[SimpleLoopUnswitch] Don't non-trivially unswitch loops that are unsafe to clone

[aeubanks]

Summary

Non-trivial unswitching can clone loops.

The legacy -loop-unswitch pass also checks for this.

Fixes PR49085.

[nathanchance]

Thanks! I can confirm that this fixes the assertion I was hitting.

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI,
   // a debugging.
   if (!NonTrivial && !EnableNonTrivialUnswitch)
     return false;
 
   // Skip non-trivial unswitching for optsize functions.
   if (L.getHeader()->getParent()->hasOptSize())
     return false;
 
+  // Skip non-trivial unswitching for loops that cannot be cloned.
+  if (!L.isSafeToClone())
+    return false;
+
   // For non-trivial unswitching, because it often creates new loops, we rely on
   // the pass manager to iterate on the loops rather than trying to immediately
   // reach a fixed point. There is no substantial advantage to iterating
   // internally, and if any of the new loops are simplified enough to contain
   // trivial unswitching we want to prefer those.
 
   // Try to unswitch the best invariant condition. We prefer this full unswitch to
   // a partial unswitch when possible below the threshold.

llvm/test/Transforms/SimpleLoopUnswitch/not-safe-to-clone.ll

+; RUN: opt -passes='unswitch<nontrivial>' %s -S | FileCheck %s
+
+declare i1 @foo()
+
+; CHECK: define {{.*}} @mem_cgroup_node_nr_lru_pages
+define i32 @mem_cgroup_node_nr_lru_pages(i1 %tree) {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %if.end8, %entry
+  br i1 %tree, label %if.end8, label %if.else
+
+if.else:                                          ; preds = %for.cond
+  callbr void asm sideeffect ".pushsection __jump_table,  \22aw\22 \0A\09.popsection \0A\09", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@mem_cgroup_node_nr_lru_pages, %for.cond5))
+          to label %if.end8 [label %for.cond5]
+
+for.cond5:                                        ; preds = %if.else, %for.cond5
+  %call6 = call i1 @foo()
+  br i1 %call6, label %if.end8.loopexit, label %for.cond5
+
+if.end8.loopexit:                                 ; preds = %for.cond5
+  br label %if.end8
+
+if.end8:                                          ; preds = %if.end8.loopexit, %if.else, %for.cond
+  br label %for.cond
+}
+