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runtime: do not execute write barrier on newly allocated slice in growslice

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The new slice created in growslice is cleared during malloc for
element types containing pointers and therefore can only contain
nil pointers. This change avoids executing write barriers for these
nil pointers by adding and using a special bulkBarrierPreWriteSrcOnly
function that does not enqueue pointers to slots in dst to the write
barrier buffer.

Change-Id: If9b18248bfeeb6a874b0132d19520adea593bfc4
Reviewed-on: https://go-review.googlesource.com/115996
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Martin Möhrmann 2018-06-03 13:00:19 +02:00
parent 96dcc4457b
commit 4363c98f62
2 changed files with 32 additions and 1 deletions
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4
src/runtime/slice.go
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@ -202,7 +202,9 @@ func growslice(et *_type, old slice, cap int) slice {
// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
p = mallocgc(capmem, et, true)
if writeBarrier.enabled {
bulkBarrierPreWrite(uintptr(p), uintptr(old.array), lenmem)
// Only shade the pointers in old.array since we know the destination slice p
// only contains nil pointers because it has been cleared during alloc.
bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem)
}
}
memmove(p, old.array, lenmem)