Document debugging POCL compilation with POCL_DEBUG

docs/make.jl

@@ -37,6 +37,7 @@ function main()
             "API" => "api.md",
             "Extras" => [
                 "extras/unrolling.md",
+                "extras/pocl_debugging.md",
             ], # Extras
             "Notes for implementations" => "implementations.md",
         ], # pages

docs/src/extras/pocl_debugging.md

@@ -0,0 +1,66 @@
+# Debugging POCL compilation
+
+The `CPU` backend of KernelAbstractions.jl is implemented on top of
+[PoCL](https://portablecl.org/) (`POCLBackend`). Kernels are compiled to SPIR-V
+and then handed to PoCL, which JIT-compiles them for the host CPU using LLVM.
+When a kernel fails to compile — or compiles but misbehaves — it can be useful
+to see what PoCL itself is doing.
+
+## Enabling PoCL's debug output
+
+PoCL's runtime honors the `POCL_DEBUG` environment variable, which takes a
+comma-separated list of message categories. A good starting point is to enable
+errors and warnings:
+
+```sh
+POCL_DEBUG=err,warn julia --project my_script.jl
+```
+
+This prints PoCL's internal error and warning messages (e.g. from SPIR-V
+ingestion, LLVM code generation, or kernel argument handling) to standard
+error, with timestamps and the source location inside PoCL that emitted them:
+
+```
+[2026-07-04 22:05:12.317942887] PoCL: in fn compile_and_link_program at line 773:  *** INFO ***  |      LLVM |     BUILDING for device: cpu
+```
+
+Since PoCL reads the variable when the library is initialized, it can also be
+set from within Julia, as long as this happens before the first kernel launch
+(or any other use of the backend):
+
+```julia
+ENV["POCL_DEBUG"] = "err,warn"
+
+using KernelAbstractions
+# ... launch kernels as usual
+```
+
+When in doubt, prefer setting the variable in the shell before starting Julia.
+
+## Other useful categories
+
+Beyond `err` and `warn`, some categories that are helpful when debugging
+compilation issues:
+
+- `llvm`: messages from PoCL's LLVM-based kernel compiler, including the
+  work-group vectorization and linking steps.
+- `cache`: shows where PoCL caches compiled kernels on disk, and whether a
+  kernel was recompiled or loaded from the cache.
+- `general`: high-level runtime activity, such as device initialization and
+  program builds.
+- `all`: everything. Very verbose, but useful as a last resort.
+
+For example:
+
+```sh
+POCL_DEBUG=err,warn,llvm julia --project my_script.jl
+```
+
+!!! note
+    PoCL caches compiled kernels on disk (by default under
+    `~/.cache/pocl/kcache`). If a compilation step you expect does not show up
+    in the debug output, the kernel was likely loaded from the cache — the
+    debug output reports this with messages such as
+    `Found cached compiled SPIRV binary at ..., skipping compilation`.
+    Set `POCL_KERNEL_CACHE=0` to disable the cache and force recompilation,
+    or `POCL_CACHE_DIR` to relocate it.