Elias Naur d9d518b248 avoid non-uniform barrier control flow when exhausting memory ...

The compute shaders have a check for the succesful completion of their
preceding stage. However, consider a shader execution path like the
following:

	void main()
		if (mem_error != NO_ERROR) {
		    return;
		}
		...
		malloc(...);
		...
		barrier();
		...
	}

and  shader execution that fails to allocate memory, thereby setting
mem_error to ERR_MALLOC_FAILED in malloc before reaching the barrier. If
another shader execution then begins execution, its mem_eror check will
make it return early and not reach the barrier.

All GPU APIs require (dynamically) uniform control flow for barriers,
and the above case may lead to GPU hangs in practice.

Fix this issue by replacing the early exits with careful checks that
don't interrupt barrier control flow.

Unfortunately, it's harder to prove the soundness of the new checks, so
this change also clears dynamic memory ranges in MEM_DEBUG mode when
memory is exhausted. The result is that accessing memory after
exhaustion triggers an error.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

2021-04-20 10:15:29 +02:00

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