cmd/link/internal/ld: fix sort comparison

Strictly speaking, the sort comparison was inconsistent
(and therefore invalid) for the sort-by-name case, if you had

	a size 0
	b size 1
	c size 0
	zerobase

That would result in the inconsistent comparison ordering:

	a < b (by name)
	b < c (by name)
	c < zerobase (by zerobase rule)
	zerobase < b (by zerobase rule)

This can't happen today because we only disable size-based
sort in a segment that has no zerobase symbol, but it's
confusing to reason through that, so clean up the code anyway.

Passes golang.org/x/tools/cmd/toolstash/buildall.

Change-Id: I21e4159cdedd2053952ba960530d1b0f28c6fb24
Reviewed-on: https://go-review.googlesource.com/c/go/+/625615
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 12 insertions, 11 deletions

diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index 421293e1f9..f07ace1447 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -2240,12 +2240,12 @@ func (state *dodataState) dodataSect(ctxt *Link, symn sym.SymKind, syms []loader
 	// from input files. Both are type sym.SELFGOT, so in that case
 	// we skip size comparison and do the name comparison instead
 	// (conveniently, .got sorts before .toc).
-	checkSize := symn != sym.SELFGOT
+	sortBySize := symn != sym.SELFGOT
 
 	for k, s := range syms {
 		ss := ldr.SymSize(s)
 		sl[k] = symNameSize{sz: ss, sym: s}
-		if !checkSize {
+		if !sortBySize {
 			sl[k].name = ldr.SymName(s)
 		}
 		ds := int64(len(ldr.Data(s)))
@@ -2286,15 +2286,16 @@ func (state *dodataState) dodataSect(ctxt *Link, symn sym.SymKind, syms []loader
 				return true
 			case sj == head, si == tail:
 				return false
-			// put zerobase right after all the zero-sized symbols,
-			// so zero-sized symbols have the same address as zerobase.
-			case si == zerobase:
-				return jsz != 0 // zerobase < nonzero-sized
-			case sj == zerobase:
-				return isz == 0 // 0-sized < zerobase
 			}
-			if checkSize {
-				if isz != jsz {
+			if sortBySize {
+				switch {
+				// put zerobase right after all the zero-sized symbols,
+				// so zero-sized symbols have the same address as zerobase.
+				case si == zerobase:
+					return jsz != 0 // zerobase < nonzero-sized, zerobase > zero-sized
+				case sj == zerobase:
+					return isz == 0 // 0-sized < zerobase, nonzero-sized > zerobase
+				case isz != jsz:
 					return isz < jsz
 				}
 			} else {
@@ -2304,7 +2305,7 @@ func (state *dodataState) dodataSect(ctxt *Link, symn sym.SymKind, syms []loader
 					return iname < jname
 				}
 			}
-			return si < sj
+			return si < sj // break ties by symbol number
 		})
 	} else {
 		// PCLNTAB was built internally, and already has the proper order.