When pragma of loop transformations is specified, follow-up metadata for
loops is generated after each transformation. On the LLVM side,
follow-up metadata is expected to be a list of properties, such as the
following:
```
!followup = !{!"llvm.loop.vectorize.followup_all", !mp, !isvectorized}
!mp = !{!"llvm.loop.mustprogress"}
!isvectorized = !{"llvm.loop.isvectorized"}
```
However, on the clang side, the generated metadata contains an MDNode
that has those properties, as shown below:
```
!followup = !{!"llvm.loop.vectorize.followup_all", !loop_id}
!loop_id = distinct !{!loop_id, !mp, !isvectorized}
!mp = !{!"llvm.loop.mustprogress"}
!isvectorized = !{"llvm.loop.isvectorized"}
```
According to the
[LangRef](https://llvm.org/docs/TransformMetadata.html#transformation-metadata-structure),
the LLVM side is correct. Due to this inconsistency, follow-up metadata
was not interpreted correctly, e.g., only one transformation is applied
when multiple pragmas are used.
This patch fixes clang side to emit followup metadata in correct format.
After unrolling, there may be additional simplifications that can be
applied. One example is removing SCALAR-STEPS for the first part where
only the first lane is demanded.
This removes redundant adds of 0 from a large number of tests (~200),
many which I am still working on updating.
In preparation for removing redundant WideIV steps added in
https://github.com/llvm/llvm-project/pull/119284.
PR: https://github.com/llvm/llvm-project/pull/123655
When pragma of loop transformations are encoded in LLVM IR, follow-up
metadata is used if multiple transformations are specified. They are
used to explicitly express the order of the transformations. However,
they are not properly processed on each transformation pass, so now only
the first one is attempted to be applied. This is a pre-commit to add a
test that causes the problem.
ref:
https://github.com/llvm/llvm-project/pull/127474#issuecomment-2717790398
