[llvm] [VPlan] Scalarize to first-lane-only directly on VPlan (PR #184267)

[Andrei Elovikov via llvm-commits]

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@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals none --version 6
+; RUN: opt -S < %s -p loop-vectorize -force-vector-width=4 | FileCheck %s
+
+define void @preserve_trunc_flags(ptr %p) {
+; CHECK-LABEL: define void @preserve_trunc_flags(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[VECTOR_PH:.*]]
+; CHECK:       [[VECTOR_PH]]:
+; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
+; CHECK:       [[VECTOR_BODY]]:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc nuw i64 [[INDEX]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr [8 x i8], ptr [[P]], i32 [[TMP0]]
+; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <4 x i64>, ptr [[TMP1]], align 8
+; CHECK-NEXT:    [[TMP2:%.*]] = add <4 x i64> [[WIDE_LOAD]], splat (i64 42)
+; CHECK-NEXT:    store <4 x i64> [[TMP2]], ptr [[TMP1]], align 8
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i64 [[INDEX_NEXT]], 100
+; CHECK-NEXT:    br i1 [[TMP3]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    br label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %header
+
+header:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %header ]
+  ; We can preserve nuw flags in the vectorized code:
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eas wrote:

The only non-auto-generated test in this diff seems to be dropping the comment which I'm not sure why. IMO, the comment brings value and doesn't cost anything.

https://github.com/llvm/llvm-project/pull/184267