[Openmp-commits] [llvm] [openmp] [openmp][wasm] Allow compiling OpenMP to WebAssembly (PR #71297)

[Jonathan Peyton via Openmp-commits]

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@@ -2286,7 +2286,27 @@ KMP_PREFIX_UNDERSCORE(__kmp_unnamed_critical_addr):
 #if KMP_OS_LINUX
 # if KMP_ARCH_ARM || KMP_ARCH_AARCH64
 .section .note.GNU-stack,"",%progbits
-# else
+# elif !KMP_ARCH_WASM
 .section .note.GNU-stack,"", at progbits
 # endif
 #endif
+
+#if KMP_ARCH_WASM
+.data
+.global .gomp_critical_user_
+.global .gomp_critical_user_.var
+.global .gomp_critical_user_.reduction.var
+.global __kmp_unnamed_critical_addr
+.gomp_critical_user_:
+.zero 4
+.size .gomp_critical_user_, 4
+.gomp_critical_user_.var:
+.zero 4
+.size .gomp_critical_user_.var, 4
+.gomp_critical_user_.reduction.var:
+.zero 4
+.size .gomp_critical_user_.reduction.var, 4
+__kmp_unnamed_critical_addr:
+    .4byte .gomp_critical_user_
+    .size __kmp_unnamed_critical_addr, 4
+#endif
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jpeyton52 wrote:

To elaborate here a bit, when a parallel region is called, the task corresponding to the parallel region (microtask_t) that is generated by the compiler is sent through:

 `__kmpc_fork_call()` -> `__kmp_fork_call()` -> `__kmp_invoke_microtask(microtask_t, gtid, tid, argc, ...)` -> call the microtask with `argc` number of "shareds"

where the varargs component is a list of pointers to shared variables (or firstprivates). There is a limited, platform-independent implementation which WASM is using inside `z_linux_util.cpp` at around line 2492 which can handle up to 15 args. Anymore than that, and the runtime will error out. The test in `openmp/runtime/test/misc_bugs/many-microtask-args.c` is an example of this limitation.

In `z_Linux_asm.S` there are platform-dependent assembly versions of this function. For WASM, it would be best if an assembly version of the `__kmp_invoke_microtask()` function is created to handle any number of "shareds" passed in.

https://github.com/llvm/llvm-project/pull/71297