cmd/link: apply DWARF relocations while doing compression

We are preparing for applying relocations to the output buffer.
However, for DWARF compression, relocations need to be applied
before compression, but we don't have an output buffer at that
time. We also cannot delay DWARF compression to when we mmap the
output file, because we need the size of the DWARF sections to
compute the file size.

Instead of applying all the relocations together, we apply
relocations in DWARF sections one symbol at a time, right before
it is writing out for compression. As the symbol content may be
in read-only memory (in the future), we use a temporary buffer
for applying the relocations, and immediately write it out.

If compression is not used, relocations are still applied all
together.

This is in preparation for mmap'ing input files read-only.

Change-Id: Iae6d2dd71313897d5054bcc458d3bb78075b30c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/171397
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/link/internal/ld/data.go

@@ -2385,6 +2385,12 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
 		log.Fatalf("NewWriterLevel failed: %s", err)
 	}
 	for _, sym := range syms {
+		// sym.P may be read-only. Apply relocations in a
+		// temporary buffer, and immediately write it out.
+		oldP := sym.P
+		ctxt.relocbuf = append(ctxt.relocbuf[:0], sym.P...)
+		sym.P = ctxt.relocbuf
+		relocsym(ctxt, sym)
 		if _, err := z.Write(sym.P); err != nil {
 			log.Fatalf("compression failed: %s", err)
 		}
@@ -2399,6 +2405,14 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
 			}
 			i -= int64(n)
 		}
+		// Restore sym.P, for 1. not holding temp buffer live
+		// unnecessarily, 2. if compression is not beneficial,
+		// we'll go back to use the uncompressed contents, in
+		// which case we still need sym.P.
+		sym.P = oldP
+		for i := range sym.R {
+			sym.R[i].Done = false
+		}
 	}
 	if err := z.Close(); err != nil {
 		log.Fatalf("compression failed: %s", err)

src/cmd/link/internal/ld/dwarf.go

@@ -2125,9 +2125,9 @@ func dwarfaddelfsectionsyms(ctxt *Link) {
 	}
 }
 
-// dwarfcompress compresses the DWARF sections. This must happen after
-// relocations are applied. After this, dwarfp will contain a
-// different (new) set of symbols, and sections may have been replaced.
+// dwarfcompress compresses the DWARF sections. Relocations are applied
+// on the fly. After this, dwarfp will contain a different (new) set of
+// symbols, and sections may have been replaced.
 func dwarfcompress(ctxt *Link) {
 	supported := ctxt.IsELF || ctxt.HeadType == objabi.Hwindows || ctxt.HeadType == objabi.Hdarwin
 	if !ctxt.compressDWARF || !supported || ctxt.LinkMode != LinkInternal {
@@ -2161,6 +2161,7 @@ func dwarfcompress(ctxt *Link) {
 		}
 	}
 	dwarfp = newDwarfp
+	ctxt.relocbuf = nil // no longer needed, don't hold it live
 
 	// Re-compute the locations of the compressed DWARF symbols
 	// and sections, since the layout of these within the file is

src/cmd/link/internal/ld/link.go

@@ -93,6 +93,8 @@ type Link struct {
 
 	compUnits         []*compilationUnit // DWARF compilation units
 	compUnitByPackage map[*sym.Library]*compilationUnit
+
+	relocbuf []byte // temporary buffer for applying relocations
 }
 
 type unresolvedSymKey struct {

src/cmd/link/internal/ld/main.go

@@ -239,8 +239,8 @@ func Main(arch *sys.Arch, theArch Arch) {
 	ctxt.symtab()
 	ctxt.dodata()
 	order := ctxt.address()
-	ctxt.reloc()
 	dwarfcompress(ctxt)
+	ctxt.reloc()
 	filesize := ctxt.layout(order)
 
 	// Write out the output file.