[LoopUnswitch] Avoid partially unswitching too aggressively.

This patch adds additional checks to avoid partial unswitching
in cases where it won't be profitable, e.g. because the path directly
exits the loop anyways.

Author: fhahn

llvm/lib/Transforms/Scalar/LoopUnswitch.cpp

@@ -725,6 +725,11 @@ hasPartialIVCondition(Loop *L, MemorySSA &MSSA, AAResults *AA) {
           WorkList.append(succ_begin(Current), succ_end(Current));
         }
 
+        // Require at least 2 blocks on a path through the loop. This skips
+        // paths that directly exit the loop.
+        if (Seen.size() < 2)
+          return false;
+
         // Next, check if there are any MemoryDefs that are on the path through
         // the loop (in the Seen set) and they may-alias any of the locations in
         // AccessedLocs. If that is the case, they may modify the condition and
@@ -761,6 +766,11 @@ hasPartialIVCondition(Loop *L, MemorySSA &MSSA, AAResults *AA) {
         return true;
       };
 
+  // If we branch to the same successor, partial unswitching will not be
+  // beneficial.
+  if (TI->getSuccessor(0) == TI->getSuccessor(1))
+    return {};
+
   if (HasNoClobbersOnPath(TI->getSuccessor(0), L->getHeader(), AccessesToCheck))
     return {ToDuplicate, ConstantInt::getTrue(TI->getContext())};
   if (HasNoClobbersOnPath(TI->getSuccessor(1), L->getHeader(), AccessesToCheck))

llvm/test/Transforms/LoopUnswitch/partial-unswitch.ll

@@ -831,9 +831,7 @@ exit:
 define i32 @no_partial_unswitch_true_successor_exit(i32* %ptr, i32 %N) {
 ; CHECK-LABEL: @no_partial_unswitch_true_successor_exit
 ; CHECK-LABEL: entry:
-; CHECK-NEXT:   [[LV:%[0-9]+]] = load i32, i32* %ptr, align 4
-; CHECK-NEXT:   [[C:%[0-9]+]] = icmp eq i32 [[LV]], 100
-; CHECK-NEXT:   br i1 [[C]],
+; CHECK-NEXT:   br label %loop.header
 ;
 entry:
   br label %loop.header
@@ -860,9 +858,7 @@ exit:
 define i32 @no_partial_unswitch_true_same_successor(i32* %ptr, i32 %N) {
 ; CHECK-LABEL: @no_partial_unswitch_true_same_successor
 ; CHECK-LABEL: entry:
-; CHECK-NEXT:   [[LV:%[0-9]+]] = load i32, i32* %ptr, align 4
-; CHECK-NEXT:   [[C:%[0-9]+]] = icmp eq i32 [[LV]], 100
-; CHECK-NEXT:   br i1 [[C]],
+; CHECK-NEXT:   br label %loop.header
 ;
 entry:
   br label %loop.header