[PATCH] D104689: [LoopDeletion] Benefit from branches by undef conditions when symbolically executing 1st iteration

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev updated this revision to Diff 353556.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104689/new/

https://reviews.llvm.org/D104689

Files:
  llvm/lib/Transforms/Scalar/LoopDeletion.cpp
  llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll


Index: llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
===================================================================
--- llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
+++ llvm/test/Transforms/LoopDeletion/eval_first_iteration.ll
@@ -881,20 +881,19 @@
   unreachable
 }
 
-; TODO: We can break the backedge here by exploiting undef.
 define i32 @test_multiple_pred_undef_3() {
 ; CHECK-LABEL: @test_multiple_pred_undef_3(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[SUM:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[SUM_NEXT:%.*]], [[BACKEDGE:%.*]] ]
+; CHECK-NEXT:    [[SUM:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    [[SUB:%.*]] = sub i32 4, [[SUM]]
 ; CHECK-NEXT:    [[IS_POSITIVE:%.*]] = icmp sgt i32 [[SUB]], 0
 ; CHECK-NEXT:    br i1 [[IS_POSITIVE]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
 ; CHECK:       if.true:
 ; CHECK-NEXT:    br i1 undef, label [[IF_TRUE_1:%.*]], label [[IF_TRUE_2:%.*]]
 ; CHECK:       if.true.1:
-; CHECK-NEXT:    br label [[BACKEDGE]]
+; CHECK-NEXT:    br label [[BACKEDGE:%.*]]
 ; CHECK:       if.true.2:
 ; CHECK-NEXT:    br label [[BACKEDGE]]
 ; CHECK:       if.false:
@@ -905,9 +904,11 @@
 ; CHECK-NEXT:    br label [[BACKEDGE]]
 ; CHECK:       backedge:
 ; CHECK-NEXT:    [[MERGE_PHI:%.*]] = phi i32 [ 0, [[IF_FALSE_1]] ], [ 0, [[IF_FALSE_2]] ], [ undef, [[IF_TRUE_1]] ], [ undef, [[IF_TRUE_2]] ]
-; CHECK-NEXT:    [[SUM_NEXT]] = add i32 [[SUM]], [[MERGE_PHI]]
+; CHECK-NEXT:    [[SUM_NEXT:%.*]] = add i32 [[SUM]], [[MERGE_PHI]]
 ; CHECK-NEXT:    [[LOOP_COND:%.*]] = icmp ne i32 [[SUM_NEXT]], 4
-; CHECK-NEXT:    br i1 [[LOOP_COND]], label [[LOOP]], label [[DONE:%.*]]
+; CHECK-NEXT:    br i1 [[LOOP_COND]], label [[BACKEDGE_LOOP_CRIT_EDGE:%.*]], label [[DONE:%.*]]
+; CHECK:       backedge.loop_crit_edge:
+; CHECK-NEXT:    unreachable
 ; CHECK:       done:
 ; CHECK-NEXT:    [[SUM_NEXT_LCSSA:%.*]] = phi i32 [ [[SUM_NEXT]], [[BACKEDGE]] ]
 ; CHECK-NEXT:    ret i32 [[SUM_NEXT_LCSSA]]
Index: llvm/lib/Transforms/Scalar/LoopDeletion.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopDeletion.cpp
+++ llvm/lib/Transforms/Scalar/LoopDeletion.cpp
@@ -333,13 +333,35 @@
     // Can we prove constant true or false for this condition?
     LHS = getValueOnFirstIteration(LHS, FirstIterValue, SQ);
     RHS = getValueOnFirstIteration(RHS, FirstIterValue, SQ);
-    auto *KnownCondition =
-        dyn_cast_or_null<ConstantInt>(SimplifyICmpInst(Pred, LHS, RHS, SQ));
+    auto *KnownCondition = SimplifyICmpInst(Pred, LHS, RHS, SQ);
     if (!KnownCondition) {
+      // Failed to simplify.
       MarkAllSuccessorsLive(BB);
       continue;
     }
-    if (KnownCondition->isAllOnesValue())
+    if (isa<UndefValue>(KnownCondition)) {
+      // TODO: According to langref, branching by undef is undefined behavior.
+      // It means that, theoretically, we should be able to just continue
+      // without marking any successors as live. However, we are not certain
+      // how correct our compiler is at handling such cases. So we are being
+      // very conservative here.
+      //
+      // If there is a non-loop successor, always assume this branch leaves the
+      // loop. Otherwise, arbitrarily take IfTrue.
+      //
+      // Once we are certain that branching by undef is handled correctly by
+      // other transforms, we should not mark any successors live here.
+      if (L->contains(IfTrue) && L->contains(IfFalse))
+        MarkLiveEdge(BB, IfTrue);
+      continue;
+    }
+    auto *ConstCondition = dyn_cast<ConstantInt>(KnownCondition);
+    if (!ConstCondition) {
+      // Non-constant condition, cannot analyze any further.
+      MarkAllSuccessorsLive(BB);
+      continue;
+    }
+    if (ConstCondition->isAllOnesValue())
       MarkLiveEdge(BB, IfTrue);
     else
       MarkLiveEdge(BB, IfFalse);


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D104689.353556.patch
Type: text/x-patch
Size: 3925 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20210622/9944c9bc/attachment.bin>