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cmd/link: consider interface conversions only in reachable code

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The linker prunes methods that are not directly reachable if the
receiver type is never converted to interface. Currently, this
"never" is too strong: it is invalidated even if the interface
conversion is in an unreachable function. This CL improves it by
only considering interface conversions in reachable code. To do
that, we introduce a marker relocation R_USEIFACE, which marks
the target symbol as UsedInIface if the source symbol is reached.

binary size    before      after
cmd/compile   18897528   18887400
cmd/go        13607372   13470652

Change-Id: I66c6b69eeff9ae02d84d2e6f2bc7f1b29dd53910
Reviewed-on: https://go-review.googlesource.com/c/go/+/256797
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
This commit is contained in:
Cherry Zhang 2020-09-20 23:29:20 -04:00
parent ad0ab812f8
commit af18bce87c
13 changed files with 129 additions and 15 deletions
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14
src/cmd/link/internal/ld/deadcode.go
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@ -153,6 +153,20 @@ func (d *deadcodePass) flood() {
// do nothing for now as we still load all type symbols.
continue
}
if t == objabi.R_USEIFACE {
// R_USEIFACE is a marker relocation that tells the linker the type is
// converted to an interface, i.e. should have UsedInIface set. See the
// comment below for why we need to unset the Reachable bit and re-mark it.
rs := r.Sym()
if !d.ldr.AttrUsedInIface(rs) {
d.ldr.SetAttrUsedInIface(rs, true)
if d.ldr.AttrReachable(rs) {
d.ldr.SetAttrReachable(rs, false)
d.mark(rs, symIdx)
}
}
continue
}
rs := r.Sym()
if isgotype && usedInIface && d.ldr.IsGoType(rs) && !d.ldr.AttrUsedInIface(rs) {
// If a type is converted to an interface, it is possible to obtain an