KosmicKrisp workarounds — The Mesa 3D Graphics Library latest documentation

# KosmicKrisp workarounds

This file documents the relevant issues found in either Metal, the MSL compiler or any other component we have no control over that needed to be worked around to accomplish Vulkan conformance.

All workarounds must be documented here and no code comment info should be provided other than the name `KK_WORKAROUND_#`.

Once a workaround was removed from the code, the code comment will be removed but the documentation here will be kept.

## Template

Use the following template to create documentation for a new workaround:

    KK_WORKAROUND_#
    ---------------
    | macOS version:
    | Metal ticket:
    | Metal ticket status:
    | CTS test failure/crash:
    | Comments:
    | Log:

`macOS version` needs to have the OS version with which it was found.

`Metal ticket` needs to be either the Metal ticket number with the GitLab handle of the user that reported the ticket or `Unreported`.

`Metal ticket status` needs to be either `Fixed in macOS # (Build hash)`,

`Waiting resolution` or empty if unreported. If Apple reported that the issue was fixed, but no user has verified the fix, append `[Untested]`. `CTS test failure/crash` (remove `failure` or `crash` based on test behavior) needs to be the name of the test or test family the issue can be reproduced with.

`Comments` needs to include as much information on the issue and how the workaround fixes it.

`Log` needs to have the dates (yyyy-mm-dd, the only correct date format) with info on what was updated.

## Workarounds

### KK_WORKAROUND_6

macOS version: 26.0.1
Metal ticket: Not reported
Metal ticket status:
CTS test failure: `dEQP-VK.spirv_assembly.instruction.*.float16.opcompositeinsert.*`
Comments:

Metal does not respect its own Memory Coherency Model (MSL spec 4.8). From the spec: `By default, memory in the device address space has threadgroup coherence.`

If we have a single thread compute dispatch so that we do (simplified version):

    for (...) {
       value = ssbo_data[0]; // ssbo_data is a device buffer
       ...
       ssbo_data[0] = new_value;
    }

`ssbo_data[0]` will not correctly store/load the values so the value written in iteration 0, will not be available in iteration 1. The workaround to this issue is marking the device memory pointer through which the memory is accessed as coherent so that the value is stored and loaded correctly. Hopefully this does not affect performance much.

Log:
2025-12-08: Workaround implemented and reported to Apple

### KK_WORKAROUND_5

macOS version: 26.0.1
Metal ticket: Not reported
Metal ticket status:
CTS test failure: `dEQP-VK.fragment_operations.early_fragment.discard_no_early_fragment_tests_depth`
Comments:

Fragment shaders that have side effects (like writing to a buffer) will be prematurely discarded if there is a `discard_fragment` that will always execute. To work around this, we just make the discard “optional” by moving it inside a run time conditional that will always be true (such as is the fragment a helper?). This tricks the MSL compiler into not optimizing it into a premature discard.

Log:
2025-12-01: Workaround implemented

### KK_WORKAROUND_4

macOS version: 26.0.1
Metal ticket: FB21124215 (@aitor)
Metal ticket status: Waiting resolution
CTS test failure: `dEQP-VK.draw.renderpass.shader_invocation.helper_invocation*` and few others
Comments:

`simd_is_helper_thread()` will always return true if the shader was started as a non-helper thread, even after `discard_fragment()` is called. The workaround is to have a variable tracking this state and update it when the fragment is discarded. This issue is present in M1 and M2 chips.

Log:
2025-11-22: Workaround implemented and reported to Apple

### KK_WORKAROUND_3

macOS version: 15.4.x
Metal ticket: FB20113490 (@aitor)
Metal ticket status: Waiting resolution
CTS test failure: `dEQP-VK.subgroups.ballot_other.*.subgroupballotfindlsb`
Comments:

`simd_is_first` does not seem to behave as documented in the MSL specification. The following code snippet misbehaves:

    if (simd_is_first())
       temp = 3u;
    else
       temp = simd_ballot(true); /* <- This will return all active threads... */

The way to fix this is by changing the conditional to:

    if (simd_is_first() && (ulong)simd_ballot(true))
       temp = 3u;
    else
       temp = (ulong)simd_ballot(true);

Log:
2025-09-09: Workaround implemented and reported to Apple

### KK_WORKAROUND_2

macOS version: 15.4.x
Metal ticket: FB21065475 (@aitor)
Metal ticket status: Waiting resolution
CTS test crash: `dEQP-VK.graphicsfuzz.cov-nested-loops-never-change-array-element-one` and `dEQP-VK.graphicsfuzz.disc-and-add-in-func-in-loop`
Comments:

We need to loop to infinite since MSL compiler crashes if we have something like (simplified version):

    while (true) {
       if (some_conditional) {
          break_loop = true;
       } else {
          break_loop = false;
       }
       if (break_loop) {
          break;
       }
    }

The issue I believe is that `some_conditional` wouldn’t change the value no matter in which iteration we are (something like fetching the same value from a buffer) and the MSL compiler doesn’t seem to like that much to the point it crashes.

The implemented solution is to change the `while(true)` loop with `for (uint64_t no_crash = 0u; no_crash < UINT64_MAX; ++no_crash)`, which tricks the MSL compiler into believing we are not doing an infinite loop (wink wink).

Log:
2025-09-08: Workaround implemented

### KK_WORKAROUND_1

macOS version: 15.4.x
Metal ticket: FB17604106 (@aitor)
Metal ticket status: [Untested] Fixed in macOS 26 Beta (25A5279m)
CTS test crash: `dEQP-VK.glsl.indexing.tmp_array.vec3_dynamic_loop_write_dynamic_loop_read_fragment`
Comments:

Uninitialized local scratch variable causes the MSL compiler to crash. Initialize scratch to avoid issue.

Log:
2025-05-14: Workaround implemented and reported to Apple
2025-06-14: Apple reported back saying it is now fixed in macOS 26 Beta (Build 25A5279m)