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cmd/cgo: mark all the C functions as go:noescape #24450

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valyala opened this issue Mar 19, 2018 · 6 comments

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@valyala
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commented Mar 19, 2018

What version of Go are you using (go version)?

go tip

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

linux/amd64

What did you do?

Build the following code with go build -gcflags=-m:

package ff

// void f1(void *p) {}
import "C"
import "unsafe"

func f2(a []byte) {
        p := unsafe.Pointer(&a[0])
        C.f1(p)
}

What did you expect to see?

a shouldn't escape when calling C.f1, since, according to cgo rules, C code may not keep a copy of a Go pointer after the call returns.

This means that all the C functions may be safely marked as //go:noescape in order to prevent from useless memory allocations when the argument to C function escapes to heap.

What did you see instead?

a escapes to heap, which leads to useless memory allocation:

./1.go:8: &a[0] escapes to heap
./1.go:7: leaking param: a

@andybons andybons added this to the Unplanned milestone Mar 19, 2018

@andybons

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commented Mar 19, 2018

@randall77

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commented Mar 19, 2018

I don't think this is safe. f1 can call back into Go, causing the Go stack to be copied. Any subsequent use of p by f1 would then be wrong.

@ianlancetaylor

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commented Mar 19, 2018

Yes, this is intentional and I don't see a way to avoid it.

Closing this bug since there is nothing we can do. Sorry.

@valyala

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commented Mar 19, 2018

@ianlancetaylor , is there a possibility to manually mark C.f1 with go:noescape?

@valyala

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commented Mar 19, 2018

Currently I use the following "trick" to avoid memory allocations when passing pointers to Go array to C function:

func f3(a []byte) {
    sh := (*reflect.SliceHeader)(unsafe.Pointer(&a))
    C.f1(unsafe.Pointer(sh.Data))
}

Unfortunately, this leads to the issue #14210 when a points to a struct with pointers unless GODEBUG=cgocheck=0 is set.

@ianlancetaylor

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commented Mar 20, 2018

There is no way to manually mark it. cgo is generating code that deliberately causes it to escape, to ensure that the value is not allocated on the stack. I don't know whether there is any better approach that will work for your application. This should probably be discussed on golang-nuts rather than in the issue tracker.

valyala added a commit to valyala/zstd that referenced this issue Mar 21, 2018
Eliminate memory allocations from Compress/Decompress.
Go allocates a copy of slice headers when they are passed to C functions.
See golang/go#24450 for details.

Avoid this by passing unsafe.Pointer as C.uintptr_t into C functions.

Benchmark results:

$ PAYLOAD=ZSTD_LICENSE go test -run=111 -bench='k[CD]' -benchmem -count=10

name             old time/op    new time/op    delta
Compression-4      21.2µs ± 5%    20.8µs ± 6%      ~     (p=0.190 n=10+10)
Decompression-4    7.37µs ± 5%    6.93µs ± 6%    -5.95%  (p=0.001 n=10+10)

name             old speed      new speed      delta
Compression-4    62.5MB/s ± 5%  63.7MB/s ± 6%      ~     (p=0.190 n=10+10)
Decompression-4   180MB/s ± 5%   191MB/s ± 6%    +6.35%  (p=0.001 n=10+10)

name             old alloc/op   new alloc/op   delta
Compression-4       96.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)
Decompression-4     96.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)

name             old allocs/op  new allocs/op  delta
Compression-4        4.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
Decompression-4      4.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

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