[LoopDeletion] Consider infinite loops alive, unless mustprogress.

The current loop or any of its sub-loops may be infinite. Unless the
function or the loops are marked as mustprogress, this in itself makes
the loop *not* dead.

This patch moves the logic to check whether the current loop is finite
or mustprogress to `isLoopDead` and also extends it to check the
sub-loops. This should fix PR50511.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D103382

diff --git a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
index fd0e983..62b145b 100644 (file)
--- a/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -100,6 +100,30 @@ static bool isLoopDead(Loop *L, ScalarEvolution &SE,
           return I.mayHaveSideEffects() && !I.isDroppable();
         }))
       return false;
+
+  // The loop or any of its sub-loops looping infinitely is legal. The loop can
+  // only be considered dead if either
+  // a. the function is mustprogress.
+  // b. all (sub-)loops are mustprogress or have a known trip-count.
+  if (L->getHeader()->getParent()->mustProgress())
+    return true;
+
+  SmallVector<Loop *, 8> WorkList;
+  WorkList.push_back(L);
+  while (!WorkList.empty()) {
+    Loop *Current = WorkList.pop_back_val();
+    if (hasMustProgress(Current))
+      continue;
+
+    const SCEV *S = SE.getConstantMaxBackedgeTakenCount(Current);
+    if (isa<SCEVCouldNotCompute>(S)) {
+      LLVM_DEBUG(
+          dbgs() << "Could not compute SCEV MaxBackedgeTakenCount and was "
+                    "not required to make progress.\n");
+      return false;
+    }
+    WorkList.append(Current->begin(), Current->end());
+  }
   return true;
 }
 
@@ -230,17 +254,6 @@ static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
                    : LoopDeletionResult::Unmodified;
   }
 
-  // Don't remove loops for which we can't solve the trip count unless the loop
-  // was required to make progress but has been determined to be dead.
-  const SCEV *S = SE.getConstantMaxBackedgeTakenCount(L);
-  if (isa<SCEVCouldNotCompute>(S) &&
-      !L->getHeader()->getParent()->mustProgress() && !hasMustProgress(L)) {
-    LLVM_DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount and was "
-                         "not required to make progress.\n");
-    return Changed ? LoopDeletionResult::Modified
-                   : LoopDeletionResult::Unmodified;
-  }
-
   LLVM_DEBUG(dbgs() << "Loop is invariant, delete it!");
   ORE.emit([&]() {
     return OptimizationRemark(DEBUG_TYPE, "Invariant", L->getStartLoc(),

diff --git a/llvm/test/Transforms/LoopDeletion/noop-loops-with-subloops.ll b/llvm/test/Transforms/LoopDeletion/noop-loops-with-subloops.ll
index 23eb324..715915b 100644 (file)
--- a/llvm/test/Transforms/LoopDeletion/noop-loops-with-subloops.ll
+++ b/llvm/test/Transforms/LoopDeletion/noop-loops-with-subloops.ll
@@ -158,7 +158,19 @@ exit:
 ; function/loop is mustprogress. Test case from PR50511.
 define void @inner_loop_may_be_infinite(i1 %c1, i1 %c2) {
 ; CHECK-LABEL: @inner_loop_may_be_infinite(
-; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK-NEXT:    br label [[LOOP1:%.*]]
+; CHECK:       loop1:
+; CHECK-NEXT:    br i1 [[C1:%.*]], label [[LOOP1_LATCH:%.*]], label [[LOOP2_PREHEADER:%.*]]
+; CHECK:       loop2.preheader:
+; CHECK-NEXT:    br label [[LOOP2:%.*]]
+; CHECK:       loop2:
+; CHECK-NEXT:    br i1 [[C2:%.*]], label [[LOOP1_LATCH_LOOPEXIT:%.*]], label [[LOOP2]]
+; CHECK:       loop1.latch.loopexit:
+; CHECK-NEXT:    br label [[LOOP1_LATCH]]
+; CHECK:       loop1.latch:
+; CHECK-NEXT:    br i1 false, label [[LOOP1_LATCH_LOOP1_CRIT_EDGE:%.*]], label [[EXIT:%.*]]
+; CHECK:       loop1.latch.loop1_crit_edge:
+; CHECK-NEXT:    unreachable
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -256,7 +268,21 @@ exit:
 ; mustprogress and can be removed.
 define void @loop2_mustprogress_but_not_sibling_loop(i1 %c1, i1 %c2, i1 %c3) {
 ; CHECK-LABEL: @loop2_mustprogress_but_not_sibling_loop(
-; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK-NEXT:    br label [[LOOP1:%.*]]
+; CHECK:       loop1:
+; CHECK-NEXT:    br i1 [[C1:%.*]], label [[LOOP1_LATCH:%.*]], label [[LOOP2_PREHEADER:%.*]]
+; CHECK:       loop2.preheader:
+; CHECK-NEXT:    br label [[LOOP3_PREHEADER:%.*]]
+; CHECK:       loop3.preheader:
+; CHECK-NEXT:    br label [[LOOP3:%.*]]
+; CHECK:       loop3:
+; CHECK-NEXT:    br i1 [[C3:%.*]], label [[LOOP1_LATCH_LOOPEXIT:%.*]], label [[LOOP3]]
+; CHECK:       loop1.latch.loopexit:
+; CHECK-NEXT:    br label [[LOOP1_LATCH]]
+; CHECK:       loop1.latch:
+; CHECK-NEXT:    br i1 false, label [[LOOP1_LATCH_LOOP1_CRIT_EDGE:%.*]], label [[EXIT:%.*]]
+; CHECK:       loop1.latch.loop1_crit_edge:
+; CHECK-NEXT:    unreachable
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -281,7 +307,27 @@ exit:
 define void @loop2_finite_but_child_is_not(i1 %c1, i1 %c2, i1 %c3) {
 ; CHECK-LABEL: @loop2_finite_but_child_is_not(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK-NEXT:    br label [[LOOP1:%.*]]
+; CHECK:       loop1:
+; CHECK-NEXT:    [[IV1:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV1_NEXT:%.*]], [[LOOP1_LATCH:%.*]] ]
+; CHECK-NEXT:    br i1 [[C1:%.*]], label [[LOOP1_LATCH]], label [[LOOP2_PREHEADER:%.*]]
+; CHECK:       loop2.preheader:
+; CHECK-NEXT:    br label [[LOOP2:%.*]]
+; CHECK:       loop2:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP2_LATCH:%.*]] ], [ 0, [[LOOP2_PREHEADER]] ]
+; CHECK-NEXT:    br label [[LOOP3:%.*]]
+; CHECK:       loop3:
+; CHECK-NEXT:    br i1 [[C2:%.*]], label [[LOOP2_LATCH]], label [[LOOP3]]
+; CHECK:       loop2.latch:
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw i32 [[IV]], 1
+; CHECK-NEXT:    [[C:%.*]] = icmp ugt i32 [[IV]], 200
+; CHECK-NEXT:    br i1 [[C]], label [[LOOP1_LATCH_LOOPEXIT:%.*]], label [[LOOP2]]
+; CHECK:       loop1.latch.loopexit:
+; CHECK-NEXT:    br label [[LOOP1_LATCH]]
+; CHECK:       loop1.latch:
+; CHECK-NEXT:    [[IV1_NEXT]] = add nuw i32 [[IV1]], 1
+; CHECK-NEXT:    [[C4:%.*]] = icmp ult i32 [[IV1_NEXT]], 200
+; CHECK-NEXT:    br i1 [[C4]], label [[LOOP1]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;

diff --git a/llvm/test/Transforms/LoopDeletion/unreachable-loops.ll b/llvm/test/Transforms/LoopDeletion/unreachable-loops.ll
index bd0d53f..6baaab6 100644 (file)
--- a/llvm/test/Transforms/LoopDeletion/unreachable-loops.ll
+++ b/llvm/test/Transforms/LoopDeletion/unreachable-loops.ll
@@ -317,7 +317,21 @@ exit:
 define void @test9(i64 %n) {
 ; CHECK-LABEL: @test9(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK-NEXT:    br label [[L1:%.*]]
+; CHECK:       L1.loopexit:
+; CHECK-NEXT:    br label [[L1_LOOPEXIT_SPLIT:%.*]]
+; CHECK:       L1.loopexit.split:
+; CHECK-NEXT:    unreachable
+; CHECK:       L1:
+; CHECK-NEXT:    br i1 true, label [[EXIT:%.*]], label [[L2_PREHEADER:%.*]]
+; CHECK:       L2.preheader:
+; CHECK-NEXT:    br label [[L3_PREHEADER:%.*]]
+; CHECK:       L3.preheader:
+; CHECK-NEXT:    [[Y_L2_LCSSA:%.*]] = phi i64 [ undef, [[L2_PREHEADER]] ]
+; CHECK-NEXT:    br label [[L3:%.*]]
+; CHECK:       L3:
+; CHECK-NEXT:    [[COND2:%.*]] = icmp slt i64 [[Y_L2_LCSSA]], [[N:%.*]]
+; CHECK-NEXT:    br i1 [[COND2]], label [[L3]], label [[L1_LOOPEXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;