Avoid shuffle self-assignment in EXPENSIVE_CHECKS builds

Some versions of libstdc++ perform self-assignment in std::shuffle. This
breaks the EXPENSIVE_CHECKS builds of TableGen due to an incorrect assertion
in libstdc++.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85828.

Fixes https://llvm.org/PR37652

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D98167

include/llvm/ADT/STLExtras.h

@@ -1332,8 +1332,15 @@ template <class Iterator, class RNG>
 void shuffle(Iterator first, Iterator last, RNG &&g) {
   // It would be better to use a std::uniform_int_distribution,
   // but that would be stdlib dependent.
-  for (auto size = last - first; size > 1; ++first, (void)--size)
-    std::iter_swap(first, first + g() % size);
+  typedef
+      typename std::iterator_traits<Iterator>::difference_type difference_type;
+  for (auto size = last - first; size > 1; ++first, (void)--size) {
+    difference_type offset = g() % size;
+    // Avoid self-assignment due to incorrect assertions in libstdc++
+    // containers (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85828).
+    if (offset != difference_type(0))
+      std::iter_swap(first, first + offset);
+  }
 }
 
 /// Find the length of an array.
@@ -1373,7 +1380,7 @@ inline unsigned presortShuffleEntropy() {
 template <class IteratorTy>
 inline void presortShuffle(IteratorTy Start, IteratorTy End) {
   std::mt19937 Generator(presortShuffleEntropy());
-  std::shuffle(Start, End, Generator);
+  llvm::shuffle(Start, End, Generator);
 }
 
 } // end namespace detail

tools/bugpoint/FindBugs.cpp

@@ -41,7 +41,7 @@ BugDriver::runManyPasses(const std::vector<std::string> &AllPasses) {
     //
     // Step 1: Randomize the order of the optimizer passes.
     //
-    std::shuffle(PassesToRun.begin(), PassesToRun.end(), randomness);
+    llvm::shuffle(PassesToRun.begin(), PassesToRun.end(), randomness);
 
     //
     // Step 2: Run optimizer passes on the program and check for success.

tools/bugpoint/ListReducer.h

@@ -92,7 +92,7 @@ template <typename ElTy> struct ListReducer {
       // distribution (improving the speed of convergence).
       if (ShufflingEnabled && NumOfIterationsWithoutProgress > MaxIterations) {
         std::vector<ElTy> ShuffledList(TheList);
-        std::shuffle(ShuffledList.begin(), ShuffledList.end(), randomness);
+        llvm::shuffle(ShuffledList.begin(), ShuffledList.end(), randomness);
         errs() << "\n\n*** Testing shuffled set...\n\n";
         // Check that random shuffle doesn't lose the bug
         Expected<TestResult> Result = doTest(ShuffledList, empty);

tools/llvm-exegesis/lib/SerialSnippetGenerator.cpp

@@ -42,7 +42,7 @@ computeAliasingInstructions(const LLVMState &State, const Instruction *Instr,
   std::vector<unsigned> Opcodes;
   Opcodes.resize(State.getInstrInfo().getNumOpcodes());
   std::iota(Opcodes.begin(), Opcodes.end(), 0U);
-  std::shuffle(Opcodes.begin(), Opcodes.end(), randomGenerator());
+  llvm::shuffle(Opcodes.begin(), Opcodes.end(), randomGenerator());
 
   std::vector<const Instruction *> AliasingInstructions;
   for (const unsigned OtherOpcode : Opcodes) {

tools/llvm-stress/llvm-stress.cpp

@@ -718,7 +718,7 @@ static void IntroduceControlFlow(Function *F, Random &R) {
       BoolInst.push_back(&Instr);
   }
 
-  std::shuffle(BoolInst.begin(), BoolInst.end(), R);
+  llvm::shuffle(BoolInst.begin(), BoolInst.end(), R);
 
   for (auto *Instr : BoolInst) {
     BasicBlock *Curr = Instr->getParent();