A new kind of git history

Discussions about rebase vs. merge are familiar territory for anybody with an interest in version control in general and git in particular. I want to finally give a more permanent home to an idea that I have expressed in the past and that I've occasionally seen others hint at in those discussions as well.

There are multiple camps in these discussions that have slightly different ideas about how and for what purposes git should be used.

The first major axis of disagreement is whether history needs to be git bisect-able. Outside of my own little hobby projects, I've always worked on projects for which bisectability was important. This has generally been because their scope was such that CI simply had no chance to cover all uses of the software. Bug reports that can be traced to regressions from weeks or even months ago are not frequent per se, but they have always been frequent enough to matter. git bisect is an essential tool for finding those regression points when they happen. Not all projects are like that, but for projects which are, the notion of an "atomic" change to the project's main development branch (or branches) is important.

The second major axis of disagreement is whether the development history of those "atomic" changes is important enough to preserve. The original git development workflow does not consider this to be important: developers send around and review multiple iterations of a change, but only the final version of the change goes into the permanent record of the git repository. I tend to agree with that view. I have very occasionally found it useful to go back and read through the comments on a pull request that lead to a change months ago (or the email thread in projects that use an email workflow), but I have never found it useful to look at older versions of a change.

Some people seem to really care about this kind of history, though. They're the people who argue for a merge-based workflow for pull requests on GitHub (but against force-pushes to the same) and who have built hacks for bisectability and readability of history like --first-parent. I'm calling that a hack because it does not compose well. It works for projects whose atomic change history is essentially linear, but it breaks down once the history becomes more complex. What if the project occasionally has a genuine merge? Now you'd want to apply --first-parent for most merge commits but not all. Things get messy.

One final observation. Even "my" camp, which generally prefers to discard development history leading up to the atomic change in a main development branch, does want to preserve a kind of history that is currently not captured by git's graph. git revert inserts the hash of the commit that was reverted into the commit message. Similarly, git cherry-pick optionally inserts the hash of the commit that was cherry-picked into the commit message.

In other words, there is a kind of history for whose preservation at least in some cases there seems to be a broad consensus. This kind of history is distinct from the history that is captured by commit parent links. Looked at in this light, the idea is almost obvious: make this history an explicit part of git commit metadata.

The gist of it would be this. Every commit has a (often empty) list of historical commit references explaining the origins of the diff that is implicitly represented by the commit; let's call them diff-parents. The diff-parents are an ordered list of references to commits, each of them with a "reverted" bit that can optionally be set.

The history of a revert can be encoded by making the reverted commit a diff-parent with the "reverted" bit set. The history of a cherry-pick can be encoded similarly, with the "reverted" bit clear. When we perform a simple rebase, each new commit has an obvious diff-parent. When commits are squashed during a rebase, the sequence of squashed commits becomes the list of diff-parents of the newly formed commit. GitHub users who like to preserve all development history can use the "squash" option when landing pull requests and have the history be preserved via the list of diff-parents. git commit --amend can similarly record the original commit as diff-parent.

This is an idea and not a fully fleshed-out plan. There are obviously a whole bunch of tricky questions to answer. For example: How does this all fit into git's admittedly often byzantine CLI? Can merge commits be diff-parents, and how would that work? Can we visualize the difference between a commit and its diff-parents? (Hint: Here's an idea)

Diff-parents are a source of potential information leaks. This is not a problem specific to the idea of diff-parents; it is a general problem with the idea of preserving all history. Imagine some developer accidentally commits some credentials in their local clone of a repository and then uses git commit --amend to remove them again. Whoops, the commit that contains the credentials is still referenced as a diff-parent. Will it (and therefore the credentials) be published to the world for all to see when the developers pushes their branch to GitHub? This needs to be taken seriously.

So there are a whole bunch of issues that would have to be addressed for this idea to work well. I believe those issues to be quite surmountable in principle, but given the state of git development (where GitHub, which to many is almost synonymous with git, doesn't even seem to be able to understand how git was originally meant to be used) I am not particularly optimistic. Still, I think it's a good idea, and I'd love to see it or something like it in git.

Building a HIP environment from scratch

HIP is a C++-based, single-source programming language for writing GPU code. "Single-source" means that a single source file can contain both the "host code" which runs on the CPU and the "device code" which runs on the GPU. In a sense, HIP is "CUDA for AMD", except that HIP can actually target both AMD and Nvidia GPUs.

If you merely want to use HIP, your best bet is to look at the documentation and download pre-built packages. (By the way, the documentation calls itself "ROCm" because that's what AMD calls its overall compute platform. It includes HIP, OpenCL, and more.)

I like to dig deep, though, so I decided I want to build at least the user space parts myself to the point where I can build a simple HelloWorld using a Clang from upstream LLVM. It's all open-source, after all!

It's a bit tricky, though, in part because of the kind of bootstrapping problems you usually get when building toolchains: Running the compiler requires runtime libraries, at least by default, but building the runtime libraries requires a compiler. Luckily, it's not quite that difficult, though, because compiling the host libraries doesn't require a HIP-enabled compiler - any C++ compiler will do. And while the device libraries do require a HIP- (and OpenCL-)enabled compiler, it is possible to build code in a "freestanding" environment where runtime libraries aren't available.

What follows is pretty much just a list of steps with running commentary on what the individual pieces do, since I didn't find an equivalent recipe in the official documentation. Of course, by the time you read this, it may well be outdated. Good luck!

Components need to be installed, but installing into some arbitrary prefix inside your $HOME works just fine. Let's call it $HOME/prefix. All packages use CMake and can be built using invocations along the lines of:

cmake -S . -B build -GNinja -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_INSTALL_PREFIX=$HOME/prefix -DCMAKE_PREFIX_PATH=$HOME/prefix
ninja -C build install

In some cases, additional variables need to be set.

Step 1: clang and lld

We're going to need a compiler and linker, so let's get llvm/llvm-project and build it with Clang and LLD enabled: -DLLVM_ENABLE_PROJECTS='clang;lld' -DLLVM_TARGETS_TO_BUILD='X86;AMDGPU'

Building LLVM is an art of its own which is luckily reasonably well documented, so I'm going to leave it at that.

Step 2: Those pesky cmake files

Build and install ROCm/rocm-cmake to avoid cryptic error messages down the road when building other components that use those CMake files without documenting the dependency clearly. Not rocket science, but man am I glad for GitHub's search function.

Step 3: libhsa-runtime64.so

This is the lowest level user space host-side library in the ROCm stack. Its services, as far as I understand them, include setting up device queues and loading "code objects" (device ELF files). All communication with the kernel driver goes through here.

Notably though, this library does not know how to dispatch a kernel! In the ROCm world, the so-called Architected Queueing Language is used for that. An AQL queue is setup with the help of the kernel driver (and that does go through libhsa-runtime64.so), and then a small ring buffer and a "door bell" associated with the queue are mapped into the application's virtual memory space. When the application wants to dispatch a kernel, it (or rather, a higher-level library like libamdhip64.so that it links against) writes an AQL packet into the ring buffer and "rings the door bell", which basically just means writing a new ring buffer head pointer to the door bell's address. The door bell virtual memory page is mapped to the device, so ringing the door bell causes a PCIe transaction (for us peasants; MI300A has slightly different details under the hood) which wakes up the GPU.

Anyway, libhsa-runtime64.so comes in two parts for what I am being told are largely historical reasons:

• ROCm/ROCT-Thunk-Interface
• ROCm/ROCR-Runtime; this one has one of those bootstrap issues and needs a -DIMAGE_SUPPORT=OFF

The former is statically linked into the latter...

Step 4: It which must not be named

For Reasons(tm), there is a fork of LLVM in the ROCm ecosystem, ROCm/llvm-project. Using upstream LLVM for the compiler seems to be fine and is what I as a compiler developer obviously want to do. However, this fork has an amd directory with a bunch of pieces that we'll need. I believe there is a desire to upstream them, but also an unfortunate hesitation from the LLVM community to accept something so AMD-specific.

In any case, the required components can each be built individually against the upstream LLVM from step 1:

• hipcc; this is a frontend for Clang which is supposed to be user-friendly, but at the cost of adding an abstraction layer. I want to look at the details under the hood, so I don't want to and don't have to use it; but some of the later components want it
• device-libs; as the name says, these are libraries of device code. I'm actually not quite sure what the intended abstraction boundary is between this one and the HIP libraries from the next step. I think these ones are meant to be tied more closely to the compiler so that other libraries, like the HIP library below, don't have to use __builtin_amdgcn_* directly? Anyway, just keep on building...
• comgr; the "code object manager". Provides a stable interface to LLVM, Clang, and LLD services, up to (as far as I understand it) invoking Clang to compile kernels at runtime. But it seems to have no direct connection to the code-related services in libhsa-runtime64.so.
  

That last one is annoying. It needs a -DBUILD_TESTING=OFF

Worse, it has a fairly large interface with the C++ code of LLVM, which is famously not stable. In fact, at least during my little adventure, comgr wouldn't build as-is against the LLVM (and Clang and LLD) build that I got from step 1. I had to hack out a little bit of code in its symbolizer. I'm sure it's fine.

Step 5: libamdhip64.so

Finally, here comes the library that implements the host-side HIP API. It also provides a bunch of HIP-specific device-side functionality, mostly by leaning on the device-libs from the previous step.

It lives in ROCm/clr, which stands for either Compute Language Runtimes or Common Language Runtime. Who knows. Either one works for me. It's obviously for compute, and it's common because it also contains OpenCL support.

You also need ROCm/HIP at this point. I'm not quite sure why stuff is split up into so many repositories. Maybe ROCm/HIP is also used when targeting Nvidia GPUs with HIP, but ROCm/CLR isn't? Not a great justification in my opinion, but at least this is documented in the README.

CLR also needs a bunch of additional CMake options: -DCLR_BUILD_HIP=ON -DHIP_COMMON_DIR=${checkout of ROCm/HIP} -DHIPCC_BIN_DIR=$HOME/prefix/bin

Step 6: Compiling with Clang

We can now build simple HIP programs with our own Clang against our own HIP and ROCm libraries:

clang -x hip --offload-arch=gfx1100 --rocm-path=$HOME/prefix -rpath $HOME/prefix/lib -lstdc++ HelloWorld.cpp
LD_LIBRARY_PATH=$HOME/prefix/lib ./a.out

Neat, huh?

Diff modulo base, a CLI tool to assist with incremental code reviews

One of the challenges of reviewing a lot of code is that many reviews require multiple iterations. I really don't want to do a full review from scratch on the second and subsequent rounds. I need to be able to see what has changed since last time.

I happen to work on projects that care about having a useful Git history. This means that authors of (without loss of generality) pull requests use amend and rebase to change commits and force-push the result. I would like to see the only the changes they made since my last review pass. Especially when the author also rebased onto a new version of the main branch, existing code review tools tend to break down.

Git has a little-known built-in subcommand, git range-diff, which I had been using for a while. It's pretty cool, really: It takes two ranges of commits, old and new, matches old and new commits, and then shows how they changed. The rather huge problem is that its output is a diff of diffs. Trying to make sense of those quickly becomes headache-inducing.

I finally broke down at some point late last year and wrote my own tool, which I'm calling diff-modulo-base. It allows you to look at the difference of the repository contents between old and new in the history below, while ignoring all the changes that are due to differences in the respective base versions A and B.

 

As a bonus, it actually does explicitly show differences between A and B that would have caused merge conflicts during rebase. This allows a fairly comfortable view of how merge conflicts were resolved.

I've been using this tool for a while now. While there are certainly still some rough edges and to dos, I did put a bunch more effort into it over the winter holidays and am now quite happy with it. I'm making it available for all to try at https://git.sr.ht/~nhaehnle/diff-modulo-base. Let me know if you find it useful!

Better integration with the larger code review flow?

One of the rough edges is that it would be great to integrate tightly with the GitHub notifications workflow. That workflow is surprisingly usable in that you can essentially treat the notifications as an inbox in which you can mark notifications as unread or completed, and can "mute" issues and pull requests all with keyboard shortcut.

What's missing in my workflow is a reliable way to remember the most recent version of a pull request that I have reviewed. My somewhat passable workaround for now is to git fetch before I do a round of reviews, and rely on the local reflog of remote refs. A Git alias allows me to say

git dmb-origin $pull_request_id

and have that become

git diff-modulo-base origin/main origin/pull/$pull_request_id/head@{1} origin/pull/$pull_request_id/head

which is usually what I want.

Ideally, I'd have a fully local way of interacting with GitHub notifications, which could then remember the reviewed version in a more reliable way. This ought to also fix the terrible lagginess of the web interface. But that's a rant for another time.

Rust

This is the first serious piece of code I've written in Rust. I have to say that experience has really been quite pleasant so far. Rust's tooling is pretty great, mostly thanks to the rust-analyzer LSP server.

The one thing I'd wish is that the borrow checker was able to better understand  "partial" borrows. I find it occasionally convenient to tie a bunch of data structures together in a general context structure, and helper functions on such aggregates can't express that they only borrow part of the structure. This can usually be worked around by changing data types, but the fact that I have to do that is annoying. It feels like having to solve a puzzle that isn't part of the inherent complexity of the underlying problem that the code is trying to solve.

And unlike, say, circular references or graph structures in general, where it's clear that expressing and proving the sort of useful lifetime facts that developers might intuitively reason about quickly becomes intractable, improving the support for partial borrows feels like it should be a tractable problem.

Software testing, and why I'm unhappy about it

Automated testing of software is great. Unfortunately, what's commonly considered best practice for how to integrate testing into the development flow is a bad fit for a lot of software. You know what I mean: You submit a pull request, some automated testing process is kicked off in the background, and some time later you get back a result that's either Green (all tests passed) or Red (at least one test failed). Don't submit the PR if the tests are red. Sounds good, doesn't quite work.

There is Software Under Test (SUT), the software whose development is the goal of what you're doing, and there's the Test Bench (TB): the tests themselves, but also additional relevant parts of the environment like perhaps some hardware device that the SUT is run against.

The above development practice works well when the SUT and TB are both defined by the same code repository and are developed together. And admittedly, that is the case for a lot of useful software. But it just so happens that I mostly tend to work on software where the SUT and TB are inherently split. Graphics drivers and shader compilers implement some spec (like Vulkan or Direct3D), and an important part of the TB are a conformance test suite and other tests, the bulk of which are developed separately from the driver itself. Not to mention the GPU itself and other software like the kernel mode driver. The point is, TB development is split from the SUT development and it is infeasible to make changes to them in lockstep.

Down with No Failures, long live No Regressions

Problem #1 with keeping all tests passing all the time is that tests can fail for reasons whose root cause is not an SUT change.

For example, a new test case is added to the conformance test suite, but that test happens to fail. Suddenly nobody can submit any changes anymore.

That clearly makes no sense, and because Tooling Sucks(tm), what folks typically do is maintain a manual list of test cases that are excluded from automated testing. This unblocks development, but are you going to remember to update that exclusion list? Bonus points if the exclusion list isn't even maintained in the same repository as the SUT, which just compounds the problem.

The situation is worse when you bring up a large new feature or perhaps a new version of the hardware supported by your driver (which is really just a super duper large new feature), where there is already a large body of tests written by somebody else. Development of the new feature may take months and typically is merged bit by bit over time. For most of that time, there are going to be some test failures. And that's fine!

Unfortunately, a typical coping mechanism is that automated testing for the feature is entirely disabled until the development process is complete. The consequences are dire, as regressions in relatively basic functionality can go unnoticed for a fairly long time.

And sometimes there are simply changes in the TB that are hard to control. Maybe you upgraded the kernel mode driver for your GPU on the test systems, and suddenly some weird corner case tests fail. Yes, you have to fix it somehow, but removing the test case from your automated testing process is almost always the wrong response.

In fact, failing tests are, given the right context, a good thing! Let's say a bug is discovered in a real application in the field. Somebody root causes the problem and writes a simplified reproducer. This reproducer should be added to the TB as soon as possible, even if it is going to fail initially!

To be fair, many of the common testing frameworks recognize this by allowing tests to be marked as "expected to fail". But they typically also assume that the TB can be changed in lockstep with the SUT and fall on their face when that isn't the case.

What is needed here is to treat testing as a truly continuous exercise, with some awareness by the automation of how test runs relate to the development history.

During day-to-day development, the important bit isn't that there are no failures. The important bit is that there are no regressions.

Automation ought to track which tests pass on the main development branch and provide pre-commit reports for pull requests relative to those results: Have there been any regressions? Have any tests been fixed? Block code submissions when they cause regressions, but don't block them for pre-existing failures, especially when those failures are caused by changes in the TB.

Changes to the TB should also be tested where possible, and when they cause regressions those should be investigated. But it is quite common that regressions caused by a TB change are legitimate and shouldn't block the TB change.

Sparse testing

Problem #2 is that good test coverage means that tests take a very long time to run.

Your first solution to this problem should be to parallelize and throw more hardware at it. Let's hope the people who control the purse care enough about quality.

There is sometimes also low-hanging fruit you should pick, like wasting lots of time in process (or other) startup and teardown overhead. Addressing that can be a double-edged sword. Changing a test suite from running every test case in a separate process to running multiple test cases sequentially in the same process reduces isolation between the tests and can therefore make the tests flakier. It can also expose genuine bugs, though, and so the effort is usually worth it.

But all these techniques have their limits.

Let me give you an example. Compilers tend to have lots of switches that subtly change the compiler's behavior without (intentionally) affecting correctness. How good is your test coverage of these?

Chances are, most of them don't see any real testing at all. You probably have a few hand-written test cases that exercise the options. But what you really should be doing is run an entire suite of end-to-end tests with each of the switches applied to make sure you aren't missing some interaction. And you really should be testing all combinations of switches as well.

The combinatorial explosion is intense. Even if you only have 10 boolean switches, testing them each individually without regressing the turn-around time of the test suite requires 11x more test system hardware. Testing all possible combinations requires 1024x more. Nobody has that kind of money.

The good news is that having extremely high confidence in the quality of your software doesn't require that kind of money. If we run the entire test suite a small number of times (maybe even just once!) and independently choose a random combination of switches for each test, then not seeing any regressions there is a great indication that there really aren't any regressions.

Why is that? Because failures are correlated! Test T failing with a default setting of switches is highly correlated with test T failing with some non-default switch S enabled.

This effect isn't restriced to taking the cross product of a test suite with a bunch of configuration switches. By design, an exhaustive conformance test suite is going to have many sets of tests with high failure correlation. For example, in the Vulkan test suite you might have a bunch of test cases that all do the same thing, but with a different combination of framebuffer format and blend function. When there is a regression affecting such tests, the specific framebuffer format or blend function might not matter at all, and all of the tests will regress. Or perhaps the regression is related to a specific framebuffer format, and so all tests using that format will regress regardless of the blend function that is used, and so on.

A good automated testing system would leverage these observations using statistical methods (aka machine learning).

Combinatorial explosion causes your full test suite to take months to run? No problem, treat testing as a genuinely continuous task. Have test systems continuously run random samplings of test cases on the latest version of the main development branch. Whenever a change is made to either the SUT or TB, switch over to testing that new version instead. When a failure is encountered, automatically determine if it is a regression by referring to earlier test results (of the exact same test if it has been run previously, or related tests otherwise) combined with a bisection over the code history.

Pre-commit testing becomes an interesting fuzzy problem. By all means have a small traditional test suite that is manually curated to run within a few minutes. But we can also apply the approach of running randomly sampled tests to pre-commit testing.

A good automated testing system would learn a statistical model of regressions and combine that with the test results obtained so far to provide an estimate of the likelihood of regression. As long as no regression is actually found, this likelihood will keep dropping as more tests are run, though it will not drop to 0 unless all tests are run (and the setup here was this would take months). The team can define a likelihood threshold that a change must reach before it can be committed based on the their appetite for risk and rate of development.

The statistical model should be augmented with source-level information about the change, such as keywords that appear in the diff and commit message and the set of files that was changed. After all, there ought to be some meaningful correlation between regressions in a raytracing test case and the fact that the regressing change affected a file with "raytracing" in its name. The model should then also be used to bias the random sampling of tests to be run to maximize the information extracted per effort spent on running test cases.

Some caveats

What I've described is largely motivated by the fact that the world is messier than commonly accepted testing "wisdom" allows. However, the world is too messy even for what I've described.

I haven't talked about flaky (randomly failing) tests at all, though a good automated testing system should be able to cope with them. Re-running a test in the same configuration is not black magic and can be used to confirm that a test is flaky. If we wanted to get fancy, we could even estimate the failure probability and treat a significant increase of the failure rate as a regression!

Along similar lines, there can be state leakage between test cases that causes failures only when test cases are run in a specific order, or when specific test cases are run in parallel. This would manifest as flaky tests, and so flaky test detection ought to try to help tease out these scenarios. That is admittedly difficult and will probably never be entirely reliable. Luckily, it doesn't happen often.

Sometimes, there are test cases that can leave a test system in such a broken state that it has to be rebooted. This is not entirely unusual in very early bringup of a driver for new hardware, when even the device's firmware may still be unstable. An automated test system can and should treat this case just like one would treat a crashing test process: Detect the failure, perhaps using some timer-based watchdog, force a reboot, possibly using a remote-controlled power switch, and resume with the next test case. But if a decent fraction of your test suite is affected, the resulting experience isn't fun and there may not be anything your team can do about it in the short term. So that's an edge case where manual exclusion of tests seems legitimate.

So no, testing perfection isn't attainable for many kinds of software projects. But even relative to what feels like it should be realistically attainable, the state of the art is depressing.

A New Type of Convergence Control Intrinsic?

Subgroup operations or wave intrinsics, such as reducing a value across the threads of a shader subgroup or wave, were introduced in GPU programming languages a while ago. They communicate with other threads of the same wave, for example to exchange the input values of a reduction, but not necessarily with all of them if there is divergent control flow.

In LLVM, we call such operations convergent. Unfortunately, LLVM does not define how the set of communicating threads in convergent operations -- the set of converged threads -- is affected by control flow.

If you're used to thinking in terms of structured control flow, this may seem trivial. Obviously, there is a tree of control flow constructs: loops, if-statements, and perhaps a few others depending on the language. Two threads are converged in the body of a child construct if and only if both execute that body and they are converged in the parent. Throw in some simple and intuitive rules about loop counters and early exits (nested return, break and continue, that sort of thing) and you're done.

In an unstructured control flow graph, the answer is not obvious at all. I gave a presentation at the 2020 LLVM Developers' Meeting that explains some of the challenges as well as a solution proposal that involves adding convergence control tokens to the IR.

Very briefly, convergent operations in the proposal use a token variable that is defined by a convergence control intrinsic. Two dynamic instances of the same static convergent operation from two different threads are converged if and only if the dynamic instances of the control intrinsic producing the used token values were converged.

(The published draft of the proposal talks of multiple threads executing the same dynamic instance. I have since been convinced that it's easier to teach this matter if we instead always give every thread its own dynamic instances and talk about a convergence equivalence relation between dynamic instances. This doesn't change the resulting semantics.)

The draft has three such control intrinsics: anchor, entry, and (loop) heart. Of particular interest here is the heart. For the most common and intuitive use cases, a heart intrinsic is placed in the header of natural loops. The token it defines is used by convergent operations in the loop. The heart intrinsic itself also uses a token that is defined outside the loop: either by another heart in the case of nested loops, or by an anchor or entry. The heart combines two intuitive behaviors:

• It uses a token in much the same way that convergent operations do: two threads are converged for their first execution of the heart if and only if they were converged at the intrinsic that defined the used token.
• Two threads are converged at subsequent executions of the heart if and only if they were converged for the first execution and they are currently at the same loop iteration, where iterations are counted by a virtual loop counter that is incremented at the heart.

Viewed from this angle, how about we define a weaker version of these rules that lies somewhere between an anchor and a loop heart? We could call it a "light heart", though I will stick with "iterating anchor". The iterating anchor defines a token but has no arguments. Like for the anchor, the set of converged threads is implementation-defined -- when the iterating anchor is first encountered. When threads encounter the iterating anchor again without leaving the dominance region of its containing basic block, they are converged if and only if they were converged during their previous encounter of the iterating anchor.

The notion of an iterating anchor came up when discussing the convergence behaviors that can be guaranteed for natural loops. Is it possible to guarantee that natural loops always behave in the natural way -- according to their loop counter -- when it comes to convergence?

Naively, this should be possible: just put hearts into loop headers! Unfortunately, that's not so straightforward when multiple natural loops are contained in an irreducible loop: 

Hearts in A and C must refer to a token defined outside the loops; that is, a token defined in E. The resulting program is ill-formed because it has a closed path that goes through two hearts that use the same token, but the path does not go through the definition of that token. This well-formedness rule exists because the rules about heart semantics are unsatisfiable if the rule is broken.

The underlying intuitive issue is that if the branch at E is divergent in a typical implementation, the wave (or subgroup) must choose whether A or C is executed first. Neither choice works. The heart in A indicates that (among the threads that are converged in E) all threads that visit A (whether immediately or via C) must be converged during their first visit of A. But if the wave executes A first, then threads which branch directly from E to A cannot be converged with those that first branch to C. The opposite conflict exists if the wave executes C first.

If we replace the hearts in A and C by iterating anchors, this problem goes away because the convergence during the initial visit of each block is implementation-defined. In practice, it should fall out of which of the blocks the implementation decides to execute first.

So it seems that iterating anchors can fill a gap in the expressiveness of the convergence control design. But are they really a sound addition? There are two main questions:

• Satisfiability: Can the constraints imposed by iterating anchors be satisfied, or can they cause the sort of logical contradiction discussed for the example above? And if so, is there a simple static rule that prevents such contradictions?
• Spooky action at a distance: Are there generic code transforms which change semantics while changing a part of the code that is distant from the iterating anchor?

The second question is important because we want to add convergence control to LLVM without having to audit and change the existing generic transforms. We certainly don't want to hurt compile-time performance by increasing the amount of code that generic transforms have to examine for making their decisions.

Satisfiability 

Consider the following simple CFG with an iterating anchor in A and a heart in B that refers back to a token defined in E:

Now consider two threads that are initially converged with execution traces:

1. E - A - A - B - X
2. E - A - B - A - X

The heart rule implies that the threads must be converged in B. The iterating anchor rule implies that if the threads are converged in their first dynamic instances of A, then they must also be converged in their second dynamic instances of A, which leads to a temporal paradox.

One could try to resolve the paradox by saying that the threads cannot be converged in A at all, but this would mean that the threads must diverge before a divergent branch occurs. That seems unreasonable, since typical implementations want to avoid divergence as long as control flow is uniform.

The example arguably breaks the spirit of the rule about convergence regions from the draft proposal linked above, and so a minor change to the definition of convergence region may be used to exclude it.

What if the CFG instead looks as follows, which does not break any rules about convergence regions:

For the same execution traces, the heart rule again implies that the threads must be converged in B. The convergence of the first dynamic instances of A are technically implementation-defined, but we'd expect most implementations to be converged there.

The second dynamic instances of A cannot be converged due to the convergence of the dynamic instances of B. That's okay: the second dynamic instance of A in thread 2 is a re-entry into the dominance region of A, and so its convergence is unrelated to any convergence of earlier dynamic instances of A.

Spooky action at a distance 

Unfortunately, we still cannot allow this second example. A program transform may find that the conditional branch in E is constant and the edge from E to B is dead. Removing that edge brings us back to the previous example which is ill-formed. However, a transform which removes the dead edge would not normally inspect the blocks A and B or their dominance relation in detail. The program becomes ill-formed by spooky action at a distance.

The following static rule forbids both example CFGs: if there is a closed path through a heart and an iterating anchor, but not through the definition of the token that the heart uses, then the heart must dominate the iterating anchor.

There is at least one other issue of spooky action at a distance. If the iterating anchor is not the first (non-phi) instruction of its basic block, then it may be preceded by a function call in the same block. The callee may contain control flow that ends up being inlined. Back edges that previously pointed at the block containing the iterating anchor will then point to a different block, which changes the behavior quite drastically. Essentially, the iterating anchor is reduced to a plain anchor.

What can we do about that? It's tempting to decree that an iterating anchor must always be the first (non-phi) instruction of a basic block. Unfortunately, this is not easily done in LLVM in the face of general transforms that might sink instructions or merge basic blocks.

Preheaders to the rescue 

We could chew through some other ideas for making iterating anchors work, but that turns out to be unnecessary. The desired behavior of iterating anchors can be obtained by inserting preheader blocks. The initial example of two natural loops contained in an irreducible loop becomes: 

Place anchors in Ap and Cp and hearts in A and C that use the token defined by their respective dominating anchor. Convergence at the anchors is implementation-defined, but relative to this initial convergence at the anchor, convergence inside the natural loops headed by A and C behaves in the natural way, based on a virtual loop counter. The transform of inserting an anchor in the preheader is easily generalized.

To sum it up: We've concluded that defining an "iterating anchor" convergence control intrinsic is problematic, but luckily also unnecessary. The control intrinsics defined in the original proposal are sufficient. I hope that the discussion that led to those conclusions helps illustrate some aspects of the convergence control proposal for LLVM as well as the goals and principles that drove it.