Commit Graph

99 Commits

Author SHA1 Message Date
Mehdi Amini e470927187 Fix auto-upgrade of TBAA tags in Bitcode Reader
If TBAA is on an intrinsic and it gets upgraded, it'll delete the call
instruction that we collected in a vector. Even if we were to use
WeakVH, it'll drop the TBAA and we'll hit the assert on the upgrade
path.

r263673 gave a shot to make sure the TBAA upgrade happens before
intrinsics upgrade, but failed to account for all cases.

Instead of collecting instructions in a vector, this patch makes it
just upgrade the TBAA on the fly, because metadata are always
already loaded at this point.

Differential Revision: https://reviews.llvm.org/D24533

llvm-svn: 281549
2016-09-14 22:29:59 +00:00
Mehdi Amini ecdab09baf [LTO] Move tests from test/tools to test/LTO, as they're testing the API functionalities (NFC)
llvm-svn: 281539
2016-09-14 21:07:32 +00:00
Mehdi Amini b2f46d1dc7 [LTO] Fix commons handling
Previously the prevailing information was not honored, and commons
symbols could override a strong definition. This patch fixes it and
propose the following semantic for commons: the client should mark
as prevailing the commons that it expects the LTO implementation to
merge (i.e. take the maximum size and alignment).
It implies that commons are allowed to have multiple prevailing
definitions.

Differential Revision: https://reviews.llvm.org/D24545

llvm-svn: 281538
2016-09-14 21:05:04 +00:00
Mehdi Amini eccffada33 [LTO] Change addSaveTemps API: do not add dot to the supplied prefix path
Summary:
It does not play well with directories (end up with a bunch of hidden
files).
Also, do not strip the 0 suffix for the first task, especially since
0 can be used by ThinLTO as well now.

Reviewers: tejohnson

Subscribers: mehdi_amini, pcc, llvm-commits

Differential Revision: https://reviews.llvm.org/D23612

llvm-svn: 279014
2016-08-18 00:12:33 +00:00
Teresa Johnson 9ba95f99f3 Restore "Resolution-based LTO API."
This restores commit r278330, with fixes for a few bot failures:
- Fix a late change I had made to the save temps output file that I
  missed due to existing files sitting on my disk
- Fix a bunch of Windows bot failures with "ambiguous call to overloaded
  function" due to confusion between llvm::make_unique vs
  std::make_unique (preface the new make_unique calls with "llvm::")
- Attempt to fix a modules bot failure by adding a missing include
  to LTO/Config.h.

Original change:

Resolution-based LTO API.

Summary:
This introduces a resolution-based LTO API. The main advantage of this API over
existing APIs is that it allows the linker to supply a resolution for each
symbol in each object, rather than the combined object as a whole. This will
become increasingly important for use cases such as ThinLTO which require us
to process symbol resolutions in a more complicated way than just adjusting
linkage.

Patch by Peter Collingbourne.

Reviewers: rafael, tejohnson, mehdi_amini

Subscribers: lhames, tejohnson, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D20268

llvm-svn: 278338
2016-08-11 14:58:12 +00:00
Teresa Johnson cbf684e6c6 Revert "Resolution-based LTO API."
This reverts commit r278330.

I made a change to the save temps output that is causing issues with the
bots. Didn't realize this because I had older output files sitting on
disk in my test output directory.

llvm-svn: 278331
2016-08-11 13:03:56 +00:00
Teresa Johnson f99573b3ee Resolution-based LTO API.
Summary:
This introduces a resolution-based LTO API. The main advantage of this API over
existing APIs is that it allows the linker to supply a resolution for each
symbol in each object, rather than the combined object as a whole. This will
become increasingly important for use cases such as ThinLTO which require us
to process symbol resolutions in a more complicated way than just adjusting
linkage.

Patch by Peter Collingbourne.

Reviewers: rafael, tejohnson, mehdi_amini

Subscribers: lhames, tejohnson, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D20268

Address review comments

llvm-svn: 278330
2016-08-11 12:56:40 +00:00
Mehdi Amini e75aa6f674 Add a libLTO API to query a memory buffer and check if it contains ObjC categories
The linker supports a feature to force load an object from a static
archive if it defines an Objective-C category.
This API supports this feature by looking at every section in the
module to find if a category is defined in the module.

llvm-svn: 275125
2016-07-11 23:10:18 +00:00
Nicolai Haehnle 84c9f9919a Add writeonly IR attribute
Summary:
This complements the earlier addition of IntrWriteMem and IntrWriteArgMem
LLVM intrinsic properties, see D18291.

Also start using the attribute for memset, memcpy, and memmove intrinsics,
and remove their special-casing in BasicAliasAnalysis.

Reviewers: reames, joker.eph

Subscribers: joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D18714

llvm-svn: 274485
2016-07-04 08:01:29 +00:00
Artur Pilipenko 6c7a8abf5c Remangle intrinsics names when types are renamed
This is a resubmittion of previously reverted rL273568.

This is a fix for the problem mentioned in "LTO and intrinsics mangling" llvm-dev mail thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-April/098387.html

Reviewers: mehdi_amini, reames

Differential Revision: http://reviews.llvm.org/D19373

llvm-svn: 273686
2016-06-24 15:10:29 +00:00
Hans Wennborg a63b50afb8 Revert r273568 "Remangle intrinsics names when types are renamed"
It broke 2008-07-15-Bswap.ll and 2009-09-01-PostRAProlog.ll

llvm-svn: 273574
2016-06-23 16:13:23 +00:00
Artur Pilipenko f0c9f81379 Remangle intrinsics names when types are renamed
This is a fix for the problem mentioned in "LTO and intrinsics mangling" llvm-dev mail thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-April/098387.html

Reviewers: mehdi_amini, reames

Differential Revision: http://reviews.llvm.org/D19373

llvm-svn: 273568
2016-06-23 15:25:09 +00:00
Peter Collingbourne 96efdd6107 IR: Introduce local_unnamed_addr attribute.
If a local_unnamed_addr attribute is attached to a global, the address
is known to be insignificant within the module. It is distinct from the
existing unnamed_addr attribute in that it only describes a local property
of the module rather than a global property of the symbol.

This attribute is intended to be used by the code generator and LTO to allow
the linker to decide whether the global needs to be in the symbol table. It is
possible to exclude a global from the symbol table if three things are true:
- This attribute is present on every instance of the global (which means that
  the normal rule that the global must have a unique address can be broken without
  being observable by the program by performing comparisons against the global's
  address)
- The global has linkonce_odr linkage (which means that each linkage unit must have
  its own copy of the global if it requires one, and the copy in each linkage unit
  must be the same)
- It is a constant or a function (which means that the program cannot observe that
  the unique-address rule has been broken by writing to the global)

Although this attribute could in principle be computed from the module
contents, LTO clients (i.e. linkers) will normally need to be able to compute
this property as part of symbol resolution, and it would be inefficient to
materialize every module just to compute it.

See:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160509/356401.html
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160516/356738.html
for earlier discussion.

Part of the fix for PR27553.

Differential Revision: http://reviews.llvm.org/D20348

llvm-svn: 272709
2016-06-14 21:01:22 +00:00
Adrian Prantl fe7a382453 Allow the LTO code generator to strip invalid debug info from the input.
This patch introduces a new option -lto-strip-invalid-debug-info, which
drops malformed debug info from the input.

The problem I'm trying to solve with this sequence of patches is that
historically we've done a really bad job at verifying debug info. We want
to be able to make the verifier stricter without having to worry about
breaking bitcode compatibility with existing producers. For example, we
don't necessarily want IR produced by an older version of clang to be
rejected by an LTO link just because of malformed debug info, and rather
provide an option to strip it. Note that merely outdated (but well-formed)
debug info would continue to be auto-upgraded in this scenario.

rdar://problem/25818489
http://reviews.llvm.org/D19987

This reapplies 268936 with a test case fix for Linux (-exported-symbol foo)

llvm-svn: 268965
2016-05-09 19:57:15 +00:00
Adrian Prantl 6d80100c6a Revert "Allow the LTO code generator to strip invalid debug info from the input."
This reverts commit 268936 while investigating buildbot breakage.

llvm-svn: 268940
2016-05-09 17:43:30 +00:00
Adrian Prantl 4a9292b127 Allow the LTO code generator to strip invalid debug info from the input.
This patch introduces a new option -lto-strip-invalid-debug-info, which
drops malformed debug info from the input.

The problem I'm trying to solve with this sequence of patches is that
historically we've done a really bad job at verifying debug info. We want
to be able to make the verifier stricter without having to worry about
breaking bitcode compatibility with existing producers. For example, we
don't necessarily want IR produced by an older version of clang to be
rejected by an LTO link just because of malformed debug info, and rather
provide an option to strip it. Note that merely outdated (but well-formed)
debug info would continue to be auto-upgraded in this scenario.

rdar://problem/25818489
http://reviews.llvm.org/D19987

llvm-svn: 268936
2016-05-09 17:37:33 +00:00
Peter Collingbourne edf8432480 LTO: Don't bother trying to mangle unnamed globals, as they can't be preserved with MustPreserveSymbols.
Summary: Should fix sanitizer-windows bot.

Reviewers: joker.eph

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19635

llvm-svn: 267820
2016-04-27 23:48:11 +00:00
Mehdi Amini c7b950171d Revert "Support "preserving" the summary information when using setModule() API in LTOCodeGenerator"
This reverts commit r267665.
ASAN shows that there is a use of undefined value.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267668
2016-04-27 05:11:44 +00:00
Mehdi Amini 360ed847bc Support "preserving" the summary information when using setModule() API in LTOCodeGenerator
Another attempt at r267655...

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267665
2016-04-27 04:24:10 +00:00
Mehdi Amini a1b8b6cd56 Revert "Support "preserving" the summary information when using setModule() API in LTOCodeGenerator"
This reverts commit r267657, r267656, and r267655.
The test does not pass on multiple bots, I'm unsure why yet but let's unbreak them.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267664
2016-04-27 03:34:28 +00:00
Mehdi Amini 4960919723 Fix the test from r267656: Support "preserving" the summary information when using setModule() API in LTOCodeGenerator
From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267657
2016-04-27 01:49:11 +00:00
Mehdi Amini 1f65039f06 Add a test for r267655: Support "preserving" the summary information when using setModule() API in LTOCodeGenerator
From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267656
2016-04-27 01:47:46 +00:00
Mehdi Amini 24d3414f06 Refactor the InternalizePass into a helper class, and expose it through a public free function (NFC)
There is really no reason to require to instanciate a pass manager to
internalize.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266167
2016-04-13 05:25:08 +00:00
Mehdi Amini f59f2bb1b5 Refactor the Internalize stage of libLTO in a separate file (NFC)
This is intended to be shared by the ThinLTOCodeGenerator.

Note that there is a change in the way the verifier is run, previously
it was ran as a Pass on the merged module during internalization.
While now the verifier is called explicitely on the merged module
outside of the internalize "pass pipeline".

What remains strange in the API is the fact that `DisableVerify` in
the API does not disable this initial verifier.

Differential Revision: http://reviews.llvm.org/D19000

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266047
2016-04-12 06:34:10 +00:00
Colin LeMahieu efe3732883 Revert r265817
lld tests need to be addressed.

llvm-svn: 265822
2016-04-08 18:15:37 +00:00
Akira Hatanaka e9148dd62f [LoopVectorize] Don't unconditionally print vectorization diagnostics
when compiling with LTO.

r244523 a new class DiagnosticInfoOptimizationRemarkAnalysisAliasing for
optimization analysis remarks related to pointer aliasing without
guarding it in isDiagnosticEnabled in LLVMContext.cpp. This caused the
diagnostic message to be printed unconditionally when compiling with
LTO.

This commit cleans up isDiagnosticEnabled and makes sure all the
vectorization optimization remarks are guarded.

rdar://problem/25382153

llvm-svn: 265084
2016-04-01 00:34:39 +00:00
Steven Wu f2fe0141ca Rename embedded bitcode section in MachO
Summary:
Rename the section embeds bitcode from ".llvmbc,.llvmbc" to "__LLVM,__bitcode".
The new name matches MachO section naming convention.

Reviewers: rafael, pcc

Subscribers: davide, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D17388

llvm-svn: 262245
2016-02-29 19:40:10 +00:00
Tobias Edler von Koch 8ecaf69291 [LTO] Restore original linkage of externals prior to splitting
Summary:
This is a companion patch for http://reviews.llvm.org/D16124.

Internalized symbols increase the size of strongly-connected components in
SCC-based module splitting and thus reduce the amount of parallelism. This
patch records the original linkage of non-local symbols prior to
internalization and then restores it just before splitting/CodeGen. This is
also useful for cases where the linker requires symbols to remain external, for
instance, so they can be placed according to linker script rules.

It's currently under its own flag (-restore-globals) but should eventually
share a common flag with D16124.

Reviewers: joker.eph, pcc

Subscribers: slarin, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D16229

llvm-svn: 258100
2016-01-18 23:24:54 +00:00
Rafael Espindola 79db917139 Don't try to check all uses if lazy loading.
This means that LTO_SYMBOL_SCOPE_DEFAULT_CAN_BE_HIDDEN will not be set
in a few cases.

This should have no impact in ld64 since it doesn't use lazy loading
when merging modules and that is when it checks
LTO_SYMBOL_SCOPE_DEFAULT_CAN_BE_HIDDEN.

llvm-svn: 257915
2016-01-15 18:23:46 +00:00
Vaivaswatha Nagaraj fb3f4907c0 Add InaccessibleMemOnly and inaccessibleMemOrArgMemOnly attributes
Summary:
This patch introduces two new function attributes 

InaccessibleMemOnly: This attribute indicates that the function may only access memory that is not accessible by the program/IR being compiled. This is a weaker form of ReadNone.
inaccessibleMemOrArgMemOnly: This attribute indicates that the function may only access memory that is either not accessible by the program/IR being compiled, or is pointed to by its pointer arguments. This is a weaker form of  ArgMemOnly

Test cases have been updated. This revision uses this (d001932f3a) as reference.

Reviewers: jmolloy, hfinkel

Subscribers: reames, joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D15499

llvm-svn: 255778
2015-12-16 16:16:19 +00:00
Tobias Edler von Koch 49c9a6e802 [LTO] Add options to llvm-lto to select output format and dump merged module
This introduces two new options:
- "llvm-lto -save-merged-module -o outfile" dumps the LTO Module to
  outfile.merged.bc prior to CodeGen and after LTO optimizations have been run.
- "llvm-lto -filetype=asm -o outfile" makes llvm-lto emit assembly instead of
  object code in outfile.

Both are intended for use in lit tests.

llvm-svn: 253624
2015-11-20 00:13:05 +00:00
James Molloy 9ad4f22538 [LTO] Add an early run of functionattrs
Because we internalize early, we can potentially mark a bunch of functions as norecurse. Do this before globalopt.

llvm-svn: 253451
2015-11-18 11:24:42 +00:00
Yunzhong Gao ea7b3a2320 Add a libLTO diagnostic handler that supports lto_get_error_message API
This is a follow-up from the previous discussion on the thread:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20151019/307763.html

The LibLTO lto_get_error_message() API reads error messages from a std::string
sLastErrorString. Instead of passing this string around as an argument, this
patch creates a diagnostic handler and then sends this handler to the
constructor of LTOCodeGenerator.

Differential Revision: http://reviews.llvm.org/D14313

llvm-svn: 252791
2015-11-11 19:59:08 +00:00
Yunzhong Gao ef436f068c llvm-lto: trivial spelling changes to distinguish custom diagnostic handler and
default diagnostic handler.

Differential Revision: http://reviews.llvm.org/D14520

llvm-svn: 252633
2015-11-10 18:52:48 +00:00
James Molloy e6f87ca812 Add a new attribute: norecurse
This attribute allows the compiler to assume that the function never recurses into itself, either directly or indirectly (transitively). This can be used among other things to demote global variables to locals.

llvm-svn: 252282
2015-11-06 10:32:53 +00:00
Mehdi Amini d178f4fc89 Make the default triple optional by allowing an empty string
When building LLVM as a (potentially dynamic) library that can be linked against
by multiple compilers, the default triple is not really meaningful.
We allow to explicitely set it to an empty string when configuring LLVM.
In this case, said "target independent" tests in the test suite that are using
the default triple are disabled by matching the newly available feature
"default_triple".

Reviewers: probinson, echristo
Differential Revision: http://reviews.llvm.org/D12660

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 247775
2015-09-16 05:34:32 +00:00
Duncan P. N. Exon Smith cff5feff6f Reapply "LTO: Disable extra verify runs in release builds"
This reverts commit r247730, effectively reapplying r247729.  This time
I have an lld commit ready to follow.

llvm-svn: 247735
2015-09-15 23:05:59 +00:00
Duncan P. N. Exon Smith 7de73e56a4 Revert "LTO: Disable extra verify runs in release builds"
This temporarily reverts commit r247729, as it caused lld build
failures.  I'll recommit once I have an lld patch ready-to-go.

llvm-svn: 247730
2015-09-15 22:47:38 +00:00
Duncan P. N. Exon Smith 236787838c LTO: Disable extra verify runs in release builds
The verifier currently runs three times in LTO: (1) after parsing, (2)
at the beginning of the optimization pipeline, and (3) at the end of it.

The first run is important, since we're not sure where the bitcode comes
from and it's nice to validate it, but in release builds the extra runs
aren't appropriate.

This commit:
  - Allows these runs to be disabled in LTOCodeGenerator.
  - Adds command-line options to llvm-lto.
  - Adds command-line options to libLTO.dylib, and disables the verifier
    by default in release builds (based on NDEBUG).

This shaves about 3.5% off the runtime of ld64 when linking
verify-uselistorder with -flto -g.

rdar://22509081

llvm-svn: 247729
2015-09-15 22:26:11 +00:00
Yunzhong Gao 46261a74db Add a non-exiting diagnostic handler for LTO.
This is in order to give LTO clients a chance to do some clean-up before
terminating the process.

llvm-svn: 247461
2015-09-11 20:01:53 +00:00
Peter Collingbourne 07dcb58f8a Add powerpc64 to parallel.ll unsupported architecture list.
llvm-svn: 246862
2015-09-04 19:45:36 +00:00
Peter Collingbourne c4a6c1f7fd Use UNSUPPORTED instead of XFAIL to disable this test, as it passes on one AArch64 bot.
llvm-svn: 246344
2015-08-28 22:17:29 +00:00
Peter Collingbourne 6cba53ec6c Tweak XFAIL line for mips.
llvm-svn: 246269
2015-08-28 04:07:52 +00:00
Peter Collingbourne 834f937c93 XFAIL parallel.ll test on MIPS and AArch64 until test failures can be investigated.
llvm-svn: 246261
2015-08-28 02:14:15 +00:00
Peter Collingbourne c269ed5115 CodeGen: Introduce splitCodeGen and teach LTOCodeGenerator to use it.
llvm::splitCodeGen is a function that implements the core of parallel LTO
code generation. It uses llvm::SplitModule to split the module into linkable
partitions and spawning one code generation thread per partition. The function
produces multiple object files which can be linked in the usual way.

This has been threaded through to LTOCodeGenerator (and llvm-lto for testing
purposes). Separate patches will add parallel LTO support to the gold plugin
and lld.

Differential Revision: http://reviews.llvm.org/D12260

llvm-svn: 246236
2015-08-27 23:37:36 +00:00
David Majnemer 7fddeccb8b Move the personality function from LandingPadInst to Function
The personality routine currently lives in the LandingPadInst.

This isn't desirable because:
- All LandingPadInsts in the same function must have the same
  personality routine.  This means that each LandingPadInst beyond the
  first has an operand which produces no additional information.

- There is ongoing work to introduce EH IR constructs other than
  LandingPadInst.  Moving the personality routine off of any one
  particular Instruction and onto the parent function seems a lot better
  than have N different places a personality function can sneak onto an
  exceptional function.

Differential Revision: http://reviews.llvm.org/D10429

llvm-svn: 239940
2015-06-17 20:52:32 +00:00
David Blaikie b5865f2868 Revert changes to LTO test case since llvm-lto can't handle textual IR inputs
llvm-svn: 235738
2015-04-24 18:13:27 +00:00
David Blaikie f8aca46512 Skip extra LLVM IR assemble/disassemble steps in some tests
llvm-svn: 235736
2015-04-24 18:06:09 +00:00
David Blaikie 23af64846f [opaque pointer type] Add textual IR support for explicit type parameter to the call instruction
See r230786 and r230794 for similar changes to gep and load
respectively.

Call is a bit different because it often doesn't have a single explicit
type - usually the type is deduced from the arguments, and just the
return type is explicit. In those cases there's no need to change the
IR.

When that's not the case, the IR usually contains the pointer type of
the first operand - but since typed pointers are going away, that
representation is insufficient so I'm just stripping the "pointerness"
of the explicit type away.

This does make the IR a bit weird - it /sort of/ reads like the type of
the first operand: "call void () %x(" but %x is actually of type "void
()*" and will eventually be just of type "ptr". But this seems not too
bad and I don't think it would benefit from repeating the type
("void (), void () * %x(" and then eventually "void (), ptr %x(") as has
been done with gep and load.

This also has a side benefit: since the explicit type is no longer a
pointer, there's no ambiguity between an explicit type and a function
that returns a function pointer. Previously this case needed an explicit
type (eg: a function returning a void() function was written as
"call void () () * @x(" rather than "call void () * @x(" because of the
ambiguity between a function returning a pointer to a void() function
and a function returning void).

No ambiguity means even function pointer return types can just be
written alone, without writing the whole function's type.

This leaves /only/ the varargs case where the explicit type is required.

Given the special type syntax in call instructions, the regex-fu used
for migration was a bit more involved in its own unique way (as every
one of these is) so here it is. Use it in conjunction with the apply.sh
script and associated find/xargs commands I've provided in rr230786 to
migrate your out of tree tests. Do let me know if any of this doesn't
cover your cases & we can iterate on a more general script/regexes to
help others with out of tree tests.

About 9 test cases couldn't be automatically migrated - half of those
were functions returning function pointers, where I just had to manually
delete the function argument types now that we didn't need an explicit
function type there. The other half were typedefs of function types used
in calls - just had to manually drop the * from those.

import fileinput
import sys
import re

pat = re.compile(r'((?:=|:|^|\s)call\s(?:[^@]*?))(\s*$|\s*(?:(?:\[\[[a-zA-Z0-9_]+\]\]|[@%](?:(")?[\\\?@a-zA-Z0-9_.]*?(?(3)"|)|{{.*}}))(?:\(|$)|undef|inttoptr|bitcast|null|asm).*$)')
addrspace_end = re.compile(r"addrspace\(\d+\)\s*\*$")
func_end = re.compile("(?:void.*|\)\s*)\*$")

def conv(match, line):
  if not match or re.search(addrspace_end, match.group(1)) or not re.search(func_end, match.group(1)):
    return line
  return line[:match.start()] + match.group(1)[:match.group(1).rfind('*')].rstrip() + match.group(2) + line[match.end():]

for line in sys.stdin:
  sys.stdout.write(conv(re.search(pat, line), line))

llvm-svn: 235145
2015-04-16 23:24:18 +00:00
Peter Collingbourne 070843d60b libLTO, llvm-lto, gold: Introduce flag for controlling optimization level.
This change also introduces a link-time optimization level of 1. This
optimization level runs only the globaldce pass as well as cleanup passes for
passes that run at -O0, specifically simplifycfg which cleans up lowerbitsets.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150316/266951.html

llvm-svn: 232769
2015-03-19 22:01:00 +00:00