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

Fix PR47390.

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.

Differential Revision: https://reviews.llvm.org/D92017

Author: azaks

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7132,6 +7132,12 @@ void LoopVectorizationPlanner::collectTriviallyDeadInstructions(
   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));
         }))

llvm/test/Transforms/LoopVectorize/dead_instructions.ll

@@ -40,3 +40,32 @@ 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
+}