[llvm] [VPlan] Hoist loads with invariant addresses using noalias metadata. (PR #166247)

[David Sherwood via llvm-commits]

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@@ -156,15 +156,15 @@ define void @trunc_store(ptr %dst, ptr %src, i16 %x) #1 {
 ; DEFAULT-NEXT:    [[VEC_EPILOG_RESUME_VAL:%.*]] = phi i64 [ 992, [[VEC_EPILOG_ITER_CHECK]] ], [ 0, [[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
 ; DEFAULT-NEXT:    [[BROADCAST_SPLATINSERT4:%.*]] = insertelement <8 x i16> poison, i16 [[X]], i64 0
 ; DEFAULT-NEXT:    [[BROADCAST_SPLAT5:%.*]] = shufflevector <8 x i16> [[BROADCAST_SPLATINSERT4]], <8 x i16> poison, <8 x i32> zeroinitializer
-; DEFAULT-NEXT:    [[TMP7:%.*]] = trunc <8 x i16> [[BROADCAST_SPLAT5]] to <8 x i8>
-; DEFAULT-NEXT:    br label [[VEC_EPILOG_VECTOR_BODY:%.*]]
-; DEFAULT:       vec.epilog.vector.body:
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david-arm wrote:

Hmm, I must be missing something because I don't see any metadata on the original scalar loop:
```
entry:
  br label %loop
loop:
  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
  %x.ext = zext i16 %x to i64
  %l = load i64, ptr %src, align 8
  %and = and i64 %l, %x.ext
  %trunc = trunc i64 %and to i8
  %gep = getelementptr i8, ptr %dst, i64 %iv
  store i8 %trunc, ptr %gep, align 1
  %iv.next = add i64 %iv, 1
  %ec = icmp eq i64 %iv.next, 1000
  br i1 %ec, label %exit, label %loop
exit:
  ret void
```

and the function pointer arguments do not have `noalias` attached either.

Is it the loop vectoriser itself that is first adding the metadata, then using it to perform the transformation?

https://github.com/llvm/llvm-project/pull/166247