Update description

Add two more test cases

Update to address review comments

@erichkeane Thanks for the comments!

More examples have been identified for the adaptation of IBM XL compiler's -qstatsym option for the hot patch use case, which was mentioned previously. Therefore, this option is extended to cover the following cases.

1. Function-local static variables
2. Thread-local variables

The test case is updated to add more coverage accordingly.
Since the option is now covering not only the variables with static storage duration, but also those with thread storage duration, the option name is changed to -fkeep-persistent-storage-variables. Any suggestions would be appreciated.

Update the option text to be more descriptive

Gentle ping.

Refactor as suggested

This is useful in keeping the static variables in a patchable function (https://clang.llvm.org/docs/AttributeReference.html#patchable-function-entry), so that they can be directly addressed by a hot patch when the optimization to merge them is enabled (https://llvm.org/docs/doxygen/GlobalMerge_8cpp_source.html).

In D147964#4336010, @fhahn wrote:

Thanks for the reproducers. This was an issue with epilogue vectorization and I pushed a workaround for now: 3d4eed01338d. I'll work on migrating the code to generate induction resume values based on VPlan ASAP so we can remove the workaround.

We also see the same error introduced by this patch. A reproducer:

$> opt -passes='loop-vectorize' -epilogue-vectorization-force-VF=4 test.ll

I open https://github.com/llvm/llvm-project/issues/58454 for a new failed assertion on SCEV verify in LoopUnrollAndJam, starting with this patch.
Thanks!

LGTM

Added some minor NFC comments.

When the loop idiom transformation processes a memset instruction in a loop, currently it only handles the memset with a compile-time constant size. The motivation of this work is to relax this limitation, so that a memset with a variable size in a loop may still be processed and promoted to a larger memset if it passes all the eligibility checks. Performance-wise, promoting the memset in a loop to a larger memset reduces the number of calls to memset; hence reducing the overall call overhead.
A similar technique may also apply to the memcpy with a variable size in a loop.

Removed "if (SI)" as comment suggested. Thanks for the review!