, try to highlight that even
these rules are simply *guidance* which may be trumped by some other
circumstance or the local conventions of code.
I'm not trying to change the thrust of this part of the document, and if
folks think this does so, I'm happy to re-wordsmith it. I just don't
want it to be so self-contradicting.
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a more modern host C++ toolchain for Linux distros where folks sometimes
don't have a good option to get one as part of their system.
This is a first cut, so feedback, testing, and suggestions are very,
very welcom. This is one of the last real documentation changes that was
specifically requested prior to switching LLVM and Clang to build in
C++11 mode by default.
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seems unlikely to be added. It also doesn't seem like it should be part
of the build system at all (consider out-of-tree builds).
We should probably add nice, easy tool for this that works both for svn
client trees and git-svn client trees, but it probably won't be spelled
"make update".
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toolchain of LLVM. These are already being enforced by the build system
and have been discussed quite a few times on the lists, but
documentation is important. =]
Also, garbage collect the majority of the information about broken host
GCC toolchains. These aren't really relevant any more as they're all
older than the minimum requirement. I've left a few notes about
compilers one step older than the current requirement as these compilers
are at least conceivable to use, and it's better to preserve this kind
of hard-won institutional knowledge.
The next step will be some specific docs on how to set up a sufficiently
modern host toolchain if your system doesn't come with one. But that'll
be tomorrow. =]
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bits of software and to use a modern GCC version.
The Subversion bit was weird anyways -- it has nothing to do with
compiling LLVM. Also, there are many other ways to get at the trunk
source (git, git-svn, etc).
The TeXinfo thing... I have no idea about. But you can get a working
LLVM w/o it pretty easily. If man pages or something are missing, that
hardly seems like a problem. If folks really want this back, let me
know, but it seems mostly like a distraction.
I'd still like to separate this into:
- Required software to compile.
- Optional software to compile.
- Required software for certain *contributor* activities (like
regenerating configure scripts).
Also we need to mention that there are multiple options for build
systems, and the differences.
Also we should mention Windows.
Also probably other stuff I'm forgetting.
I'm wondering if this whole thing needs to be shot in the head and we
should just start a new, simpler getting started that doesn't have so
many years of accumulated stuff that is no longer relevant.
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