From f19876c5366136da942b733d6e4b435fb19863a3 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo@fhahn.com>
Date: Sun, 6 Dec 2020 19:58:33 +0000
Subject: [PATCH] [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 |  3 +
 llvm/lib/Analysis/ConstraintSystem.cpp        |  3 +
 .../large-system-growth.ll                    | 86 +++++++++++++++++++
 3 files changed, 92 insertions(+)
 create mode 100644 llvm/test/Transforms/ConstraintElimination/large-system-growth.ll

diff --git a/llvm/include/llvm/Analysis/ConstraintSystem.h b/llvm/include/llvm/Analysis/ConstraintSystem.h
index 2e55d5d45674..83c1fb4485fd 100644
--- a/llvm/include/llvm/Analysis/ConstraintSystem.h
+++ b/llvm/include/llvm/Analysis/ConstraintSystem.h
@@ -79,6 +79,9 @@ public:
   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
 
diff --git a/llvm/lib/Analysis/ConstraintSystem.cpp b/llvm/lib/Analysis/ConstraintSystem.cpp
index f3a28347ded4..1d582802f8e7 100644
--- a/llvm/lib/Analysis/ConstraintSystem.cpp
+++ b/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);
diff --git a/llvm/test/Transforms/ConstraintElimination/large-system-growth.ll b/llvm/test/Transforms/ConstraintElimination/large-system-growth.ll
new file mode 100644
index 000000000000..e2d603ed7462
--- /dev/null
+++ b/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)