[LoopUnroll] Respect the convergent attribute.

Summary:
Specifically, when we perform runtime loop unrolling of a loop that
contains a convergent op, we can only unroll k times, where k divides
the loop trip multiple.

Without this change, we'll happily unroll e.g. the following loop

  for (int i = 0; i < N; ++i) {
    if (i == 0) convergent_op();
    foo();
  }

into

  int i = 0;
  if (N % 2 == 1) {
    convergent_op();
    foo();
    ++i;
  }
  for (; i < N - 1; i += 2) {
    if (i == 0) convergent_op();
    foo();
    foo();
  }.

This is unsafe, because we've just added a control-flow dependency to
the convergent op in the prelude.

In general, runtime unrolling loops that contain convergent ops is safe
only if we don't have emit a prelude, which occurs when the unroll count
divides the trip multiple.

Reviewers: resistor

Subscribers: llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D17526

llvm-svn: 263509

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -273,7 +273,23 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
   // flag is specified.
   bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);
 
-  if (RuntimeTripCount &&
+  // Loops containing convergent instructions must have a count that divides
+  // their TripMultiple.
+  DEBUG(
+    bool HasConvergent = false;
+    for (auto &BB : L->blocks())
+      for (auto &I : *BB)
+        if (auto CS = CallSite(&I))
+          HasConvergent |= CS.isConvergent();
+    assert(
+        !HasConvergent || TripMultiple % Count == 0 &&
+        "Unroll count must divide trip multiple if loop contains a convergent "
+        "operation.");
+  );
+  // Don't output the runtime loop prolog if Count is a multiple of
+  // TripMultiple.  Such a prolog is never needed, and is unsafe if the loop
+  // contains a convergent instruction.
+  if (RuntimeTripCount && TripMultiple % Count != 0 &&
       !UnrollRuntimeLoopProlog(L, Count, AllowExpensiveTripCount, LI, SE, DT,
                                PreserveLCSSA))
     return false;