Document debugging POCL compilation with POCL_DEBUG

[vchuravy]

Adds a docs page (under Extras) on debugging POCL compilation for the CPU backend (POCLBackend):

• How to enable PoCL's debug output with POCL_DEBUG=err,warn, either in the shell or via ENV["POCL_DEBUG"] before the first kernel launch.
• Other useful categories (llvm, cache, general, all) with a sample output line.
• A note on the on-disk kernel cache (~/.cache/pocl/kcache, POCL_KERNEL_CACHE=0, POCL_CACHE_DIR), since cache hits can hide the compilation messages one is looking for.

Verified against the vendored pocl_standalone_jll (debug output confirmed for both ways of setting the variable) and that the docs build renders the page and nav entry.

🤖 Generated with Claude Code

[github-actions]

Benchmark Results

Show table

	main	f69d924...	main / f69d924...
saxpy/default/Float32/1024	29.9 ± 18 μs	0.0318 ± 0.022 ms	0.939 ± 0.86
saxpy/default/Float32/1048576	0.359 ± 0.031 ms	0.343 ± 0.032 ms	1.05 ± 0.13
saxpy/default/Float32/16384	0.0452 ± 0.022 ms	0.0441 ± 0.022 ms	1.03 ± 0.72
saxpy/default/Float32/2048	0.0439 ± 0.021 ms	0.0431 ± 0.021 ms	1.02 ± 0.7
saxpy/default/Float32/256	29.1 ± 15 μs	26.3 ± 8.5 μs	1.11 ± 0.69
saxpy/default/Float32/262144	0.112 ± 0.017 ms	0.11 ± 0.017 ms	1.02 ± 0.22
saxpy/default/Float32/32768	0.0457 ± 0.023 ms	0.0464 ± 0.022 ms	0.986 ± 0.68
saxpy/default/Float32/4096	0.0537 ± 0.025 ms	0.0507 ± 0.023 ms	1.06 ± 0.69
saxpy/default/Float32/512	26.8 ± 10 μs	26.7 ± 11 μs	1 ± 0.58
saxpy/default/Float32/64	26.4 ± 7.5 μs	26.2 ± 7.8 μs	1.01 ± 0.42
saxpy/default/Float32/65536	0.0566 ± 0.024 ms	0.0546 ± 0.022 ms	1.04 ± 0.61
saxpy/default/Float64/1024	0.0371 ± 0.023 ms	0.0343 ± 0.024 ms	1.08 ± 1
saxpy/default/Float64/1048576	0.646 ± 0.057 ms	0.608 ± 0.048 ms	1.06 ± 0.13
saxpy/default/Float64/16384	0.0452 ± 0.023 ms	0.044 ± 0.022 ms	1.03 ± 0.73
saxpy/default/Float64/2048	0.0485 ± 0.024 ms	0.0459 ± 0.023 ms	1.06 ± 0.74
saxpy/default/Float64/256	26.2 ± 9.4 μs	25.4 ± 7.2 μs	1.03 ± 0.47
saxpy/default/Float64/262144	0.183 ± 0.023 ms	0.179 ± 0.023 ms	1.03 ± 0.18
saxpy/default/Float64/32768	0.0521 ± 0.024 ms	0.0511 ± 0.024 ms	1.02 ± 0.67
saxpy/default/Float64/4096	0.0538 ± 0.024 ms	0.0514 ± 0.023 ms	1.05 ± 0.66
saxpy/default/Float64/512	29.2 ± 13 μs	26.2 ± 9.7 μs	1.12 ± 0.65
saxpy/default/Float64/64	28 ± 10 μs	25.9 ± 6.6 μs	1.08 ± 0.48
saxpy/default/Float64/65536	0.0681 ± 0.024 ms	0.0688 ± 0.025 ms	0.99 ± 0.5
saxpy/static workgroup=(1024,)/Float32/1024	0.0333 ± 0.018 ms	0.0327 ± 0.018 ms	1.02 ± 0.78
saxpy/static workgroup=(1024,)/Float32/1048576	0.444 ± 0.075 ms	0.429 ± 0.072 ms	1.04 ± 0.25
saxpy/static workgroup=(1024,)/Float32/16384	0.0483 ± 0.023 ms	0.0459 ± 0.021 ms	1.05 ± 0.69
saxpy/static workgroup=(1024,)/Float32/2048	0.047 ± 0.021 ms	0.0425 ± 0.019 ms	1.11 ± 0.71
saxpy/static workgroup=(1024,)/Float32/256	31.6 ± 12 μs	31.4 ± 13 μs	1.01 ± 0.56
saxpy/static workgroup=(1024,)/Float32/262144	0.147 ± 0.015 ms	0.141 ± 0.015 ms	1.04 ± 0.15
saxpy/static workgroup=(1024,)/Float32/32768	0.0513 ± 0.021 ms	0.0513 ± 0.021 ms	1 ± 0.58
saxpy/static workgroup=(1024,)/Float32/4096	0.0509 ± 0.02 ms	0.0529 ± 0.02 ms	0.962 ± 0.52
saxpy/static workgroup=(1024,)/Float32/512	0.0316 ± 0.013 ms	0.032 ± 0.014 ms	0.989 ± 0.59
saxpy/static workgroup=(1024,)/Float32/64	0.0316 ± 0.011 ms	31.4 ± 9.9 μs	1.01 ± 0.47
saxpy/static workgroup=(1024,)/Float32/65536	0.0661 ± 0.021 ms	0.0643 ± 0.02 ms	1.03 ± 0.46
saxpy/static workgroup=(1024,)/Float64/1024	0.0396 ± 0.022 ms	0.0342 ± 0.02 ms	1.16 ± 0.93
saxpy/static workgroup=(1024,)/Float64/1048576	0.677 ± 0.1 ms	0.668 ± 0.1 ms	1.01 ± 0.22
saxpy/static workgroup=(1024,)/Float64/16384	0.0457 ± 0.021 ms	0.0446 ± 0.02 ms	1.03 ± 0.66
saxpy/static workgroup=(1024,)/Float64/2048	0.0478 ± 0.024 ms	0.0483 ± 0.024 ms	0.989 ± 0.69
saxpy/static workgroup=(1024,)/Float64/256	0.0318 ± 0.011 ms	31.5 ± 11 μs	1.01 ± 0.49
saxpy/static workgroup=(1024,)/Float64/262144	0.192 ± 0.019 ms	0.193 ± 0.017 ms	0.995 ± 0.13
saxpy/static workgroup=(1024,)/Float64/32768	0.0538 ± 0.022 ms	0.0544 ± 0.021 ms	0.989 ± 0.56
saxpy/static workgroup=(1024,)/Float64/4096	0.0513 ± 0.019 ms	0.0527 ± 0.019 ms	0.973 ± 0.5
saxpy/static workgroup=(1024,)/Float64/512	0.0331 ± 0.015 ms	31.2 ± 9.8 μs	1.06 ± 0.59
saxpy/static workgroup=(1024,)/Float64/64	0.032 ± 0.012 ms	0.0328 ± 0.012 ms	0.976 ± 0.51
saxpy/static workgroup=(1024,)/Float64/65536	0.0742 ± 0.02 ms	0.0721 ± 0.02 ms	1.03 ± 0.39
time_to_load	0.976 ± 0.015 s	0.957 ± 0.013 s	1.02 ± 0.021

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).