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¶
dEQP-VK.spirv_assembly.instruction.*.float16.opcompositeinsert.*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.
KK_WORKAROUND_5¶
dEQP-VK.fragment_operations.early_fragment.discard_no_early_fragment_tests_depthFragment 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.
KK_WORKAROUND_4¶
dEQP-VK.draw.renderpass.shader_invocation.helper_invocation* and few otherssimd_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.
KK_WORKAROUND_3¶
dEQP-VK.subgroups.ballot_other.*.subgroupballotfindlsbsimd_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);
KK_WORKAROUND_2¶
dEQP-VK.graphicsfuzz.cov-nested-loops-never-change-array-element-one and dEQP-VK.graphicsfuzz.disc-and-add-in-func-in-loopWe 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).
KK_WORKAROUND_1¶
dEQP-VK.glsl.indexing.tmp_array.vec3_dynamic_loop_write_dynamic_loop_read_fragmentUninitialized local scratch variable causes the MSL compiler to crash. Initialize scratch to avoid issue.