MiniEngine blur shader has a data race

[jenatali]

The blur shader has a data race. This race is specifically problematic on WARP, but can reproduce on any GPU with small wave sizes. The access pattern for the groupshared memory is:

• Store 2xf16 in every slot of the arrays.
• Sync
• Read 2xf16
• Compute horizontal blur
• Store 1xf32 in every slot of the arrays
• Sync
• Read 1xf32

This pattern correctly inserts barriers to prevent hazards from write -> read (readers must wait until writes complete), but is missing barriers to prevent hazards from read -> write (writers must wait until all readers complete before overwriting data).

Since WARP executes 4-channel waves sequentially, it will deterministically hit a problematic case where some readers try to load 2xf16 data, but instead they read 1xf32 data. Trying to unpack this f32 as f16s produces nans and other garbage. Theoretically any GPU with a wave size smaller than 64 (since the blur uses 8x8 thread groups) can hit this.