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runtime: _cgoCheckPointer0 extreme overhead #14265
The overhead is 22x in practice and 36000 in this contrived example:
$ GODEBUG=cgocheck=1 go test -bench=. -run=-
The code is at https://github.com/tgulacsi/go/tree/master/cgo22
changed the title from
_cgoCheckPointer0 extreme overhead
runtime/cgo: _cgoCheckPointer0 extreme overhead
Feb 8, 2016
Thanks for the test case. I don't know how to do much better for this kind of example. The pointer passing rules say that if you pass the address of an element of a slice, the C code is permitted to examine every element of the slice. You are passing the address of a 1000000 slice of pointers, so each call needs to check 1000000 pointers to make sure that they are not Go pointers.
Thanks Ian for the clear description!
Now I think I understand another strange thing: with
Back to the original issue: this understanding allows a not-too-ugly workaround:
By calling with a one-element-length array (capped), it works:
Here the "Naive" is the old version, the "Sliced" is the carr := arr[i:i+1] version, and the "Capped" is the above carr := arr[i:i+1:i+1] version.
(same repo has the new code).
I think at least we should document it more explicitly, that the cgo checker checks all reachable pointers in the struct/array/slice level.