[polly] r252750 - ScopInfo: Limit the number of disjuncts in assumed context

[Johannes Doerfert via llvm-commits]

On 11/11, Tobias Grosser via llvm-commits wrote:
> Author: grosser
> Date: Wed Nov 11 10:22:36 2015
> New Revision: 252750
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=252750&view=rev
> Log:
> ScopInfo: Limit the number of disjuncts in assumed context
> 
> In certain rare cases (mostly -polly-process-unprofitable on large sequences
> of conditions - often without any loop), we see some compile-time timeouts due
> to the construction of an overly complex assumption context. This change limits
> the number of disjuncts to 150 (adjustable), to prevent us from creating
> assumptions contexts that are too large for even the compilation to finish.
> 
> The limit has been choosen as large as possible to make sure we do not
> unnecessarily drop test coverage. If such cases also appear in
> -polly-process-unprofitable=false mode we may need to think about this again,
> as the current limitations may still allow assumptions that are way to complex
> to be checked profitably at run-time.
> 
> There is also certainly room for improvement regarding how (and how efficient)
> we construct an assumed context, but this requires some more thinking.
> 
> This completes llvm.org/PR25458
> 
> Added:
>     polly/trunk/test/ScopInfo/long-sequence-of-error-blocks-2.ll
> Modified:
>     polly/trunk/lib/Analysis/ScopInfo.cpp
> 
> Modified: polly/trunk/lib/Analysis/ScopInfo.cpp
> URL: http://llvm.org/viewvc/llvm-project/polly/trunk/lib/Analysis/ScopInfo.cpp?rev=252750&r1=252749&r2=252750&view=diff
> ==============================================================================
> --- polly/trunk/lib/Analysis/ScopInfo.cpp (original)
> +++ polly/trunk/lib/Analysis/ScopInfo.cpp Wed Nov 11 10:22:36 2015
> @@ -91,6 +91,12 @@ static cl::opt<bool> DetectReductions("p
>                                        cl::Hidden, cl::ZeroOrMore,
>                                        cl::init(true), cl::cat(PollyCategory));
>  
> +static cl::opt<int> MaxDisjunctsAssumed(
> +    "polly-max-disjuncts-assumed",
> +    cl::desc("The maximal number of disjuncts we allow in the assumption "
> +             "context (this bounds compile time)"),
> +    cl::Hidden, cl::ZeroOrMore, cl::init(150), cl::cat(PollyCategory));
> +
>  //===----------------------------------------------------------------------===//
>  
>  // Create a sequence of two schedules. Either argument may be null and is
> @@ -2851,6 +2857,16 @@ bool Scop::hasFeasibleRuntimeContext() c
>  
>  void Scop::addAssumption(__isl_take isl_set *Set) {
>    AssumedContext = isl_set_intersect(AssumedContext, Set);
> +  isl_basic_set_list *List = isl_set_get_basic_set_list(AssumedContext);
> +  int NSets = isl_basic_set_list_n_basic_set(List);
> +  isl_basic_set_list_free(List);
> +
> +  if (NSets >= MaxDisjunctsAssumed) {
> +    isl_space *Space = isl_set_get_space(AssumedContext);
> +    isl_set_free(AssumedContext);
> +    AssumedContext = isl_set_universe(Space);
This should be an empty set and not the universe, right? Otherwise you
just drop assumptions which is not sound.
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