[ConstraintElimination] Bail out if system gets too big.

For some inputs, the constraint system can grow quite large during
solving, because it replaces complex constraints with one or more
simpler constraints. This adds a cut-off to avoid compile-time explosion
on problematic inputs.

llvm/include/llvm/Analysis/ConstraintSystem.h

@@ -79,6 +79,9 @@ class ConstraintSystem {
   bool isConditionImplied(SmallVector<int64_t, 8> R);
 
   void popLastConstraint() { Constraints.pop_back(); }
+
+  /// Returns the number of rows in the constraint system.
+  unsigned size() const { return Constraints.size(); }
 };
 } // namespace llvm
 

llvm/lib/Analysis/ConstraintSystem.cpp

@@ -84,6 +84,9 @@ bool ConstraintSystem::eliminateUsingFM() {
                      .getZExtValue();
       }
       NewSystem.push_back(std::move(NR));
+      // Give up if the new system gets too big.
+      if (NewSystem.size() > 500)
+        return false;
     }
   }
   Constraints = std::move(NewSystem);

llvm/test/Transforms/ConstraintElimination/large-system-growth.ll

@@ -0,0 +1,86 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -constraint-elimination -S %s | FileCheck %s
+
+; The system for the function below grows quite large. Check to make sure
+; we can handle that scenario.
+define void @test(i64 %x, i8* %y, i8* %z, i8* %w) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:    [[TMP22:%.*]] = getelementptr inbounds i8, i8* [[Y:%.*]], i64 [[X:%.*]]
+; CHECK-NEXT:    [[TMP26:%.*]] = icmp ult i8* [[TMP22]], [[Z:%.*]]
+; CHECK-NEXT:    br i1 [[TMP26]], label [[BB28:%.*]], label [[EARLY_EXIT:%.*]]
+; CHECK:       early.exit:
+; CHECK-NEXT:    unreachable
+; CHECK:       bb28:
+; CHECK-NEXT:    [[TMP29:%.*]] = getelementptr inbounds i8, i8* [[Y]], i64 [[X]]
+; CHECK-NEXT:    [[TMP30:%.*]] = icmp ult i8* [[TMP29]], [[Z]]
+; CHECK-NEXT:    br i1 true, label [[EARLY_EXIT]], label [[BB32:%.*]]
+; CHECK:       bb32:
+; CHECK-NEXT:    [[TMP33:%.*]] = icmp ult i8* [[TMP29]], [[Z]]
+; CHECK-NEXT:    br i1 true, label [[BB35:%.*]], label [[EARLY_EXIT]]
+; CHECK:       bb35:
+; CHECK-NEXT:    [[TMP36:%.*]] = icmp ult i8* [[Y]], [[Z]]
+; CHECK-NEXT:    br i1 true, label [[EARLY_EXIT]], label [[BB38:%.*]]
+; CHECK:       bb38:
+; CHECK-NEXT:    [[TMP41:%.*]] = icmp ult i8* [[Y]], [[Z]]
+; CHECK-NEXT:    br i1 true, label [[EARLY_EXIT]], label [[BB43:%.*]]
+; CHECK:       bb43:
+; CHECK-NEXT:    [[TMP47:%.*]] = getelementptr inbounds i8, i8* [[W:%.*]], i64 [[X]]
+; CHECK-NEXT:    [[TMP48:%.*]] = icmp ult i8* [[TMP47]], [[Y]]
+; CHECK-NEXT:    br i1 [[TMP48]], label [[EARLY_EXIT]], label [[BB50:%.*]]
+; CHECK:       bb50:
+; CHECK-NEXT:    [[TMP52:%.*]] = getelementptr inbounds i8, i8* [[W]], i64 [[X]]
+; CHECK-NEXT:    [[TMP53:%.*]] = icmp ult i8* [[TMP52]], [[Y]]
+; CHECK-NEXT:    br i1 true, label [[EARLY_EXIT]], label [[BB55:%.*]]
+; CHECK:       bb55:
+; CHECK-NEXT:    [[TMP57:%.*]] = icmp ult i8* [[W]], [[Y]]
+; CHECK-NEXT:    br i1 true, label [[BB59:%.*]], label [[EARLY_EXIT]]
+; CHECK:       bb59:
+; CHECK-NEXT:    [[TMP60:%.*]] = icmp ult i8* [[W]], [[Y]]
+; CHECK-NEXT:    call void @use(i1 true)
+; CHECK-NEXT:    ret void
+;
+  %tmp22 = getelementptr inbounds i8, i8* %y, i64 %x
+  %tmp26 = icmp ult i8* %tmp22, %z
+  br i1 %tmp26, label %bb28, label %early.exit
+
+early.exit:
+  unreachable
+
+bb28:
+  %tmp29 = getelementptr inbounds i8, i8* %y, i64 %x
+  %tmp30 = icmp ult i8* %tmp29, %z
+  br i1 %tmp30, label %early.exit, label %bb32
+
+bb32:
+  %tmp33 = icmp ult i8* %tmp29, %z
+  br i1 %tmp33, label %bb35, label %early.exit
+
+bb35:
+  %tmp36 = icmp ult i8* %y, %z
+  br i1 %tmp36, label %early.exit, label %bb38
+
+bb38:
+  %tmp41 = icmp ult i8* %y, %z
+  br i1 %tmp41, label %early.exit, label %bb43
+
+bb43:
+  %tmp47 = getelementptr inbounds i8, i8* %w, i64 %x
+  %tmp48 = icmp ult i8* %tmp47, %y
+  br i1 %tmp48, label %early.exit, label %bb50
+
+bb50:
+  %tmp52 = getelementptr inbounds i8, i8* %w, i64 %x
+  %tmp53 = icmp ult i8* %tmp52, %y
+  br i1 %tmp53, label %early.exit, label %bb55
+
+bb55:
+  %tmp57 = icmp ult i8* %w, %y
+  br i1 %tmp57, label %bb59, label %early.exit
+
+bb59:
+  %tmp60 = icmp ult i8* %w, %y
+  call void @use(i1 %tmp60)
+  ret void
+}
+
+declare void @use(i1)