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[dev.link] cmd/compile, cmd/link: remove dead methods if type is not used in interface

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Currently, a method of a reachable type is live if it matches a
method of a reachable interface. In fact, we only need to retain
the method if the type is actually converted to an interface. If
the type is never converted to an interface, there is no way to
call the method through an interface method call (but the type
descriptor could still be used, e.g. in calling
runtime.newobject).

A type can be used in an interface in two ways:
- directly converted to interface. (Any interface counts, as it
  is possible to convert one interface to another.)
- obtained by reflection from a related type (e.g. obtaining an
  interface of T from []T).

For the former, we let the compiler emit a marker on the type
descriptor symbol when it is converted to an interface. In the
linker, we only need to check methods of marked types.

For the latter, when the linker visits a marked type, it needs to
visit all its "child" types as marked (i.e. potentially could be
converted to interface).

This reduces binary size:
cmd/compile	18792016	18706096 (-0.5%)
cmd/go		14120572	13398948 (-5.1%)

Change-Id: I4465c7eeabf575f4dc84017214c610fa05ae31fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/237298
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
This commit is contained in:
Cherry Zhang 2020-06-08 18:38:59 -04:00
parent 3187b05b87
commit 95848fc5c6
11 changed files with 142 additions and 15 deletions
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24
src/cmd/link/internal/loader/loader.go
View file

@ -244,7 +244,8 @@ type Loader struct {
attrReachable Bitmap // reachable symbols, indexed by global index
attrOnList Bitmap // "on list" symbols, indexed by global index
attrLocal Bitmap // "local" symbols, indexed by global index
attrNotInSymbolTable Bitmap // "not in symtab" symbols, indexed by glob idx
attrNotInSymbolTable Bitmap // "not in symtab" symbols, indexed by global idx
attrUsedInIface Bitmap // "used in interface" symbols, indexed by global idx
attrVisibilityHidden Bitmap // hidden symbols, indexed by ext sym index
attrDuplicateOK Bitmap // dupOK symbols, indexed by ext sym index
attrShared Bitmap // shared symbols, indexed by ext sym index
@ -768,6 +769,20 @@ func (l *Loader) SetAttrLocal(i Sym, v bool) {
}
}
// AttrUsedInIface returns true for a type symbol that is used in
// an interface.
func (l *Loader) AttrUsedInIface(i Sym) bool {
return l.attrUsedInIface.Has(i)
}
func (l *Loader) SetAttrUsedInIface(i Sym, v bool) {
if v {
l.attrUsedInIface.Set(i)
} else {
l.attrUsedInIface.Unset(i)
}
}
// SymAddr checks that a symbol is reachable, and returns its value.
func (l *Loader) SymAddr(i Sym) int64 {
if !l.AttrReachable(i) {
@ -1665,6 +1680,7 @@ func (l *Loader) growAttrBitmaps(reqLen int) {
l.attrOnList = growBitmap(reqLen, l.attrOnList)
l.attrLocal = growBitmap(reqLen, l.attrLocal)
l.attrNotInSymbolTable = growBitmap(reqLen, l.attrNotInSymbolTable)
l.attrUsedInIface = growBitmap(reqLen, l.attrUsedInIface)
}
l.growExtAttrBitmaps()
}
@ -1983,6 +1999,9 @@ func (l *Loader) preloadSyms(r *oReader, kind int) {
if osym.Local() {
l.SetAttrLocal(gi, true)
}
if osym.UsedInIface() {
l.SetAttrUsedInIface(gi, true)
}
if strings.HasPrefix(name, "go.itablink.") {
l.itablink[gi] = struct{}{}
}
@ -2025,6 +2044,9 @@ func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch) {
if osym.Local() {
l.SetAttrLocal(gi, true)
}
if osym.UsedInIface() {
l.SetAttrUsedInIface(gi, true)
}
l.preprocess(arch, gi, name)
}
}