Note that the isl::union_set(isl_ctx,std::string) constructor will
auto-convert the char* to an std::string. Converting a nullptr to
std::string is undefined in C++11 (sect. 21.4.2.9).
llvm-svn: 298259
Otherwise the isl_id NewId which ensures uniqueness of the
created space is unused. None of the tests currently uses an
nameless tuple, so there is not change in what is tested.
llvm-svn: 298258
When not adding constraints on parameters using -polly-ignore-parameter-bounds,
the context may not necessarily list all parameter dimensions. To support code
generation in this situation, we now always iterate over the actual parameter
list, rather than relying on the context to list all parameter dimensions.
llvm-svn: 298197
After this change, enabling -polly-codegen-add-debug-printing in combination
with -polly-codegen-generate-expressions allows us to instrument the compiled
binaries to not only print the values stored and loaded to a given memory
access, but also to print the accessed location with array name and
per-dimension offset:
MemRef_A[3][2]
Store to 6299784: 5.000000
MemRef_A[3][3]
Load from 6299788: 0.000000
MemRef_A[3][3]
Store to 6299788: 6.000000
This can be very helpful for debugging.
llvm-svn: 298194
In commit r219005 lifetime markers have been introduced to mark the lifetime of
the OpenMP context data structure. However, their use seems incorrect and
recently caused a miscompile in ASC_Sequoia/CrystalMk after r298053 which was
not at all related to r298053. r298053 only caused a change in the loop order,
as this change resulted in a different isl internal representation which caused
the scheduler to derive a different schedule. This change then caused the IR to
change, which apparently created a pattern in which LLVM exploites the lifetime
markers. It seems we are using the OpenMP context outside of the lifetime
markers. Even though CrystalMk could probably be fixed by expanding the scope of
the lifetime markers, it is not clear what happens in case the OpenMP function
call is in a loop which will cause a sequence of starting and ending lifetimes.
As it is unlikely that the lifetime markers give any performance benefit, we
just drop them to remove complexity.
llvm-svn: 298192
The AssumptionCache removal of r289756 has been reverted in
r290086/r290087. A different solution has been implemented in r291671
which keeps the AssumptionCache. We can therefore use it again in Polly.
This reverts r289791.
llvm-svn: 298089
In the previous default ScopInfo applied the profitability heuristic for
scalar accesses (-polly-unprofitable-scalar-accs=true) and the
-polly-prune-unprofitable was disabled by default
(-polly-enable-prune-unprofitable=false) as that pruning was already done.
This changes switches the defaults to -polly-unprofitable-scalar-accs=true
-polly-enable-prune-unprofitable=false such that the scalar access
heuristic check is done by the pass. This allows passes between ScopInfo
and PruneUnprofitable to optimize away scalar accesses.
Without enabling such intermediate passes, there is no change in
behaviour of profitability checks in a PassManagerBuilder built
pass chain, but it allows us to cover this configuration with the
buildbots.
Suggested-by: Tobias Grosser <tobias@grosser.es>
llvm-svn: 298081
ScopInfo's normal profitability heuristic considers SCoPs where all
statements have scalar writes as not profitably optimizable and
invalidate the SCoP in that case. However, -polly-delicm and
-polly-simplify may be able to remove some of the scalar writes such
that the flag -polly-unprofitable-scalar-accs=false allows disabling
that part of the heuristic.
In cases where DeLICM (or other passes after ScopInfo) are not
successful in removing scalar writes, the SCoP is still not profitably
optimizable. The schedule optimizer would again try computing another
schedule, resulting in slower compilation.
The -polly-prune-unprofitable pass applies the profitability heuristic
again before the schedule optimizer Polly can still bail out even with
-polly-unprofitable-scalar-accs=false.
Differential Revision: https://reviews.llvm.org/D31033
llvm-svn: 298080
For experiments it is sometimes helpful to provide parameter bound information
to polly and to not use these parameter bounds for simplification.
Add a new option "-polly-ignore-parameter-bounds" which does precisely this.
llvm-svn: 298077
Dependences::calculateDependences.
This ensures that we handle may-writes correctly when building
dependence information. Also add a test case checking correctness of
may-write information. Not handling it before was an oversight.
Differential Revision: https://reviews.llvm.org/D31075
llvm-svn: 298074
For experiments it is sometimes helpful to not take any inbounds assumptions.
Add a new option "-polly-ignore-inbounds" which does precisely this.
llvm-svn: 298073
In subsequent changes we will make Polly a little bit more lazy in adding
parameter dimensions to different sets. As a result, not all parameters will
always be part of the parameter space. This change ensures that we do not use
the '-1' returned when a parameter dimension cannot be found, but instead
just do not try to eliminate the anyhow non-existing dimension.
llvm-svn: 298054
Since several years, isl can perform most operations on sets with differing
parameter spaces, by expanding the parameter space on demand relying using
named isl ids to distinguish different parameter dimensions.
By not always expanding to full dimensionality the set remain smaller and can
likely be operated on faster. This change by itself did not yet result in
measurable performance benefits, but it is a step into the right direction
needed to ensure that subsequent changes indeed can work with lower-dimensional
sets and these sets do not get blown up by accident when later intersected with
the domain context.
llvm-svn: 298053
Introduce ScopStmt::getSurroundingLoop() to replace getFirstNonBoxedLoopFor.
getSurroundingLoop() returns the precomputed surrounding/first non-boxed
loop. Except in ScopDetection, the list of boxed loops is only used to
get the surrounding loop. getFirstNonBoxedLoopFor also requires LoopInfo
at every use which is not necessarily available everywhere where we may
want to use it.
Differential Revision: https://reviews.llvm.org/D30985
llvm-svn: 297899
The bindings currently need to be generated manually, as they are not yet
part of the official isl distribution. Hence, we keep them across updates
assuming they only need to be updated when new functions or functionality
should be exposed.
llvm-svn: 297710
In ScheduleOptimizer::isTileableBand(), allow the case in which
the band node's child is an isl_schedule_sequence_node and its
grandchildren isl_schedule_leaf_nodes. This case can arise when
two or more statements are fused by the isl scheduler.
The tile_after_fusion.ll test has two statements in separate
loop nests and checks whether they are tiled after being fused
when polly-opt-fusion equals "max".
Reviewers: grosser
Subscribers: gareevroman, pollydev
Tags: #polly
Contributed-by: Theodoros Theodoridis <theodort@student.ethz.ch>
Differential Revision: https://reviews.llvm.org/D30815
llvm-svn: 297587
If a SCoP is most probably sequential, then it's better to run it on a CPU.
Hence, there's no point in running it on a GPU.
Reviewers: grosser
Subscribers: nemanjai
Tags: #polly
Contributed-by: Singapuram Sanjay <singapuram.sanjay@gmail.com>
Differential Revision: https://reviews.llvm.org/D30864
llvm-svn: 297578
Currently the isl::val constructor only takes a signed long as parameter, which
on Windows is only 32 bit large and can consequently not be used to obtain the
same result when loading from the expression '(1ull << 32) - 1)' that we get
when loading this value via isl_val_int_from_ui or when loading the value
on Linux systems with 64 bit long data types. We avoid this issue by performing
the shift and subtractiong within the isl::val.
It would be nice to teach the isl++ bindings to construct isl::val from other
integer types, but the current interface is not sufficient to do so. If
constructors from both signed long and unsigned long are exposed, then integer
literals that are of type 'int' and which must be converted to 'long' to match
the constructor have two ambigious constructors to choose from, which result
in a compiler error. The right solution is likely to additionally expose
constructors from signed and unsigned int, but these are not yet available in
the isl C interface and adding those adhoc to our bindings is something I would
like to avoid for now. We should address this issue with a proper discussion
on the isl side.
llvm-svn: 297522
As most discussions about these bindings have concluded and only the final
patch review on the isl mailing list is missing, we drop the experimental
warning tag to match the patchset we will submit to isl, which is expected to
not change notably any more.
llvm-svn: 297519
Instead of declaring a function as:
inline val plain_get_val_if_fixed(enum dim type, unsigned int pos) const;
we use:
inline isl::val plain_get_val_if_fixed(isl::dim type, unsigned int pos) const;
The first argument caused the following compile time error on windows:
"error C3431: 'dim': a scoped enumeration cannot be redeclared as an
unscoped enumeration"
In some cases it is sufficient to just drop the 'enum' prefix, but for example
for isl::set the 'enum class dim' type collides with the function name
isl::set::dim and can consequently not be referenced. To avoid such kind of
ambiguities in the future we add the isl:: prefix consistently to all types
used.
Reported-by: Michael Kruse <llvm@meinersbur.de>
llvm-svn: 297478
This new pass removes unnecessary accesses and writes. It currently
supports 2 simplifications, but more are planned.
It removes write accesses that write a loaded value back to the location
it was loaded from. It is a typical artifact from DeLICM. Removing it
will get rid of bogus dependencies later in dependency analysis.
It also removes statements without side-effects. ScopInfo already
removes these, but the removal of unnecessary writes can result in
more side-effect free statements.
Differential Revision: https://reviews.llvm.org/D30820
llvm-svn: 297473
This pass is a small and self-contained example of a piece of code that was
written with the isl C interface. The diff of this change nicely shows how the
C++ bindings can improve the readability of the code by avoiding the long C
function names and by avoiding any need for memory management.
As you will see, no calls to isl_*_copy or isl_*_free are needed anymore.
Instead the C++ interface takes care of automatically managing the objects.
This may introduce internally additional copies, but due to the isl reference
counting, such copies are expected to be cheap. For performance critical
operations, we will later exploit move semantics to eliminate unnecessary
copies that have shown to be costly.
Below we give a set of examples that shows the benefit of the C++ interface vs.
the pure C interface.
Check properties
----------------
Before:
if (isl_aff_is_zero(aff) || isl_aff_is_one(aff))
return true;
After:
if (Aff.is_zero() || Aff.is_one())
return true;
Type conversion
---------------
Before:
isl_union_pw_multi_aff *UPMA = isl_union_pw_multi_aff_from_union_map(umap);
After:
isl::union_pw_multi_aff UPMA = UMap;
Type construction
-----------------
Before:
auto *Empty = isl_union_map_empty(space);
After:
auto Empty = isl::union_map::empty(Space);
Operations
----------
Before:
set = isl_union_set_intersect(set, set2);
After:
Set = Set.intersect(Set2);
The use of isl::boolean in return types also adds an increases the robustness
of Polly, as on conversion to true or false, we verify that no isl_bool_error
has been returned and assert in case an error was returned. Before this change
we would have just ignored the error and proceeded with (some) exection path.
Tags: #polly
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D30619
llvm-svn: 297466
For this translation we introduce two functions, valFromAPInt and APIntFromVal,
to convert between isl::val and APInt. For now these are just proxies, but in
the future they will replace the current isl_val* based conversion functions.
The isl unit test cases benefit most from the new isl::boolean (from Michael
Kruse), which can be explicitly casted to bool and which -- as part of this cast
-- emits a check that no error condition has been triggered so far. This allows
us to simplify
EXPECT_EQ(isl_bool_true, isl_val_is_zero(IslZero));
to
EXPECT_TRUE(IslZero.is_zero());
This simplification also becomes very clear in operator==, which changes from
auto IsEqual = isl_set_is_equal(LHS.keep(), RHS.keep());
EXPECT_NE(isl_bool_error, IsEqual);
return IsEqual;
to just
return bool(LHS.is_equal(RHS));
Some background for non-isl users. The isl C interface has an isl_bool type,
which can be either true, false, or error. Hence, whenever a function returns
a value of type isl_bool, an explicit error check should be considered. By
using isl::boolean, we can just cast the isl::boolean to 'bool' or simply use
the isl::boolean in a context where it will automatically be casted to bool
(e.g., in an if-condition). When doing so, the C++ bindings automatically add
code that verifies that the return value is not an error code. If it is, the
program will warn about this and abort. For cases where errors are expected,
isl::boolean provides checks such as boolean::is_true_or_error() or
boolean::is_true_no_error() to explicitly control program behavior in case of
error conditions.
Thanks to the new automatic memory management, we also can avoid many calls to
isl_*_free. For code that had previously been managed by IslPtr<>, many calls
to give/take/copy are eliminated.
Tags: #polly
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D30618
llvm-svn: 297464
Translate the full algorithm to use the new isl C++ bindings
This is a large piece of code that has been written with the Polly IslPtr<>
memory management tool, which only performed memory management, but did not
provide a method interface. As such the code was littered with calls to
give(), copy(), keep(), and take(). The diff of this change should give a
good example how the new method interface simplifies the code by removing the
need for switching between managed types and C functions all the time
and consequently also the need to use the long C function names.
These are a couple of examples comparing the old IslPtr memory management
interface with the complete method interface.
Check properties
----------------
Before:
if (isl_aff_is_zero(Aff.get()) || isl_aff_is_one(Aff.get()))
return true;
After:
if (Aff.is_zero() || Aff.is_one())
return true;
Type conversion
---------------
Before:
isl_union_pw_multi_aff *UPMA =
give(isl_union_pw_multi_aff_from_union_map(UMap.copy());
After:
isl::union_pw_multi_aff UPMA = UMap;
Type construction
-----------------
Before:
auto Empty = give(isl_union_map_empty(Space.copy());
After:
auto Empty = isl::union_map::empty(Space);
Operations
----------
Before:
Set = give(isl_union_set_intersect(Set.copy(), Set2.copy());
After:
Set = Set.intersect(Set2);
Tags: #polly
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D30617
llvm-svn: 297463
The isl C++ binding method interface introduces a thin C++ layer that allows
to call isl methods directly on the memory managed C++ objects. This makes the
relevant methods directly available via code-completion interfaces, allows for
the use of overloading, conversion constructors, and many other nice C++
features that make using isl a lot easier.
The individual features will be highlighted in the subsequent commits.
Tags: #polly
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D30616
llvm-svn: 297462
Over the last couple of months several authors of independent isl C++ bindings
worked together to jointly design an official set of isl C++ bindings which
combines their experience in developing isl C++ bindings. The new bindings have
been designed around a value pointer style interface and remove the need for
explicit pointer managenent and instead use C++ language features to manage isl
objects.
This commit introduces the smart-pointer part of the isl C++ bindings and
replaces the current IslPtr<T> classes, which served the very same purpose, but
had to be manually maintained. Instead, we now rely on automatically generated
classes for each isl object, which provide value_ptr semantics.
An isl object has the following smart pointer interface:
inline set manage(__isl_take isl_set *ptr);
class set {
friend inline set manage(__isl_take isl_set *ptr);
isl_set *ptr = nullptr;
inline explicit set(__isl_take isl_set *ptr);
public:
inline set();
inline set(const set &obj);
inline set &operator=(set obj);
inline ~set();
inline __isl_give isl_set *copy() const &;
inline __isl_give isl_set *copy() && = delete;
inline __isl_keep isl_set *get() const;
inline __isl_give isl_set *release();
inline bool is_null() const;
}
The interface and behavior of the new value pointer style classes is inspired
by http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3339.pdf, which
proposes a std::value_ptr, a smart pointer that applies value semantics to its
pointee.
We currently only provide a limited set of public constructors and instead
require provide a global overloaded type constructor method "isl::obj
isl::manage(isl_obj *)", which allows to convert an isl_set* to an isl::set by
calling 'S = isl::manage(s)'. This pattern models the make_unique() constructor
for unique pointers.
The next two functions isl::obj::get() and isl::obj::release() are taken
directly from the std::value_ptr proposal:
S.get() extracts the raw pointer of the object managed by S.
S.release() extracts the raw pointer of the object managed by S and sets the
object in S to null.
We additionally add std::obj::copy(). S.copy() returns a raw pointer refering
to a copy of S, which is a shortcut for "isl::obj(oldobj).release()", a
functionality commonly needed when interacting directly with the isl C
interface where all methods marked with __isl_take require consumable raw
pointers.
S.is_null() checks if S manages a pointer or if the managed object is currently
null. We add this function to provide a more explicit way to check if the
pointer is empty compared to a direct conversion to bool.
This commit also introduces a couple of polly-specific extensions that cover
features currently not handled by the official isl C++ bindings draft, but
which have been provided by IslPtr<T> and are consequently added to avoid code
churn. These extensions include:
- operator bool() : Conversion from objects to bool
- construction from nullptr_t
- get_ctx() method
- take/keep/give methods, which match the currently used naming
convention of IslPtr<T> in Polly. They just forward to
(release/get/manage).
- raw_ostream printers
We expect that these extensions are over time either removed or upstreamed to
the official isl bindings.
We also export a couple of classes that have not yet been exported in isl (e.g.,
isl::space)
As part of the code review, the following two questions were asked:
- Why do we not use a standard smart pointer?
std::value_ptr was a proposal that has not been accepted. It is consequently
not available in the standard library. Even if it would be available, we want
to expand this interface with a complete method interface that is conveniently
available from each managed pointer. The most direct way to achieve this is to
generate a specialiced value style pointer class for each isl object type and
add any additional methods to this class. The relevant changes follow in
subsequent commits.
- Why do we not use templates or macros to avoid code duplication?
It is certainly possible to use templates or macros, but as this code is
auto-generated there is no need to make writing this code more efficient. Also,
most of these classes will be specialized with individual member functions in
subsequent commits, such that there will be little code reuse to exploit. Hence,
we decided to do so at the moment.
These bindings are not yet officially part of isl, but the draft is already very
stable. The smart pointer interface itself did not change since serveral months.
Adding this code to Polly is against our normal policy of only importing
official isl code. In this case however, we make an exception to showcase a
non-trivial use case of these bindings which should increase confidence in these
bindings and will help upstreaming them to isl.
Tags: #polly
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D30325
llvm-svn: 297452
This pass allows writing the LLVM-IR just before and after the Polly
passes to a file.
Dumping the IR before Polly helps reproducing bugs that occur in code
generated by clang. It is the only reliable way to get the IR that
triggers a bug. The alternative is to emit the IR with
clang -c -emit-llvm -S -o dump.ll
then pass it through all optimization passes
opt dump.ll -basicaa -sroa ... -S -o optdump.ll
to then reproduce the error with
opt optdump.ll -polly-opt-isl -polly-codegen -analyze
However, the IR is not the same. -O3 uses a PassBuilder than creates passes
with different parameters than the default.
Dumping the IR after Polly is useful to compare a miscompilation with
a known-good configuration.
Differential Revision: https://reviews.llvm.org/D30788
llvm-svn: 297415
Generate a PollyConfig.cmake for use with Cmake's find_package in
out-of-tree projects.
Contributed-by: Philip Pfaffe <philip.pfaffe@gmail.com>
Differential Revision: https://reviews.llvm.org/D30495
llvm-svn: 297395
In case LLVM pointers are annotated with !dereferencable attributes/metadata
or LLVM can look at the allocation from which a pointer is derived, we can know
that dereferencing pointers is safe and can be done unconditionally. We use this
information to proof certain pointers as save to hoist and then hoist them
unconditionally.
llvm-svn: 297375
One of the current limitations of DeLICM is that it only creates
PHI WRITEs that it knows are read by some PHI. Such writes may not span
all instances of a statement. Polly's code generator currently does not
support MemoryAccesses that are not executed in all instances
('partial accesses') and so has to give up on a possible mapping.
This workaround has once been suggested by Tobias Grosser: Try to
interpolate an arbitrary expansion to all instances. It will be checked
for possible conflicts with the existing Knowledge and can be applied if
the conflict checking result is that no semantics are changed.
Expansion is done by simplifying the mapping by coalescing with the hope
that coalescing will find a polyhedral 'rule' of the relevant map. It is
then 'gist'-ed using the domain of the relevant instances such that the
rule is expanded to the universe and finally intersected with the domain
of all statement instances.
The expansion makes conflicts become more likely, the found rule may
still not encompass all statement instances and the found rule exposes
internals of isl's implementation of coalesce and gist. The latter means
that the result depends on how much effort the implementation invests
into finding a rule which may change between versions of isl. Trivial
implementations of gist and coalesce just return the input arguments.
A patch that makes codegen support partial accesses is in preparation
as well.
Differential Revision: https://reviews.llvm.org/D30763
llvm-svn: 297373
Simplify ScopDetection::isInvariant(). Essentially deny everything that
is defined within the SCoP and is not load-hoisted.
The previous understanding of "invariant" has a few holes:
- Expressions without side-effects with only invariant arguments, but
are defined withing the SCoP's region with the exception of selects
and PHIs. These should be part of the index expression derived by
ScalarEvolution and not of the base pointer.
- Function calls with that are !mayHaveSideEffects() (typically
functions with "readnone nounwind" attributes). An example is given
below.
@C = external global i32
declare float* @getNextBasePtr(float*) readnone nounwind
...
%ptr = call float* @getNextBasePtr(float* %A, float %B)
The call might return:
* %A, so %ptr aliases with it in the SCoP
* %B, so %ptr aliases with it in the SCoP
* @C, so %ptr aliases with it in the SCoP
* a new pointer everytime it is called, such as malloc()
* a pointer into the allocated block of one of the aforementioned
* any of the above, at random at each call
Hence and contrast to a comment in the base_pointer.ll regression
test, %ptr is not necessarily the same all the time. It might also
alias with anything and no AliasAnalysis can tell otherwise if the
definition is external. It is hence not suitable in the role of a
base pointer.
The practical problem with base pointers defined in SCoP statements is
that it is not available globally in the SCoP. The statement instance
must be executed first before the base pointer can be used. This is no
problem if the base pointer is transferred as a scalar value between
statements. Uses of MemoryAccess::setNewAccessRelation may add a use of
the base pointer anywhere in the array. setNewAccessRelation is used by
JSONImporter, DeLICM and D28518. Indeed, BlockGenerator currently
assumes that base pointers are available globally and generates invalid
code for new access relation (referring to the base pointer of the
original code) if not, even if the base pointer would be available in
the statement.
This could be fixed with some added complexity and restrictions. The
ExprBuilder must lookup the local BBMap and code that call
setNewAccessRelation must check whether the base pointer is available
first.
The code would still be incorrect in the presence of aliasing. There
is the switch -polly-ignore-aliasing to explicitly allow this, but
it is hardly a justification for the additional complexity. It would
still be mostly useless because in most cases either getNextBasePtr()
has external linkage in which case the readnone nounwind attributes
cannot be derived in the translation unit itself, or is defined in the
same translation unit and gets inlined.
Reviewed By: grosser
Differential Revision: https://reviews.llvm.org/D30695
llvm-svn: 297281
Only when load-hoisted we can be sure the base pointer is invariant
during the SCoP's execution. Most of the time it would be added to
the required hoists for the alias checks anyway, except with
-polly-ignore-aliasing, -polly-use-runtime-alias-checks=0 or if
AliasAnalysis is already sure it doesn't alias with anything
(for instance if there is no other pointer to alias with).
Two more parts in Polly assume that this load-hoisting took place:
- setNewAccessRelation() which contains an assert which tests this.
- BlockGenerator which would use to the base ptr from the original
code if not load-hoisted (if the access expression is regenerated)
Differential Revision: https://reviews.llvm.org/D30694
llvm-svn: 297195
There is no point in optimizing unreachable code, hence our test cases should
always return.
This commit is part of a series that makes Polly more robust on the presence of
unreachables.
llvm-svn: 297158
These test cases should work in combination with
https://reviews.llvm.org/D12676, but became outdated over time. Update them
in preparation of discussions with Daniel Berlin on how to represent unreachable
in the post-dominator tree.
llvm-svn: 297157
Our current scop modeling enters an infinite loop when trying to model code
that has unreachable instructions (e.g.,
test/ScopInfo/BoundChecks/single-loop.ll), as the number of basic blocks
returned by the LLVM Loop* does not include unreachable basic blocks that
branch off from the core loop body. This arises for example in the following
piece of code:
for (i = 0; i < N; i++) {
if (i > 1024)
abort(); <- this abort might be translated to an
unreachable
A[i] = ...
}
This patch adds these unreachable basic blocks in our per loop basic block
count to ensure that the schedule construction does not assume a loop has been
processed completely, despite certain unreachable basic blocks still remaining.
The infinite loop is only observable in combination with
https://reviews.llvm.org/D12676 or a similar patch.
llvm-svn: 297156
Scops that exit with an unreachable are today still permitted, but make little
sense to optimize. We therefore can already skip them during scop detection.
This speeds up scop detection in certain cases and also ensures that bugpoint
does not introduce unreachables when reducing test cases.
In practice this change should have little impact, as the performance of
unreachable code is unlikely to matter.
This commit is part of a series that makes Polly more robust in the presence
of unreachables.
llvm-svn: 297151
