[llvm] Iterate SmallPtrSet in reverse order to uncover non-determinism in codegen

Summary:
Given a flag (-mllvm -reverse-iterate) this patch will enable iteration of SmallPtrSet in reverse order.
The idea is to compile the same source with and without this flag and expect the code to not change.
If there is a difference in codegen then it would mean that the codegen is sensitive to the iteration order of SmallPtrSet.
This is enabled only with LLVM_ENABLE_ABI_BREAKING_CHECKS.

Reviewers: chandlerc, dexonsmith, mehdi_amini

Subscribers: mgorny, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D26718

llvm-svn: 289619

4 files changed, 104 insertions, 1 deletions

diff --git a/llvm/include/llvm/ADT/SmallPtrSet.h b/llvm/include/llvm/ADT/SmallPtrSet.h
index 16e5ed8933f..438f10b11a2 100644
--- a/llvm/include/llvm/ADT/SmallPtrSet.h
+++ b/llvm/include/llvm/ADT/SmallPtrSet.h
@@ -15,6 +15,7 @@
 #ifndef LLVM_ADT_SMALLPTRSET_H
 #define LLVM_ADT_SMALLPTRSET_H
 
+#include "llvm/Config/abi-breaking.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
 #include <cassert>
@@ -25,6 +26,13 @@
 #include <iterator>
 #include <utility>
 
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+namespace llvm {
+template <class T = void> struct ReverseIterate { static bool value; };
+template <class T> bool ReverseIterate<T>::value = true;
+}
+#endif
+
 namespace llvm {
 
 /// SmallPtrSetImplBase - This is the common code shared among all the
@@ -206,6 +214,12 @@ protected:
 public:
   explicit SmallPtrSetIteratorImpl(const void *const *BP, const void*const *E)
     : Bucket(BP), End(E) {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate<bool>::value) {
+      RetreatIfNotValid();
+      return;
+    }
+#endif
     AdvanceIfNotValid();
   }
 
@@ -227,6 +241,17 @@ protected:
             *Bucket == SmallPtrSetImplBase::getTombstoneMarker()))
       ++Bucket;
   }
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+  void RetreatIfNotValid() {
+    --Bucket;
+    assert(Bucket <= End);
+    while (Bucket != End &&
+           (*Bucket == SmallPtrSetImplBase::getEmptyMarker() ||
+            *Bucket == SmallPtrSetImplBase::getTombstoneMarker())) {
+      --Bucket;
+    }
+  }
+#endif
 };
 
 /// SmallPtrSetIterator - This implements a const_iterator for SmallPtrSet.
@@ -252,13 +277,27 @@ public:
   }
 
   inline SmallPtrSetIterator& operator++() {          // Preincrement
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate<bool>::value) {
+      RetreatIfNotValid();
+      return *this;
+    }
+#endif
     ++Bucket;
     AdvanceIfNotValid();
     return *this;
   }
 
   SmallPtrSetIterator operator++(int) {        // Postincrement
-    SmallPtrSetIterator tmp = *this; ++*this; return tmp;
+    SmallPtrSetIterator tmp = *this;
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate<bool>::value) {
+      --*this;
+      return tmp;
+    }
+#endif
+    ++*this;
+    return tmp;
   }
 };
 
@@ -343,9 +382,22 @@ public:
   }
 
   inline iterator begin() const {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate<bool>::value)
+      return endPtr();
+#endif
     return iterator(CurArray, EndPointer());
   }
   inline iterator end() const {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate<bool>::value)
+      return iterator(CurArray, CurArray);
+#endif
+    return endPtr();
+  }
+
+private:
+  inline iterator endPtr() const {
     const void *const *End = EndPointer();
     return iterator(End, End);
   }
diff --git a/llvm/lib/Support/CommandLine.cpp b/llvm/lib/Support/CommandLine.cpp
index c9751c093d6..f4d501aceba 100644
--- a/llvm/lib/Support/CommandLine.cpp
+++ b/llvm/lib/Support/CommandLine.cpp
@@ -45,6 +45,17 @@ using namespace cl;
 
 #define DEBUG_TYPE "commandline"
 
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+namespace llvm {
+// If LLVM_ENABLE_ABI_BREAKING_CHECKS is set the flag -mllvm -reverse-iterate
+// can be used to toggle forward/reverse iteration of unordered containers.
+// This will help uncover differences in codegen caused due to undefined
+// iteration order.
+static cl::opt<bool, true> ReverseIteration("reverse-iterate",
+  cl::location(ReverseIterate<bool>::value), cl::init(true));
+}
+#endif
+
 //===----------------------------------------------------------------------===//
 // Template instantiations and anchors.
 //
diff --git a/llvm/unittests/ADT/CMakeLists.txt b/llvm/unittests/ADT/CMakeLists.txt
index 9e10e92de49..738f6efe92d 100644
--- a/llvm/unittests/ADT/CMakeLists.txt
+++ b/llvm/unittests/ADT/CMakeLists.txt
@@ -41,6 +41,7 @@ set(ADTSources
   PostOrderIteratorTest.cpp
   PriorityWorklistTest.cpp
   RangeAdapterTest.cpp
+  ReverseIterationTest.cpp
   SCCIteratorTest.cpp
   STLExtrasTest.cpp
   ScopeExitTest.cpp
diff --git a/llvm/unittests/ADT/ReverseIterationTest.cpp b/llvm/unittests/ADT/ReverseIterationTest.cpp
new file mode 100644
index 00000000000..9235ecd7432
--- /dev/null
+++ b/llvm/unittests/ADT/ReverseIterationTest.cpp
@@ -0,0 +1,39 @@
+//===- llvm/unittest/ADT/ReverseIterationTest.cpp ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// ReverseIteration unit tests.
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "llvm/ADT/SmallPtrSet.h"
+
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+using namespace llvm;
+
+TEST(ReverseIterationTest, SmallPtrSetTest) {
+
+  SmallPtrSet<void*, 4> Set;
+  void *Ptrs[] = { (void*)0x1, (void*)0x2, (void*)0x3, (void*)0x4 };
+  void *ReversePtrs[] = { (void*)0x4, (void*)0x3, (void*)0x2, (void*)0x1 };
+
+  for (auto *Ptr: Ptrs)
+    Set.insert(Ptr);
+
+  // Check forward iteration.
+  ReverseIterate<bool>::value = false;
+  for (const auto &Tuple : zip(Set, Ptrs))
+    ASSERT_EQ(std::get<0>(Tuple), std::get<1>(Tuple));
+
+  // Check reverse iteration.
+  ReverseIterate<bool>::value = true;
+  for (const auto &Tuple : zip(Set, ReversePtrs))
+    ASSERT_EQ(std::get<0>(Tuple), std::get<1>(Tuple));
+}
+#endif