ScopInfo: Bail out in case of complex branch structures

Scops that contain many complex branches are likely to result in complex domain
conditions that consist of a large (> 100) number of conjucts.  Transforming
such domains is expensive and unlikely to result in efficient code.  To avoid
long compile times we detect this case and skip such scops. In the future we may
improve this by either using non-affine subregions to hide such complex
condition structures or by exploiting in certain cases properties (e.g.,
dominance) that allow us to construct the domains of a scop in a way that
results in a smaller number improving conjuncts.

Example of a code that results in complex iteration spaces:

      loop.header
     /    |    \ \
   A0    A2    A4 \
     \  /  \  /    \
      A1    A3      \
     /  \  /  \     |
   B0    B2    B4   |
     \  /  \  /     |
      B1    B3      ^
     /  \  /  \     |
   C0    C2    C4   |
     \  /  \  /    /
      C1    C3    /
       \   /     /
    loop backedge

llvm-svn: 256123

diff --git a/polly/include/polly/ScopInfo.h b/polly/include/polly/ScopInfo.h
index 16a5275..d70d13d 100644 (file)
--- a/polly/include/polly/ScopInfo.h
+++ b/polly/include/polly/ScopInfo.h
@@ -77,6 +77,7 @@ enum AssumptionKind {
   INFINITELOOP,
   INVARIANTLOAD,
   DELINEARIZATION,
+  ERROR_DOMAINCONJUNCTS,
 };
 
 /// Maps from a loop to the affine function expressing its backedge taken count.

diff --git a/polly/lib/Analysis/ScopInfo.cpp b/polly/lib/Analysis/ScopInfo.cpp
index a617e1a..6a933f1 100644 (file)
--- a/polly/lib/Analysis/ScopInfo.cpp
+++ b/polly/lib/Analysis/ScopInfo.cpp
@@ -62,6 +62,11 @@ using namespace polly;
 STATISTIC(ScopFound, "Number of valid Scops");
 STATISTIC(RichScopFound, "Number of Scops containing a loop");
 
+// The maximal number of basic sets we allow during domain construction to
+// be created. More complex scops will result in very high compile time and
+// are also unlikely to result in good code
+static int const MaxConjunctsInDomain = 20;
+
 static cl::opt<bool> ModelReadOnlyScalars(
     "polly-analyze-read-only-scalars",
     cl::desc("Model read-only scalar values in the scop description"),
@@ -2176,6 +2181,12 @@ void Scop::buildDomainsWithBranchConstraints(Region *R) {
         SuccDomain = isl_set_union(SuccDomain, CondSet);
 
       SuccDomain = isl_set_coalesce(SuccDomain);
+      if (isl_set_n_basic_set(SuccDomain) > MaxConjunctsInDomain) {
+        auto *Empty = isl_set_empty(isl_set_get_space(SuccDomain));
+        isl_set_free(SuccDomain);
+        SuccDomain = Empty;
+        invalidate(ERROR_DOMAINCONJUNCTS, DebugLoc());
+      }
       DEBUG(dbgs() << "\tSet SuccBB: " << SuccBB->getName() << " : "
                    << SuccDomain << "\n");
     }
@@ -3043,6 +3054,8 @@ static std::string toString(AssumptionKind Kind) {
     return "Invariant load";
   case DELINEARIZATION:
     return "Delinearization";
+  case ERROR_DOMAINCONJUNCTS:
+    return "Low number of domain conjuncts";
   }
   llvm_unreachable("Unknown AssumptionKind!");
 }

diff --git a/polly/test/ScopInfo/complex-branch-structure.ll b/polly/test/ScopInfo/complex-branch-structure.ll
new file mode 100644 (file)
index 0000000..201a410
--- /dev/null
+++ b/polly/test/ScopInfo/complex-branch-structure.ll
@@ -0,0 +1,195 @@
+; RUN: opt %loadPolly -pass-remarks-analysis="polly-scops" -polly-scops \
+; RUN:     < %s 2>&1 | FileCheck %s
+
+; We build a scop of the following form to check that the domain construction
+; does not take a huge amount of time, but that we instead just bail out.
+;
+;       loop.header
+;      /    |    \ \
+;    A0    A2    A4 \
+;      \  /  \  /    \
+;       A1    A3      \
+;      /  \  /  \     |
+;    B0    B2    B4   |
+;      \  /  \  /     |
+;       B1    B3      ^
+;      /  \  /  \     |
+;    C0    C2    C4   |
+;      \  /  \  /    /
+;       C1    C3    /
+;        \   /     /
+;     loop backedge
+
+; CHECK: Low number of domain conjuncts assumption: {  : 1 = 0 }
+
+define void @foo(float* %A, float* %B, float* %C, float* %D, float* %E,
+                 i64 %A1.p, i64 %A2.p, i64 %A3.p,
+                 i64 %B1.p, i64 %B2.p, i64 %B3.p,
+                 i64 %C1.p, i64 %C2.p, i64 %C3.p,
+                 i64 %D1.p, i64 %D2.p, i64 %D3.p,
+                 i64 %E1.p, i64 %E2.p, i64 %E3.p) {
+entry:
+  br label %loop.header
+
+loop.header:
+  %indvar = phi i64 [0, %entry], [%indvar.next, %loop.backedge]
+  switch i2 0, label %A0 [i2 1, label %A2 i2 2, label %A4]
+
+A0:
+  %val.A0 = load float, float* %A
+  store float %val.A0, float* %A
+  br label %A1
+
+A2:
+  %val.A2 = load float, float* %A
+  store float %val.A2, float* %A
+  %A2.cmp = icmp eq i64 %A2.p, 0
+  br i1 %A2.cmp, label %A1, label %A3
+
+A4:
+  %val.A4 = load float, float* %A
+  store float %val.A4, float* %A
+  br label %A3
+
+A1:
+  %val.A1 = load float, float* %A
+  store float %val.A1, float* %A
+  %A1.cmp = icmp eq i64 %A1.p, 0
+  br i1 %A1.cmp, label %B0, label %B2
+
+A3:
+  %val.A3 = load float, float* %A
+  store float %val.A3, float* %A
+  %A3.cmp = icmp eq i64 %A3.p, 0
+  br i1 %A3.cmp, label %B2, label %B4
+
+B0:
+  %val.B0 = load float, float* %B
+  store float %val.B0, float* %B
+  br label %B1
+
+B2:
+  %val.B2 = load float, float* %B
+  store float %val.B2, float* %B
+  %B2.cmp = icmp eq i64 %B2.p, 0
+  br i1 %B2.cmp, label %B1, label %B3
+
+B4:
+  %val.B4 = load float, float* %B
+  store float %val.B4, float* %B
+  br label %B3
+
+B1:
+  %val.B1 = load float, float* %B
+  store float %val.B1, float* %B
+  %B1.cmp = icmp eq i64 %B1.p, 0
+  br i1 %B1.cmp, label %C0, label %C2
+
+B3:
+  %val.B3 = load float, float* %A
+  store float %val.B3, float* %A
+  %B3.cmp = icmp eq i64 %A3.p, 0
+  br i1 %B3.cmp, label %C2, label %C4
+
+C0:
+  %val.C0 = load float, float* %C
+  store float %val.C0, float* %C
+  br label %C1
+
+C2:
+  %val.C2 = load float, float* %C
+  store float %val.C2, float* %C
+  %C2.cmp = icmp eq i64 %C2.p, 0
+  br i1 %C2.cmp, label %C1, label %C3
+
+C4:
+  %val.C4 = load float, float* %C
+  store float %val.C4, float* %C
+  br label %C3
+
+C1:
+  %val.C1 = load float, float* %C
+  store float %val.C1, float* %C
+  %C1.cmp = icmp eq i64 %C1.p, 0
+  br i1 %C1.cmp, label %D0, label %D2
+
+C3:
+  %val.C3 = load float, float* %C
+  store float %val.C3, float* %C
+  %C3.cmp = icmp eq i64 %C3.p, 0
+  br i1 %C3.cmp, label %D2, label %D4
+
+D0:
+  %val.D0 = load float, float* %D
+  store float %val.D0, float* %D
+  br label %D1
+
+D2:
+  %val.D2 = load float, float* %D
+  store float %val.D2, float* %D
+  %D2.cmp = icmp eq i64 %D2.p, 0
+  br i1 %D2.cmp, label %D1, label %D3
+
+D4:
+  %val.D4 = load float, float* %D
+  store float %val.D4, float* %D
+  br label %D3
+
+D1:
+  %val.D1 = load float, float* %D
+  store float %val.D1, float* %D
+  %D1.cmp = icmp eq i64 %D1.p, 0
+  br i1 %D1.cmp, label %E0, label %E2
+
+D3:
+  %val.D3 = load float, float* %D
+  store float %val.D3, float* %D
+  %D3.cmp = icmp eq i64 %D3.p, 0
+  br i1 %D3.cmp, label %E2, label %E4
+
+E0:
+  %val.E0 = load float, float* %E
+  store float %val.E0, float* %E
+  br label %E1
+
+E2:
+  %val.E2 = load float, float* %E
+  store float %val.E2, float* %E
+  %E2.cmp = icmp eq i64 %E2.p, 0
+  br i1 %E2.cmp, label %E1, label %E3
+
+E4:
+  %val.E4 = load float, float* %E
+  store float %val.E4, float* %E
+  br label %E3
+
+E1:
+  %val.E1 = load float, float* %E
+  store float %val.E1, float* %E
+  %E1.cmp = icmp eq i64 %E1.p, 0
+  br i1 %E1.cmp, label %F0, label %F2
+
+E3:
+  %val.E3 = load float, float* %E
+  store float %val.E3, float* %E
+  %E3.cmp = icmp eq i64 %E3.p, 0
+  br i1 %E3.cmp, label %F2, label %F4
+
+F0:
+  br label %loop.backedge
+
+F2:
+  br label %loop.backedge
+
+F4:
+  br label %loop.backedge
+
+loop.backedge:
+  %indvar.next = add i64 %indvar, 1
+  %cmp = icmp ne i64 %indvar, 1000
+  br i1 %cmp, label %loop.header, label %exit
+
+exit:
+  ret void
+
+}