[llvm] [VPlan] Handle FirstActiveLane when unrolling. (PR #145394)

[David Sherwood via llvm-commits]

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@@ -47,8 +74,28 @@ define i64 @same_exit_block_pre_inc_use1() #0 {
 ; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 510, [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[LOOP_END:%.*]], label [[SCALAR_PH]]
 ; CHECK:       vector.early.exit:
+; CHECK-NEXT:    [[TMP63:%.*]] = call i64 @llvm.vscale.i64()
+; CHECK-NEXT:    [[TMP42:%.*]] = mul nuw i64 [[TMP63]], 16
+; CHECK-NEXT:    [[TMP43:%.*]] = mul i64 1, [[TMP42]]
+; CHECK-NEXT:    [[TMP44:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.nxv16i1(<vscale x 16 x i1> [[TMP32]], i1 true)
+; CHECK-NEXT:    [[TMP62:%.*]] = mul i64 [[TMP42]], 3
+; CHECK-NEXT:    [[TMP45:%.*]] = add i64 [[TMP62]], [[TMP44]]
+; CHECK-NEXT:    [[TMP46:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.nxv16i1(<vscale x 16 x i1> [[TMP31]], i1 true)
+; CHECK-NEXT:    [[TMP58:%.*]] = mul i64 [[TMP42]], 2
+; CHECK-NEXT:    [[TMP50:%.*]] = add i64 [[TMP58]], [[TMP46]]
+; CHECK-NEXT:    [[TMP47:%.*]] = icmp ne i64 [[TMP50]], [[TMP43]]
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david-arm wrote:

I don't understand the logic behind this. We're comparing TMP43 (the number of lanes in a vector with VF=vscale x 16) with TMP50 ((2 * number of lanes in a vector) + first active lane of part 2). The result is always going to be false.

https://github.com/llvm/llvm-project/pull/145394