[llvm] [VPlan] Simplify VPPhi with all-equal incoming (PR #157414)

[Ramkumar Ramachandra via llvm-commits]

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@@ -9568,6 +9568,16 @@ preparePlanForEpilogueVectorLoop(VPlan &Plan, Loop *L,
   // vectorizing the epilogue loop.
   for (VPRecipeBase &R : Header->phis()) {
     if (auto *IV = dyn_cast<VPCanonicalIVPHIRecipe>(&R)) {
+      // If we didn't find any PHIs, due to a simplification where all incoming
+      // values were equal (and necessarily zero), it means that the vector trip
+      // count is zero.
+      // TODO: We should not choose VF * UF so the main vector loop is known to
+      // be dead.
+      if (L->getLoopPreheader()->phis().empty()) {
+        EPI.VectorTripCount = ConstantInt::get(IV->getScalarType(), 0);
+        continue;
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artagnon wrote:

Oh, it looks like @fhahn's patch isn't sufficient to insert the assert: the epilogue scalar.ph is still not removed. This is the test that fails:

```diff
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/epilog-vectorization-factors.ll b/llvm/test/Transforms/LoopVectorize/AArch64/epilog-vectorization-factors.ll
index b83d3af3a0d6..8586e126b75b 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/epilog-vectorization-factors.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/epilog-vectorization-factors.ll
@@ -374,10 +374,9 @@ define void @small_trip_count_loop(ptr %arg, ptr %arg2) {
 ; CHECK:       vec.epilog.iter.check:
 ; CHECK-NEXT:    br i1 false, label [[VEC_EPILOG_SCALAR_PH]], label [[VEC_EPILOG_PH]]
 ; CHECK:       vec.epilog.ph:
-; CHECK-NEXT:    [[VEC_EPILOG_RESUME_VAL:%.*]] = phi i32 [ 0, [[VEC_EPILOG_ITER_CHECK]] ], [ 0, [[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
 ; CHECK-NEXT:    br label [[VEC_EPILOG_VECTOR_BODY:%.*]]
 ; CHECK:       vec.epilog.vector.body:
-; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ [[VEC_EPILOG_RESUME_VAL]], [[VEC_EPILOG_PH]] ], [ [[INDEX_NEXT:%.*]], [[VEC_EPILOG_VECTOR_BODY]] ]
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VEC_EPILOG_PH]] ], [ [[INDEX_NEXT:%.*]], [[VEC_EPILOG_VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[TMP11:%.*]] = getelementptr inbounds i8, ptr [[ARG]], i32 [[INDEX]]
 ; CHECK-NEXT:    [[WIDE_LOAD7:%.*]] = load <16 x i8>, ptr [[TMP11]], align 1
 ; CHECK-NEXT:    [[TMP12:%.*]] = add <16 x i8> [[WIDE_LOAD7]], splat (i8 10)
```

https://github.com/llvm/llvm-project/pull/157414