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cmd/compile: skip ptr writeback for appends which don't grow #14969

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randall77 opened this issue Mar 25, 2016 · 5 comments
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cmd/compile: skip ptr writeback for appends which don't grow #14969

randall77 opened this issue Mar 25, 2016 · 5 comments
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FrozenDueToAge Performance
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@randall77
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func(t *[]int) {
    *t = append(*t, 7)
}

We only need to write the pointer (and the capacity) back when we take the growslice path. For the standard path, only the length needs to be written back.

See #14921 and #14855 .
@randall77 randall77 mentioned this issue Mar 29, 2016
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@randall77
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Looks like this is related to the following in SSA (ssa.go:OAS case):

        if rhs != nil && rhs.Op == OAPPEND {
            // Yuck!  The frontend gets rid of the write barrier, but we need it!
            // At least, we need it in the case where growslice is called.
            // TODO: Do the write barrier on just the growslice branch.
            // TODO: just add a ptr graying to the end of growslice?
            // TODO: check whether we need to do this for ODOTTYPE and ORECV also.
            // They get similar wb-removal treatment in walk.go:OAS.
            needwb = true
        }

So we need to be less aggressive in adding back the write barrier. The frontend basically punts to the backend to decide if write barriers are needed (walk.go, OAS case):

            if r.Op == OAPPEND {
                // Left in place for back end.
                // Do not add a new write barrier.
                break opswitch
            }

For *p = append(*p, x) we need to generate code like:

a := *p
newlen := a.len+1
if newlen > a.cap {
   a = growslice()...
   p.ptr = a.ptr  // with a write barrier
   p.cap = a.cap
}
p.len = newlen
a.ptr[newlen-1] = x // with a write barrier, if needed

This code is similar to what we generate for the generic OAPPEND case, maybe we can share some code.

See #15023 for another case of this same problem.

@randall77 randall77 mentioned this issue Mar 29, 2016
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@josharian
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I'll take a look at this.

@bradfitz bradfitz added this to the Unplanned milestone Apr 9, 2016
@gopherbot
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CL https://golang.org/cl/21813 mentions this issue.

@gopherbot
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CL https://golang.org/cl/21891 mentions this issue.

@josharian josharian reopened this Apr 12, 2016
gopherbot pushed a commit that referenced this issue Apr 12, 2016
@josharian
cmd/compile: temporarily disable inplace append special case
811ebb6 
Fixes #15246
Re-opens #14969

Change-Id: Ic0b41c5aa42bbb229a0d62b7f3e5888c6b29293d
Reviewed-on: https://go-review.googlesource.com/21891
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
@gopherbot
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CL https://golang.org/cl/22197 mentions this issue.

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