[Polly][ScopInfo] Remove RunTimeChecksMaxAccessDisjuncts bail-out condition.

[Meinersbur]

Summary

The condition was introduced in r267142 to mitigate a long compile-time case. In r306087, a max-computation limit was introduced that should handle the same case while leaving the max disjuncts heuristic it should have replaced intact.

Today, the max disjuncts bail-out causes problems in that it prematurely stops SCoPs from being detected, e.g. in SPEC's lbm. This would hit less like if isl_set_coalesce would be called after isl_set_remove_divs (which makes more basic_set likely to be coalescable) instead of before.

This patch removes max disjuncts bail-out as having been superseded by the max-computations limit.

[grosser]

Hi Michael,

as far as I remember I left this check in originally to save compile time, as we would bail out before having spent a lot of time in exploding lexmin/lexmax computations. Would calling isl_set_remove_divs at the right place not fix this issue?

Best,
Tobias

[Meinersbur]

I'd rather remove the number of magic constants that make impactful decisions. If the goal is to reduce the computational overhead, I'd recommend to take the convex hull before lexmin/lexmin, not bail-out.

[grosser]

I am afraid taking a convex hull might also be costly (we could take a simple/affine hull instead).

I feel that in general, we should still decide to bail out early (before potentially costly functions) rather than waiting for compute outs to trigger. So I don't see how we can get rid of magic constants. However, we can potentially just take a hull rather than erroring out here.

[grosser]

Ping, as emails got lost at some point earlier in time.

[Meinersbur]

Deprecated in favor of D45066.

lib/Analysis/ScopInfo.cpp

OptComputeOut("polly-analysis-computeout",
                   cl::Hidden, cl::init(800000), cl::ZeroOrMore,
                   cl::cat(PollyCategory));
 
 static cl::opt<bool> PollyRemarksMinimal(
     "polly-remarks-minimal",
     cl::desc("Do not emit remarks about assumptions that are known"),
     cl::Hidden, cl::ZeroOrMore, cl::init(false), cl::cat(PollyCategory));
 
-static cl::opt<int> RunTimeChecksMaxAccessDisjuncts(
-    "polly-rtc-max-array-disjuncts",
-    cl::desc("The maximal number of disjunts allowed in memory accesses to "
-             "to build RTCs."),
-    cl::Hidden, cl::ZeroOrMore, cl::init(8), cl::cat(PollyCategory));
-
 static cl::opt<unsigned> RunTimeChecksMaxParameters(
     "polly-rtc-max-parameters",
     cl::desc("The maximal number of parameters allowed in RTCs."), cl::Hidden,
     cl::ZeroOrMore, cl::init(8), cl::cat(PollyCategory));
 
 static cl::opt<unsigned> RunTimeChecksMaxArraysPerGroup(
     "polly-rtc-max-arrays-per-group",
     cl::desc("The maximal number of arrays to compare in each alias group."),
if (isl_set_n_param(Set.get()) > RunTimeChecksMaxParameters) {
     for (unsigned u = 0, e = isl_set_n_param(Set.get()); u < e; u++)
       if (Set.involves_dims(isl::dim::param, u, 1))
         InvolvedParams++;
 
     if (InvolvedParams > RunTimeChecksMaxParameters)
       return isl::stat::error;
   }
 
-  if (isl_set_n_basic_set(Set.get()) > RunTimeChecksMaxAccessDisjuncts)
-    return isl::stat::error;
-
   MinPMA = Set.lexmin_pw_multi_aff();
   MaxPMA = Set.lexmax_pw_multi_aff();
 
   if (isl_ctx_last_error(Ctx.get()) == isl_error_quota)
     return isl::stat::error;
 
   MinPMA = MinPMA.coalesce();
   MaxPMA = MaxPMA.coalesce();