cmd/link: improve comments for relocsym

This patch contains the remnants of CL (122482), which was intended to
reduce memory allocation in 'relocsym'. Another CL (113637) went in
first that included pretty much all of the code changes in 122482,
however there are some changes to comments that are worth preserving.

Change-Id: Iacdbd2bfe3b7ca2656596570f06ce9a646211913
Reviewed-on: https://go-review.googlesource.com/122482Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -111,7 +111,20 @@ func trampoline(ctxt *Link, s *sym.Symbol) {
 
 }
 
-// resolve relocations in s.
+// relocsym resolve relocations in "s". The main loop walks through
+// the list of relocations attached to "s" and resolves them where
+// applicable. Relocations are often architecture-specific, requiring
+// calls into the 'archreloc' and/or 'archrelocvariant' functions for
+// the architecture. When external linking is in effect, it may not be
+// possible to completely resolve the address/offset for a symbol, in
+// which case the goal is to lay the groundwork for turning a given
+// relocation into an external reloc (to be applied by the external
+// linker). For more on how relocations work in general, see
+//
+//  "Linkers and Loaders", by John R. Levine (Morgan Kaufmann, 1999), ch. 7
+//
+// This is a performance-critical function for the linker; be careful
+// to avoid introducing unnecessary allocations in the main loop.
 func relocsym(ctxt *Link, s *sym.Symbol) {
 	for ri := int32(0); ri < int32(len(s.R)); ri++ {
 		r := &s.R[ri]

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

@@ -104,8 +104,27 @@ type Arch struct {
 	Solarisdynld   string
 	Adddynrel      func(*Link, *sym.Symbol, *sym.Reloc) bool
 	Archinit       func(*Link)
-	Archreloc        func(*Link, *sym.Reloc, *sym.Symbol, int64) (int64, bool)
-	Archrelocvariant func(*Link, *sym.Reloc, *sym.Symbol, int64) int64
+	// Archreloc is an arch-specific hook that assists in
+	// relocation processing (invoked by 'relocsym'); it handles
+	// target-specific relocation tasks. Here "rel" is the current
+	// relocation being examined, "sym" is the symbol containing the
+	// chunk of data to which the relocation applies, and "off" is the
+	// contents of the to-be-relocated data item (from sym.P). Return
+	// value is the appropriately relocated value (to be written back
+	// to the same spot in sym.P) and a boolean indicating
+	// success/failure (a failing value indicates a fatal error).
+	Archreloc func(link *Link, rel *sym.Reloc, sym *sym.Symbol,
+		offset int64) (relocatedOffset int64, success bool)
+	// Archrelocvariant is a second arch-specific hook used for
+	// relocation processing; it handles relocations where r.Type is
+	// insufficient to describe the relocation (r.Variant !=
+	// sym.RV_NONE). Here "rel" is the relocation being applied, "sym"
+	// is the symbol containing the chunk of data to which the
+	// relocation applies, and "off" is the contents of the
+	// to-be-relocated data item (from sym.P). Return is an updated
+	// offset value.
+	Archrelocvariant func(link *Link, rel *sym.Reloc, sym *sym.Symbol,
+		offset int64) (relocatedOffset int64)
 	Trampoline  func(*Link, *sym.Reloc, *sym.Symbol)
 	Asmb        func(*Link)
 	Elfreloc1   func(*Link, *sym.Reloc, int64) bool