SubgroupBroadcast test bug for subgroup sizes < 16

[jenatali]

Test case webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:compute,all_active:wgSize=[3,3,3] produces a thread group with 333=27 threads. Test subcase 3 attempts to subgroupBroadcast from subgroup lane 3. When subgroup size is 4 or 8, the last subgroup to execute will not have a thread executing at subgroup index 3.

Subgroup size 4 will have:

• 0, 1, 2, 3
• 4, 5, 6, 7
• 8, 9, 10, 11
• 12, 13, 14, 15
• 16, 17, 18, 19
• 20, 21, 22, 23
• 24, 25, 26

Subgroup size 8 will have:

• 0 - 7
• 8 - 15
• 16 - 23
• 24, 25, 26

It's not until subgroup size 16 where all active subgroups include an active lane at index 3:

• 0 - 15
• 16 - 26

[jenatali]

This also impacts webgpu:shader,execution,expression,call,builtin,subgroupBroadcast:compute,split:predicate="every_odd" in the subtest for ID=3 and group size = [3,3,3].

[dneto0]

Thanks for the bug report. We're looking at it.

[petermcneeleychromium]

Discussed more with david. We suspect, while an extreme edge case, this is a legitimate error in the CTS.