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[dev.link] cmd/link: stop setting ReadOnly attribute in late stage

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The ReadOnly attribute was used to do copy on write when applying
relocations to symbols with read-only backing stores. Now that we
always apply relocations in the output buffer (mmap or heap), it
is always writeable. No need to tamper with the ReadOnly
attribute anymore.

Wasm is an exception, where we don't copy symbol contents to the
output buffer first. Do copy-on-write there.

This is in preparation of converting reloc to using the loader.

Change-Id: I15e53b7c162b9124e6689dfd8eb45cbe2ffd7153
Reviewed-on: https://go-review.googlesource.com/c/go/+/229991
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
This commit is contained in:
Cherry Zhang 2020-04-24 20:22:39 -04:00
parent 51ac260e5a
commit 2a874562bf
2 changed files with 9 additions and 8 deletions
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16
src/cmd/link/internal/ld/data.go
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@ -140,11 +140,14 @@ func relocsym(target *Target, ldr *loader.Loader, err *ErrorReporter, syms *Arch
if len(s.R) == 0 {
return
}
if s.Attr.ReadOnly() {
if target.IsWasm() && s.Attr.ReadOnly() {
// The symbol's content is backed by read-only memory.
// Copy it to writable memory to apply relocations.
// Only need to do this on Wasm. On other platforms we
// apply relocations to the output buffer, which is
// always writeable.
s.P = append([]byte(nil), s.P...)
s.Attr.Set(sym.AttrReadOnly, false)
// No need to unset AttrReadOnly because it will not be used.
}
for ri := int32(0); ri < int32(len(s.R)); ri++ {
r := &s.R[ri]
@ -579,8 +582,10 @@ func (ctxt *Link) reloc() {
syms := &ctxt.ArchSyms
wg.Add(3)
go func() {
for _, s := range ctxt.Textp {
relocsym(target, ldr, reporter, syms, s)
if !ctxt.IsWasm() { // On Wasm, text relocations are applied in Asmb2.
for _, s := range ctxt.Textp {
relocsym(target, ldr, reporter, syms, s)
}
}
wg.Done()
}()
@ -2503,8 +2508,6 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
if len(s.R) != 0 && wasReadOnly {
relocbuf = append(relocbuf[:0], s.P...)
s.P = relocbuf
// TODO: This function call needs to be parallelized when the loader wavefront gets here.
s.Attr.Set(sym.AttrReadOnly, false)
}
relocsym(target, ldr, reporter, archSyms, s)
if _, err := z.Write(s.P); err != nil {
@ -2526,7 +2529,6 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
// contents, in which case we still need s.P.
if len(s.R) != 0 && wasReadOnly {
s.P = oldP
s.Attr.Set(sym.AttrReadOnly, wasReadOnly)
for i := range s.R {
s.R[i].Done = false
}