[LoopSimplify] don't separate nested loops with convergent calls

Summary:
When a loop has multiple backedges, loop simplification attempts to
separate them out into nested loops. This results in incorrect control
flow in the presence of some functions like a GPU barrier. This change
skips the transformation when such "convergent" function calls are
present in the loop body.

Reviewed By: nhaehnle

Differential Revision: https://reviews.llvm.org/D80078

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopSimplify.cpp b/llvm/lib/Transforms/Utils/LoopSimplify.cpp
index 28f88f3..14c6950 100644
--- a/llvm/lib/Transforms/Utils/LoopSimplify.cpp
+++ b/llvm/lib/Transforms/Utils/LoopSimplify.cpp
@@ -230,6 +230,27 @@ static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
   if (!Preheader)
     return nullptr;
 
+  // Treat the presence of convergent functions conservatively. The
+  // transformation is invalid if calls to certain convergent
+  // functions (like an AMDGPU barrier) get included in the resulting
+  // inner loop. But blocks meant for the inner loop will be
+  // identified later at a point where it's too late to abort the
+  // transformation. Also, the convergent attribute is not really
+  // sufficient to express the semantics of functions that are
+  // affected by this transformation. So we choose to back off if such
+  // a function call is present until a better alternative becomes
+  // available. This is similar to the conservative treatment of
+  // convergent function calls in GVNHoist and JumpThreading.
+  for (auto BB : L->blocks()) {
+    for (auto &II : *BB) {
+      if (auto CI = dyn_cast<CallBase>(&II)) {
+        if (CI->isConvergent()) {
+          return nullptr;
+        }
+      }
+    }
+  }
+
   // The header is not a landing pad; preheader insertion should ensure this.
   BasicBlock *Header = L->getHeader();
   assert(!Header->isEHPad() && "Can't insert backedge to EH pad");