In 2020, Apple released the M1 with a custom GPU. We got to work reverse-engineering the hardware and porting Linux. Today, you can run Linux on a range of M1 and M2 Macs, with almost all hardware working: wireless, audio, and full graphics acceleration. Our story begins in December 2020, when Hector Martin kicked off Asahi Linux. I was working for Collabora working on Panfrost, the open source Mesa3D driver for Arm Mali GPUs. Hector put out a public call for guidance from upstream open source maintainers, and I bit. I just intended to give some quick pointers. Instead, I bought myself a Christmas present and got to work. In between my university coursework and Collabora work, I poked at the shader instruction set. One thing led to another. Within a few weeks, I drew a triangle. In 3D graphics, once you can draw a triangle, you can do anything. Pretty soon, I started work on a shader compiler. After my final exams that semester, I took a few days off from Collabora to bring up an OpenGL driver capable of spinning gears with my new compiler. Over the next year, I kept reverse-engineering and improving the driver until it could run 3D games on macOS. Meanwhile, Asahi Lina wrote a kernel driver for the Apple GPU. My userspace OpenGL driver ran on macOS, leaving her kernel driver as the missing piece for an open source graphics stack. In December 2022, we shipped graphics acceleration in Asahi Linux. In January 2023, I started my final semester in my Computer Science program at the University of Toronto. For years I juggled my courses with my part-time job and my hobby driver. I faced the same question as my peers: what will I do after graduation? Maybe Panfrost? I started reverse-engineering of the Mali Midgard GPU back in 2017, when I was still in high school. That led to an internship at Collabora in 2019 once I graduated, turning into my job throughout four years of university. During that time, Panfrost grew from a kid’s pet project based on blackbox reverse-engineering, to a professional driver engineered by a team with Arm’s backing and hardware documentation. I did what I set out to do, and the project succeeded beyond my dreams. It was time to move on. What did I want to do next? Finish what I started with the M1. Ship a great driver. Bring full, conformant OpenGL drivers to the M1. Apple’s drivers are not conformant, but we should strive for the industry standard. Bring full, conformant Vulkan to Apple platforms, disproving the myth that Vulkan isn’t suitable for Apple hardware. Bring Proton gaming to Asahi Linux. Thanks to Valve’s work for the Steam Deck, Windows games can run better on Linux than even on Windows. Why not reap those benefits on the M1? Panfrost was my challenge until we “won”. My next challenge? Gaming on Linux on M1. Once I finished my coursework, I started full-time on gaming on Linux. Within a month, we shipped OpenGL 3.1 on Asahi Linux. A few weeks later, we passed official conformance for OpenGL ES 3.1. That put us at feature parity with Panfrost. I wanted to go further. OpenGL (ES) 3.2 requires geometry shaders, a legacy feature not supported by either Arm or Apple hardware. The proprietary OpenGL drivers emulate geometry shaders with compute, but there was no open source prior art to borrow. Even though multiple Mesa drivers need geometry/tessellation emulation, nobody did the work to get there. My early progress on OpenGL was fast thanks to the mature common code in Mesa. It was time to pay it forward. Over the rest of the year, I implemented geometry/tessellation shader emulation. And also the rest of the owl. In January 2024, I passed conformance for the full OpenGL 4.6 specification, finishing up OpenGL. Vulkan wasn’t too bad, either. I polished the OpenGL driver for a few months, but once I started typing a Vulkan driver, I passed 1.3 conformance in a few weeks. What remained was wiring up the geometry/tessellation emulation to my shiny new Vulkan driver, since those are required for Direct3D. Et voilà, Proton games. Along the way, Karol Herbst passed OpenCL 3.0 conformance on the M1, running my compiler atop his “rusticl” frontend. Meanwhile, when the Vulkan 1.4 specification was published, we were ready and shipped a conformant implementation on the same day. After that, I implemented sparse texture support, unlocking Direct3D 12 via Proton. …Now what? Ship a great driver? Check. Conformant OpenGL 4.6, OpenGL ES 3.2, and OpenCL 3.0? Check. Conformant Vulkan 1.4? Check. Proton gaming? Check. That’s a wrap. We’ve succeeded beyond my dreams. The challenges I chased, I have tackled. The drivers are fully upstream in Mesa. Performance isn’t too bad. With the Vulkan on Apple myth busted, conformant Vulkan is now coming to macOS via LunarG’s KosmicKrisp project building on my work. Satisfied, I am now stepping away from the Apple ecosystem. My friends in the Asahi Linux orbit will carry the torch from here. As for me? Onto the next challenge!

English version follows. Aujourd’hui, Khronos Group a sorti la spécification 1.4 de l’API graphique standard Vulkan. Le projet Asahi Linux est fier d’annoncer le premier pilote Vulkan 1.4 pour le matériel d’Apple. En effet, notre pilote graphique Honeykrisp est reconnu par Khronos comme conforme à cette nouvelle version dès aujourd’hui. Ce pilote est déjà disponible dans nos dépôts officiels. Après avoir installé Fedora Asahi Remix, executez dnf upgrade --refresh pour obtenir la dernière version du pilote. Vulkan 1.4 standardise plusieurs fonctionnalités importantes, y compris les horodatages et la lecture locale avec le rendu dynamique. L’industrie suppose que ces fonctionnalités devront être plus courantes, et nous y sommes préparés. Sortir un pilote conforme reflète notre engagement en faveur des standards graphiques et du logiciel libre. Asahi Linux est aussi compatible avec OpenGL 4.6, OpenGL ES 3.2, et OpenCL 3.0, tous conformes aux spécifications pertinentes. D’ailleurs, les notres sont les seules pilotes conformes pour le materiel d’Apple de n’importe quel standard graphique. Même si le pilote est sorti, il faut encore compiler une version expérimentale de Vulkan-Loader pour accéder à la nouvelle version de Vulkan. Toutes les nouvelles fonctionnalités sont néanmoins disponsibles comme extensions à notre pilote Vulkan 1.3 pour en profiter tout de suite. Pour plus d’informations, consultez l’article de blog de Khronos. Today, the Khronos Group released the 1.4 specification of Vulkan, the standard graphics API. The Asahi Linux project is proud to announce the first Vulkan 1.4 driver for Apple hardware. Our Honeykrisp driver is Khronos-recognized as conformant to the new version since day one. That driver is already available in our official repositories. After installing Fedora Asahi Remix, run dnf upgrade --refresh to get the latest drivers. Vulkan 1.4 standardizes several important features, including timestamps and dynamic rendering local read. The industry expects that these features will become more common, and we are prepared. Releasing a conformant driver reflects our commitment to graphics standards and software freedom. Asahi Linux is also compatible with OpenGL 4.6, OpenGL ES 3.2, and OpenCL 3.0, all conformant to the relevant specifications. For that matter, ours are the only conformant drivers on Apple hardware for any graphics standard graphics. Although the driver is released, you still need to build an experimental version of Vulkan-Loader to access the new Vulkan version. Nevertheless, you can immediately use all the new features as extensions in Vulkan 1.3 driver. For more information, see the Khronos blog post.

u{text-decoration-thickness:0.09em;text-decoration-color:skyblue} Finally, conformant Vulkan for the M1! The new “Honeykrisp” driver is the first conformant Vulkan® for Apple hardware on any operating system, implementing the full 1.3 spec without “portability” waivers. Honeykrisp is not yet released for end users. We’re continuing to add features, improve performance, and port to more hardware. Source code is available for developers. HoloCure running on Honeykrisp ft. DXVK, FEX, and Proton. Honeykrisp is not based on prior M1 Vulkan efforts, but rather Faith Ekstrand’s open source NVK driver for NVIDIA GPUs. In her words: All Vulkan drivers in Mesa trace their lineage to the Intel Vulkan driver and started by copying+pasting from it. My hope is that NVK will eventually become the driver that everyone copies and pastes from. To that end, I’m building NVK with all the best practices we’ve developed for Vulkan drivers over the last 7.5 years and trying to keep the code-base clean and well-organized. Why spend years implementing features from scratch when we can reuse NVK? There will be friction starting out, given NVIDIA’s desktop architecture differs from the M1’s mobile roots. In exchange, we get a modern driver designed for desktop games. We’ll need to pass a half-million tests ensuring correctness, submit the results, and then we’ll become conformant after 30 days of industry review. Starting from NVK and our OpenGL 4.6 driver… can we write a driver passing the Vulkan 1.3 conformance test suite faster than the 30 day review period? It’s unprecedented… Challenge accepted. April 2 It begins with a text. Faith… I think I want to write a Vulkan driver. Her advice? Just start typing. Thre’s no copy-pasting yet – we just add M1 code to NVK and remove NVIDIA as we go. Since the kernel mediates our access to the hardware, we begin connecting “NVK” to Asahi Lina’s kernel driver using code shared with OpenGL. Then we plug in our shader compiler and hit the hay. April 3 To access resources, GPUs use “descriptors” containing the address, format, and size of a resource. Vulkan bundles descriptors into “sets” per the application’s “descriptor set layout”. When compiling shaders, the driver lowers descriptor accesses to marry the set layout with the hardware’s data structures. As our descriptors differ from NVIDIA’s, our next task is adapting NVK’s descriptor set lowering. We start with a simple but correct approach, deleting far more code than we add. April 4 With working descriptors, we can compile compute shaders. Now we program the fixed-function hardware to dispatch compute. We first add bookkeeping to map Vulkan command buffers to lists of M1 “control streams”, then we generate a compute control stream. We copy that code from our OpenGL driver, translate the GL into Vulkan, and compute works. That’s enough to move on to “copies” of buffers and images. We implement Vulkan’s copies with compute shaders, internally dispatched with Vulkan commands as if we were the application. The first copy test passes. April 5 Fleshing out yesterday’s code, all copy tests pass. April 6 We’re ready to tackle graphics. The novelty is handling graphics state like depth/stencil. That’s straightforward, but there’s a lot of state to handle. Faith’s code collects all “dynamic state” into a single structure, which we translate into hardware control words. As usual, we grab that translation from our OpenGL driver, blend with NVK, and move on. April 7 What makes state “dynamic”? Dynamic state can change without recompiling shaders. By contrast, static state is baked into shader binaries called “pipelines”. If games create all their pipelines during a loading screen, there is no compiler “stutter” during gameplay. The idea hasn’t quite panned out: many game developers don’t know their state ahead-of-time so cannot create pipelines early. In response, Vulkan has made ever more state dynamic, punctuated with the EXT_shader_object extension that makes pipelines optional. We want full dynamic state and shader objects. Unfortunately, the M1 bakes random state into shaders: vertex attributes, fragment outputs, blending, even linked interpolation qualifiers. Like most of the industry in the 2010s, the M1’s designers bet on pipelines. Faced with this hardware, a reasonable driver developer would double-down on pipelines. DXVK would stutter, but we’d pass conformance. I am not reasonable. To eliminate stuttering in OpenGL, we make state dynamic with four strategies: Conditional code. Precompiled variants. Indirection. Prologs and epilogs. Wait, what-a-logs? AMD also bakes state into shaders… with a twist. They divide the hardware binary into three parts: a prolog, the shader, and an epilog. Confining dynamic state to the periphery eliminates shader variants. They compile prologs and epilogs on the fly, but that’s fast and doesn’t stutter. Linking shader parts is a quick concatenation, or long jumps avoid linking altogether. This strategy works for the M1, too. For Honeykrisp, let’s follow NVK’s lead and treat all state as dynamic. No other Vulkan driver has implemented full dynamic state and shader objects this early on, but it avoids refactoring later. Today we add the code to build, compile, and cache prologs and epilogs. Putting it together, we get a (dynamic) triangle: April 8 Guided by the list of failing tests, we wire up the little bits missed along the way, like translating border colours. /* Translate an American VkBorderColor into a Canadian agx_border_colour */ enum agx_border_colour translate_border_color(VkBorderColor color) { switch (color) { case VK_BORDER_COLOR_INT_TRANSPARENT_BLACK: return AGX_BORDER_COLOUR_TRANSPARENT_BLACK; ... } } Test results are getting there. Pass: 149770, Fail: 7741, Crash: 2396 That’s good enough for vkQuake. April 9 Lots of little fixes bring us to a 99.6% pass rate… for Vulkan 1.1. Why stop there? NVK is 1.3 conformant, so let’s claim 1.3 and skip to the finish line. Pass: 255209, Fail: 3818, Crash: 599 98.3% pass rate for 1.3 on our 1 week anniversary. Not bad. April 10 SuperTuxKart has a Vulkan renderer. April 11 Zink works too. April 12 I tracked down some fails to a test bug, where an arbitrary verification threshold was too strict to pass on some devices. I filed a bug report, and it’s resolved within a few weeks. April 16 The tests for “descriptor indexing” revealed a compiler bug affecting subgroup shuffles in non-uniform control flow. The M1’s shuffle instruction is quirky, but it’s easy to workaround. Fixing that fixes the descriptor indexing tests. April 17 A few tests crash inside our register allocator. Their shaders contain a peculiar construction: if (condition) { while (true) { } } condition is always false, but the compiler doesn’t know that. Infinite loops are nominally invalid since shaders must terminate in finite time, but this shader is syntactically valid. “All loops contain a break” seems obvious for a shader, but it’s false. It’s straightforward to fix register allocation, but what a doozy. April 18 Remember copies? They’re slow, and every frame currently requires a copy to get on screen. For “zero copy” rendering, we need enough Linux window system integration to negotiate an efficient surface layout across process boundaries. Linux uses “modifiers” for this purpose, so we implement the EXT_image_drm_format_modifier extension. And by implement, I mean copy. Copies to avoid copies. April 20 “I’d like a 4K x86 Windows Direct3D PC game on a 16K arm64 Linux Vulkan Mac.” … “Ma’am, this is a Wendy’s.” April 22 As bug fixing slows down, we step back and check our driver architecture. Since we treat all state as dynamic, we don’t pre-pack control words during pipeline creation. That adds theoretical CPU overhead. Is that a problem? After some optimization, vkoverhead says we’re pushing 100 million draws per second. I think we’re okay. April 24 Time to light up YCbCr. If we don’t use special YCbCr hardware, this feature is “software-only”. However, it touches a lot of code. It touches so much code that Mohamed Ahmed spent an entire summer adding it to NVK. Which means he spent a summer adding it to Honeykrisp. Thanks, Mohamed ;-) April 25 Query copies are next. In Vulkan, the application can query the number of samples rendered, writing the result into an opaque “query pool”. The result can be copied from the query pool on the CPU or GPU. For the CPU, the driver maps the pool’s internal data structure and copies the result. This may require nontrivial repacking. For the GPU, we need to repack in a compute shader. That’s harder, because we can’t just run C code on the GPU, right? …Actually, we can. A little witchcraft makes GPU query copies as easy as C. void copy_query(struct params *p, int i) { uintptr_t dst = p->dest + i * p->stride; int query = p->first + i; if (p->available[query] || p->partial) { int q = p->index[query]; write_result(dst, p->_64, p->results[q]); } ... } April 26 The final boss: border colours, hard mode. Direct3D lets the application choose an arbitrary border colour when creating a sampler. By contrast, Vulkan only requires three border colours: (0, 0, 0, 0) – transparent black (0, 0, 0, 1) – opaque black (1, 1, 1, 1) – opaque white We handled these on April 8. Unfortunately, there are two problems. First, we need custom border colours for Direct3D compatibility. Both DXVK and vkd3d-proton require the EXT_custom_border_color extension. Second, there’s a subtle problem with our hardware, causing dozens of fails even without custom border colours. To understand the issue, let’s revisit texture descriptors, which contain a pixel format and a component reordering swizzle. Some formats are implicitly reordered. Common “BGRA” formats swap red and blue for historical reasons. The M1 does not directly support these formats. Instead, the driver composes the swizzle with the format’s reordering. If the application uses a BARB swizzle with a BGRA format, the driver uses an RABR swizzle with an RGBA format. There’s a catch: swizzles apply to the border colour, but formats do not. We need to undo the format reordering when programming the border colour for correct results after the hardware applies the composed swizzle. Our OpenGL driver implements border colours this way, because it knows the texture format when creating the sampler. Unfortunately, Vulkan doesn’t give us that information. Without custom border colour support, we “should” be okay. Swapping red and blue doesn’t change anything if the colour is white or black. There’s an even subtler catch. Vulkan mandates support for a packed 16-bit format with 4-bit components. The M1 supports a similar format… but with reversed “endianness”, swapping red and alpha. That still seems okay. For transparent black (all zero) and opaque white (all one), swapping components doesn’t change the result. The problem is opaque black: (0, 0, 0, 1). Swapping red and alpha gives (1, 0, 0, 0). Transparent red? Uh-oh. We’re stuck. No known hardware configuration implements correct Vulkan semantics. Is hope lost? Do we give up? A reasonable person would. I am not reasonable. Let’s jump into the deep end. If we implement custom border colours, opaque black becomes a special case. But how? The M1’s custom border colours entangle the texture format with the sampler. A reasonable person would skip Direct3D support. As you know, I am not reasonable. Although the hardware is unsuitable, we control software. Whenever a shader samples a texture, we’ll inject code to fix up the border colour. This emulation is simple, correct, and slow. We’ll use dirty driver tricks to speed it up later. For now, we eat the cost, advertise full custom border colours, and pass the opaque black tests. April 27 All that’s left is some last minute bug fixing, and… Pass: 686930, Fail: 0 Success. The future The next task is implementing everything that DXVK and vkd3d-proton require to layer Direct3D. That includes esoteric extensions like transform feedback. Then Wine and an open source x86 emulator will run Windows games on Asahi Linux. That’s getting ahead of ourselves. In the mean time, enjoy Linux games with our conformant OpenGL 4.6 drivers… and stay tuned. Baby Storm running on Honeykrisp ft. DXVK, FEX, and Proton.

For years, the M1 has only supported OpenGL 4.1. That changes today – with our release of full OpenGL® 4.6 and OpenGL® ES 3.2! Install Fedora for the latest M1/M2-series drivers. Already installed? Just dnf –refresh upgrade. Unlike the vendor’s non-conformant 4.1 drivers, our open source Linux drivers are conformant to the latest OpenGL versions, finally promising broad compatibility with modern OpenGL workloads, like Blender, Ryujinx, and Citra. Conformant 4.6/3.2 drivers must pass over 100,000 tests to ensure correctness. The official list of conformant drivers now includes our OpenGL 4.6 and ES 3.2. While the vendor doesn’t yet support graphics standards like modern OpenGL, we do. For this Valentine’s Day, we want to profess our love for interoperable open standards. We want to free users and developers from lock-in, enabling applications to run anywhere the heart wants without special ports. For that, we need standards conformance. Six months ago, we became the first conformant driver for any standard graphics API for the M1 with the release of OpenGL ES 3.1 drivers. Today, we’ve finished OpenGL with the full 4.6… and we’re well on the road to Vulkan. Compared to 4.1, OpenGL 4.6 adds dozens of required features, including: Robustness SPIR-V Clip control Cull distance Compute shaders Upgraded transform feedback Regrettably, the M1 doesn’t map well to any graphics standard newer than OpenGL ES 3.1. While Vulkan makes some of these features optional, the missing features are required to layer DirectX and OpenGL on top. No existing solution on M1 gets past the OpenGL 4.1 feature set. How do we break the 4.1 barrier? Without hardware support, new features need new tricks. Geometry shaders, tessellation, and transform feedback become compute shaders. Cull distance becomes a transformed interpolated value. Clip control becomes a vertex shader epilogue. The list goes on. For a taste of the challenges we overcame, let’s look at robustness. Built for gaming, GPUs traditionally prioritize raw performance over safety. Invalid application code, like a shader that reads a buffer out-of-bounds, can trigger undefined behaviour. Drivers exploit that to maximize performance. For applications like web browsers, that trade-off is undesirable. Browsers handle untrusted shaders, which they must sanitize to ensure stability and security. Clicking a malicious link should not crash the browser. While some sanitization is necessary as graphics APIs are not security barriers, reducing undefined behaviour in the API can assist “defence in depth”. “Robustness” features can help. Without robustness, out-of-bounds buffer access in a shader can crash. With robustness, the application can opt for defined out-of-bounds behaviour, trading some performance for less attack surface. All modern cross-vendor APIs include robustness. Many games even (accidentally?) rely on robustness. Strangely, the vendor’s proprietary API omits buffer robustness. We must do better for conformance, correctness, and compatibility. Let’s first define the problem. Different APIs have different definitions of what an out-of-bounds load returns when robustness is enabled: Zero (Direct3D, Vulkan with robustBufferAccess2) Either zero or some data in the buffer (OpenGL, Vulkan with robustBufferAccess) Arbitrary values, but can’t crash (OpenGL ES) OpenGL uses the second definition: return zero or data from the buffer. One approach is to return the last element of the buffer for out-of-bounds access. Given the buffer size, we can calculate the last index. Now consider the minimum of the index being accessed and the last index. That equals the index being accessed if it is valid, and some other valid index otherwise. Loading the minimum index is safe and gives a spec-compliant result. As an example, a uniform buffer load without robustness might look like: load.i32 result, buffer, index Robustness adds a single unsigned minimum (umin) instruction: umin idx, index, last load.i32 result, buffer, idx Is the robust version slower? It can be. The difference should be small percentage-wise, as arithmetic is faster than memory. With thousands of threads running in parallel, the arithmetic cost may even be hidden by the load’s latency. There’s another trick that speeds up robust uniform buffers. Like other GPUs, the M1 supports “preambles”. The idea is simple: instead of calculating the same value in every thread, it’s faster to calculate once and reuse the result. The compiler identifies eligible calculations and moves them to a preamble executed before the main shader. These redundancies are common, so preambles provide a nice speed-up. We usually move uniform buffer loads to the preamble when every thread loads the same index. Since the size of a uniform buffer is fixed, extra robustness arithmetic is also moved to the preamble. The robustness is “free” for the main shader. For robust storage buffers, the clamping might move to the preamble even if the load or store cannot. Armed with robust uniform and storage buffers, let’s consider robust “vertex buffers”. In graphics APIs, the application can set vertex buffers with a base GPU address and a chosen layout of “attributes” within each buffer. Each attribute has an offset and a format, and the buffer has a “stride” indicating the number of bytes per vertex. The vertex shader can then read attributes, implicitly indexing by the vertex. To do so, the shader loads the address: Some hardware implements robust vertex fetch natively. Other hardware has bounds-checked buffers to accelerate robust software vertex fetch. Unfortunately, the M1 has neither. We need to implement vertex fetch with raw memory loads. One instruction set feature helps. In addition to a 64-bit base address, the M1 GPU’s memory loads also take an offset in elements. The hardware shifts the offset and adds to the 64-bit base to determine the address to fetch. Additionally, the M1 has a combined integer multiply-add instruction imad. Together, these features let us implement vertex loads in two instructions. For example, a 32-bit attribute load looks like: imad idx, stride/4, vertex, offset/4 load.i32 result, base, idx The hardware load can perform an additional small shift. Suppose our attribute is a vector of 4 32-bit values, densely packed into a buffer with no offset. We can load that attribute in one instruction: load.v4i32 result, base, vertex << 2 …with the hardware calculating the address: What about robustness? We want to implement robustness with a clamp, like we did for uniform buffers. The problem is that the vertex buffer size is given in bytes, while our optimized load takes an index in “vertices”. A single vertex buffer can contain multiple attributes with different formats and offsets, so we can’t convert the size in bytes to a size in “vertices”. Let’s handle the latter problem. We can rewrite the addressing equation as: That is: one buffer with many attributes at different offsets is equivalent to many buffers with one attribute and no offset. This gives an alternate perspective on the same data layout. Is this an improvement? It avoids an addition in the shader, at the cost of passing more data – addresses are 64-bit while attribute offsets are 16-bit. More importantly, it lets us translate the vertex buffer size in bytes into a size in “vertices” for each vertex attribute. Instead of clamping the offset, we clamp the vertex index. We still make full use of the hardware addressing modes, now with robustness: umin idx, vertex, last valid load.v4i32 result, base, idx << 2 We need to calculate the last valid vertex index ahead-of-time for each attribute. Each attribute has a format with a particular size. Manipulating the addressing equation, we can calculate the last byte accessed in the buffer (plus 1) relative to the base: The load is valid when that value is bounded by the buffer size in bytes. We solve the integer inequality as: The driver calculates the right-hand side and passes it into the shader. One last problem: what if a buffer is too small to load anything? Clamping won’t save us – the code would clamp to a negative index. In that case, the attribute is entirely invalid, so we swap the application’s buffer for a small buffer of zeroes. Since we gave each attribute its own base address, this determination is per-attribute. Then clamping the index to zero correctly loads zeroes. Putting it together, a little driver math gives us robust buffers at the cost of one umin instruction. In addition to buffer robustness, we need image robustness. Like its buffer counterpart, image robustness requires that out-of-bounds image loads return zero. That formalizes a guarantee that reasonable hardware already makes. …But it would be no fun if our hardware was reasonable. Running the conformance tests for image robustness, there is a single test failure affecting “mipmapping”. For background, mipmapped images contain multiple “levels of detail”. The base level is the original image; each successive level is the previous level downscaled. When rendering, the hardware selects the level closest to matching the on-screen size, improving efficiency and visual quality. With robustness, the specifications all agree that image loads return… Zero if the X- or Y-coordinate is out-of-bounds Zero if the level is out-of-bounds Meanwhile, image loads on the M1 GPU return… Zero if the X- or Y-coordinate is out-of-bounds Values from the last level if the level is out-of-bounds Uh-oh. Rather than returning zero for out-of-bounds levels, the hardware clamps the level and returns nonzero values. It’s a mystery why. The vendor does not document their hardware publicly, forcing us to rely on reverse engineering to build drivers. Without documentation, we don’t know if this behaviour is intentional or a hardware bug. Either way, we need a workaround to pass conformance. The obvious workaround is to never load from an invalid level: if (level <= levels) { return imageLoad(x, y, level); } else { return 0; } That involves branching, which is inefficient. Loading an out-of-bounds level doesn’t crash, so we can speculatively load and then use a compare-and-select operation instead of branching: vec4 data = imageLoad(x, y, level); return (level <= levels) ? data : 0; This workaround is okay, but it could be improved. While the M1 GPU has combined compare-and-select instructions, the instruction set is scalar. Each thread processes one value at a time, not a vector of multiple values. However, image loads return a vector of four components (red, green, blue, alpha). While the pseudo-code looks efficient, the resulting assembly is not: image_load R, x, y, level ulesel R[0], level, levels, R[0], 0 ulesel R[1], level, levels, R[1], 0 ulesel R[2], level, levels, R[2], 0 ulesel R[3], level, levels, R[3], 0 Fortunately, the vendor driver has a trick. We know the hardware returns zero if either X or Y is out-of-bounds, so we can force a zero output by setting X or Y out-of-bounds. As the maximum image size is 16384 pixels wide, any X greater than 16384 is out-of-bounds. That justifies an alternate workaround: bool valid = (level <= levels); int x_ = valid ? x : 20000; return imageLoad(x_, y, level); Why is this better? We only change a single scalar, not a whole vector, compiling to compact scalar assembly: ulesel x_, level, levels, x, #20000 image_load R, x_, y, level If we preload the constant to a uniform register, the workaround is a single instruction. That’s optimal – and it passes conformance. Blender “Wanderer” demo by Daniel Bystedt, licensed CC BY-SA.