[LV] Do not try to sink dead instructions.

Dead instructions do not need to be sunk. Currently we try and record
the recipies for them, but there are no recipes emitted for them and
there's nothing to sink. They can be removed from SinkAfter while
marking them for recording.

Fixes PR44634.

Reviewers: rengolin, hsaito, fhahn, Ayal, gilr

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D73423

(cherry picked from commit a911fef)

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h

@@ -279,9 +279,10 @@ class LoopVectorizationPlanner {
 
   /// Build a VPlan using VPRecipes according to the information gather by
   /// Legal. This method is only used for the legacy inner loop vectorizer.
-  VPlanPtr
-  buildVPlanWithVPRecipes(VFRange &Range, SmallPtrSetImpl<Value *> &NeedDef,
-                          SmallPtrSetImpl<Instruction *> &DeadInstructions);
+  VPlanPtr buildVPlanWithVPRecipes(
+      VFRange &Range, SmallPtrSetImpl<Value *> &NeedDef,
+      SmallPtrSetImpl<Instruction *> &DeadInstructions,
+      const DenseMap<Instruction *, Instruction *> &SinkAfter);
 
   /// Build VPlans for power-of-2 VF's between \p MinVF and \p MaxVF inclusive,
   /// according to the information gathered by Legal when it checked if it is

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7118,25 +7118,29 @@ void LoopVectorizationPlanner::buildVPlansWithVPRecipes(unsigned MinVF,
   SmallPtrSet<Instruction *, 4> DeadInstructions;
   collectTriviallyDeadInstructions(DeadInstructions);
 
+  DenseMap<Instruction *, Instruction *> &SinkAfter = Legal->getSinkAfter();
+  // Dead instructions do not need sinking. Remove them from SinkAfter.
+  for (Instruction *I : DeadInstructions)
+    SinkAfter.erase(I);
+
   for (unsigned VF = MinVF; VF < MaxVF + 1;) {
     VFRange SubRange = {VF, MaxVF + 1};
-    VPlans.push_back(
-        buildVPlanWithVPRecipes(SubRange, NeedDef, DeadInstructions));
+    VPlans.push_back(buildVPlanWithVPRecipes(SubRange, NeedDef,
+                                             DeadInstructions, SinkAfter));
     VF = SubRange.End;
   }
 }
 
 VPlanPtr LoopVectorizationPlanner::buildVPlanWithVPRecipes(
     VFRange &Range, SmallPtrSetImpl<Value *> &NeedDef,
-    SmallPtrSetImpl<Instruction *> &DeadInstructions) {
+    SmallPtrSetImpl<Instruction *> &DeadInstructions,
+    const DenseMap<Instruction *, Instruction *> &SinkAfter) {
 
   // Hold a mapping from predicated instructions to their recipes, in order to
   // fix their AlsoPack behavior if a user is determined to replicate and use a
   // scalar instead of vector value.
   DenseMap<Instruction *, VPReplicateRecipe *> PredInst2Recipe;
 
-  DenseMap<Instruction *, Instruction *> &SinkAfter = Legal->getSinkAfter();
-
   SmallPtrSet<const InterleaveGroup<Instruction> *, 1> InterleaveGroups;
 
   VPRecipeBuilder RecipeBuilder(OrigLoop, TLI, Legal, CM, Builder);

llvm/test/Transforms/LoopVectorize/first-order-recurrence.ll

@@ -607,3 +607,41 @@ cond.end:                                         ; preds = %for.body4, %cond.tr
 for.end12.loopexit:                               ; preds = %cond.end
   ret void
 }
+
+; Dead instructions, like the exit condition are not part of the actual VPlan
+; and do not need to be sunk. PR44634.
+define void @sink_dead_inst() {
+; SINK-AFTER-LABEL: define void @sink_dead_inst(
+; SINK-AFTER-LABEL: vector.body:                                      ; preds = %vector.body, %vector.ph
+; SINK-AFTER-NEXT:    %index = phi i32 [ 0, %vector.ph ], [ %index.next, %vector.body ]
+; SINK-AFTER-NEXT:    %vec.ind = phi <4 x i16> [ <i16 -27, i16 -26, i16 -25, i16 -24>, %vector.ph ], [ %vec.ind.next, %vector.body ]
+; SINK-AFTER-NEXT:    %vector.recur = phi <4 x i16> [ <i16 undef, i16 undef, i16 undef, i16 0>, %vector.ph ], [ %3, %vector.body ]
+; SINK-AFTER-NEXT:    %vector.recur2 = phi <4 x i32> [ <i32 undef, i32 undef, i32 undef, i32 -27>, %vector.ph ], [ %1, %vector.body ]
+; SINK-AFTER-NEXT:    %0 = add <4 x i16> %vec.ind, <i16 1, i16 1, i16 1, i16 1>
+; SINK-AFTER-NEXT:    %1 = zext <4 x i16> %0 to <4 x i32>
+; SINK-AFTER-NEXT:    %2 = shufflevector <4 x i32> %vector.recur2, <4 x i32> %1, <4 x i32> <i32 3, i32 4, i32 5, i32 6>
+; SINK-AFTER-NEXT:    %3 = add <4 x i16> %0, <i16 5, i16 5, i16 5, i16 5>
+; SINK-AFTER-NEXT:    %4 = shufflevector <4 x i16> %vector.recur, <4 x i16> %3, <4 x i32> <i32 3, i32 4, i32 5, i32 6>
+; SINK-AFTER-NEXT:    %5 = sub <4 x i16> %4, <i16 10, i16 10, i16 10, i16 10>
+; SINK-AFTER-NEXT:    %index.next = add i32 %index, 4
+; SINK-AFTER-NEXT:    %vec.ind.next = add <4 x i16> %vec.ind, <i16 4, i16 4, i16 4, i16 4>
+; SINK-AFTER-NEXT:    %6 = icmp eq i32 %index.next, 40
+; SINK-AFTER-NEXT:    br i1 %6, label %middle.block, label %vector.body, !llvm.loop !43
+;
+entry:
+  br label %for.cond
+
+for.cond:
+  %iv = phi i16 [ -27, %entry ], [ %iv.next, %for.cond ]
+  %rec.1 = phi i16 [ 0, %entry ], [ %rec.1.prev, %for.cond ]
+  %rec.2 = phi i32 [ -27, %entry ], [ %rec.2.prev, %for.cond ]
+  %use.rec.1 = sub i16 %rec.1, 10
+  %cmp = icmp eq i32 %rec.2, 15
+  %iv.next = add i16 %iv, 1
+  %rec.2.prev = zext i16 %iv.next to i32
+  %rec.1.prev = add i16 %iv.next, 5
+  br i1 %cmp, label %for.end, label %for.cond
+
+for.end:
+  ret void
+}