added some commentary notes for the review.

Modified the tests to work at opt. Replaced initialization of return values with literals to simplify the IR.

Formatted the RUN steps.

I'll format the RUN steps as suggested but with respect to the other suggestions, I prefer to keep the test as-is.

I think the review title "ByVal formal argument support: passing on the stack" implies that it covers the reg/stack split case. I noted you described it better in the review description. I think a slightly more descriptive abstract would be useful for the commit message.

Added MIR expect output for tests that expect calls to memcpy to ensure the memcpy calls are generated independently from caller register setup.

Addressed unnecessary iterator check while looping on expect byval argument registers.

Rebased to account for changes committed in D76902.

My comments are only suggestions/discussion and can be addressed as you see fit on the commit.

I had intended to move the 5-8 byte byval callee cases out of aix64-cc-byval.ll and put them here. I don't want aix64-cc-byval.ll to exist when we're done. If the 5-8 byte tests aren't useful for the callee I'd rather delete them. They were of interest to the caller handling to make sure that we implement the left justification correctly.

Fixed expected output for 64-byte byval arg test.

Added a test for the homogeneous float stuct Sean suggested.

Tweaked the lambda capture on GetLoad.

Discussion ensued post-approval regarding use of CHECK-DAG on the ADJSTACKDOWN lines so I thought I'd post the final version before committing.

Clang format.

Forgot the 64 byte byval test.

I split out some of the suggested test improvements to https://reviews.llvm.org/D76875

Summary:
-Test tidy ups
-FYI that I noted a 32-bit 8 byte aligned by-val (struct S { double d; }) that will need special handling in future
-Requested some comments in the code / tests to explain the suboptimal codegen, which otherwise looks odd.

I still have to update the tests as per Sean's suggestion but I'll post the updated code for now.

Updated code to address comments.

Addressed Sean's comments:
-General test tidy up, remove an unnecessary assertion, tweaked a variable rename.

-Added skipping of 0 size by val args.
-Tidied up tests.

-Add CC_AIX logic to pass over 0 size by-val arguments.
-Tidy the by val tests added.
-Some expected output changes required in aix-cc-byval.ll related to use of ZEXTLOAD now that I've added the run step.

Added the missing run steps to aix-cc-byval.ll

-Use ZEXTLOAD rather than EXTLOAD. This changed expected output because we're able to use rotate left word immediate with mask form instructions.
-Split byval cc tests into new test files.

1. Split byval cc tests into a separate file.
2. Use ZEXTLOAD

One comment update. Should be ready to commit.

Some of the prior test changes have been committed as NFC patches. I've updated the diff and readers can now see the instruction changes for widening arguments to register width before writing to the stack.

I think we need some changes to ensure frame objects are created with their proper size and not word size. i64 is passed as two i32s but we should coalesce these into a single 8-byte object. Like-wise arguments smaller than word size are generating frame objects that are word size.

In D75126#1891669, @ZarkoCA wrote:

It would be better if the test cleanup was done in a separate patch to make it clearer what the expected output of the patch. But that said, it's not difficult to find added testcase and it would be ok with me to leave the modified test cases.

In D74015#1882933, @sfertile wrote:

Patch LGTM as far a formatting/naming/testing etc. C++ specifics is outside my wheelhouse though, so I can't confirm things like the tail padding rules are correct for AIX. Because of that I'm not comfortable being the one to accept the patch.

All minor comments except for the concern on whether or not a truncation node is needed when ValSize < LocSize.

From my perspective, the only issue holding this up is settling on the name. I'd like to hammer that out and get this committed.

I believe the current patch is sound and correct. I had one test suggestion and one suggestion to use a lambda to reuse some of the cr spill logic.

Code semantics look ok. If you can post a revision to address the minor concerns I need to have one more pass-through of the test updates in detail.

I would like to see some test coverage for the AIX CRSave assembly, whether we adapt the tests you've already updated or add a separate test. It seems 64-bit PPC Linux has a better way of handling a single CR save. If we choose not to use the same approach on AIX I'd like to understand why.

Looks fine, but we need to settle on the name for the ABI. My preference would be "XLC++11", or perhaps "XLCXX11" (I propose the latter because of the common reference CXXABI.)

This concludes my initial review. Mostly tidy up comments. The logic seems correct so semantically I think it's ok (for the incremental support that it adds.) I'll see if I can break it on the next round.

Did a passthrough of the test changes with a number of comments. I expect you wanted to make some test updates, nonetheless I thought I'd document what I'd like to see in the test coverage. Will review code changes next.

There's some trailing whitespace that emits warnings when I apply the patch. Probably a good idea to tidy it up. I'll continue to review the content of that patch.

Used custom mem to tag memLocs for float args that also pass in register so the LowerFormalArguments_AIX can skip them easily.

Addressed Sean's comments. Notable change: when a float arg passes in FPR as well as PSA memory we will mark the memLoc custom. LowerFormalArguments_AIX can ignore custom memLoc. If a float arg passes in PSA memory and there was no FPR available, then the memLoc will be normal (non-custom.) I've updated LowerFormalArguments_AIX such that it will pass over custom memloc but continue to report_fatal_error on non-custom memloc. Zarko will have a patch coming to remove the report_fatal_error and implement memLoc for LowerFormalArguments_AIX.

Only minor test questions / requests. The change looks ok. I think we should explain the stack adj differences with the XL compiler before committing, specifically: do we need to allocate the PSA for the current frame even if there are no calls?

Some early issues. I need more time to go through the AIX assembly in detail to ensure it's correct.

Added a test for a double that passes in fpr and 2 gpr.

Preliminary comments. I still need to go through the test changes more thoroughly.

Already approved but I've addressed the nits and posted the final commit here because I'll need someone to commit it for me.

Rewrote comments in aix-cc-altivec.ll test, fixed the vector diagnostic bug Zarko pointed out and addressed small nits.

I've decided to use custom regs for all gpr inits for float varargs. I discovered that f64 passing in 64-bit gpr was missing a required bitcast to int to handle the initialization. The new logic ensures the bitcast occurs for any custom handling. Custom handling is only expected for gpr inits of float args.

Recent problems have come up generating the GPR initializations for floating point args, which may require some rework. Hold off on review until revision is posted.

Minor tweaks and test upates to come. I'll add an assembly test to validate that we don't need to store to memory to initiailze the two GPRs for f64 in PPC32 mode on newer hardware.

Clang formatted the change and updated the float reg to use LocVT.