[LV] Keep Primary Induction alive when folding tail by masking

[Ayal]

Summary

The primary induction should be considered alive when folding tail by masking, because it will be used by said masking; even when it may otherwise appear useless: feeding only its own 'bump', which is correctly considered dead, and as the 'bump' of another induction variable, which may wrongfully want to consider its bump = the primary induction, dead.

Fixes PR47390.

[dmgreen]

Seems good to me. Thanks

llvm/test/Transforms/LoopVectorize/dead_instructions.ll

49

if -> is

[fhahn]

LGTM, thanks!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

void LoopVectorizationPlanner::collectTriviallyDeadInstructions(
   }
 
   // We create new "steps" for induction variable updates to which the original
   // induction variables map. An original update instruction will be dead if
   // all its users except the induction variable are dead.
   for (auto &Induction : Legal->getInductionVars()) {
     PHINode *Ind = Induction.first;
     auto *IndUpdate = cast<Instruction>(Ind->getIncomingValueForBlock(Latch));
+
+    // If the tail is to be folded by masking, the primary induction variable,
+    // if exists, isn't dead: it will be used for masking. Don't kill it.
+    if (CM.foldTailByMasking() && IndUpdate == Legal->getPrimaryInduction())
+      continue;
+
     if (llvm::all_of(IndUpdate->users(), [&](User *U) -> bool {
           return U == Ind || DeadInstructions.count(cast<Instruction>(U));
         }))
       DeadInstructions.insert(IndUpdate);
 
     // We record as "Dead" also the type-casting instructions we had identified
     // during induction analysis. We don't need any handling for them in the
     // vectorized loop because we have proven that, under a proper runtime

llvm/test/Transforms/LoopVectorize/dead_instructions.ll

for.body:
   %i.next = add nuw nsw i64 %i, 1
   %cond = icmp slt i64 %i.next, %n
   br i1 %cond, label %for.body, label %for.end
 
 for.end:
   %tmp3  = phi i64 [ %tmp2, %for.body ]
   ret i64 %tmp3
 }
+
+
+; CHECK-LABEL: @pr47390
+;
+; This test ensures that the primary induction is not considered dead when
+; acting as the 'add' of another induction, and otherwise feeding only its own
+; 'add' (recognized earlier as 'dead'), when the tail of the loop is folded by
+; masking. Such masking uses the primary induction.
+;
+; CHECK:     vector.body:
+;
+define void @pr47390(i32 *%a) {
+entry:
+  br label %loop
+
+exit:
+  ret void
+
+loop:
+  %primary = phi i32 [ 0, %entry ], [ %primary_add, %loop ]
+  %use_primary = phi i32 [ -1, %entry ], [ %primary, %loop ]
+  %secondary = phi i32 [ 1, %entry ], [ %secondary_add, %loop ]
+  %primary_add = add i32 %primary, 1
+  %secondary_add = add i32 %secondary, 1
+  %gep = getelementptr inbounds i32, i32* %a, i32 %secondary
+  %load = load i32, i32* %gep, align 8
+  %cmp = icmp eq i32 %secondary, 5
+  br i1 %cmp, label %exit, label %loop
+}