cmd/link/internal/wasm: do not generate more than 100000 data segments

Some WebAssembly runtimes (e.g. Node.js) fail to load a wasm binary if
it has more than 100000 data segments. Do not skip zero regions any more
if the limit was reached.

Change-Id: I14c4c2aba142d1d2b887bce6d03b8c1c1746c5ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/168884
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/wasm/asm.go

@@ -427,8 +427,11 @@ func writeDataSec(ctxt *ld.Link) {
 	// overhead of adding a new segment (same as wasm-opt's memory-packing optimization uses).
 	const segmentOverhead = 8
 
+	// Generate at most this many segments. A higher number of segments gets rejected by some WebAssembly runtimes.
+	const maxNumSegments = 100000
+
 	var segments []*dataSegment
-	for _, sec := range sections {
+	for secIndex, sec := range sections {
 		data := ld.DatblkBytes(ctxt, int64(sec.Vaddr), int64(sec.Length))
 		offset := int32(sec.Vaddr)
 
@@ -441,6 +444,10 @@ func writeDataSec(ctxt *ld.Link) {
 		for len(data) > 0 {
 			dataLen := int32(len(data))
 			var segmentEnd, zeroEnd int32
+			if len(segments)+(len(sections)-secIndex) == maxNumSegments {
+				segmentEnd = dataLen
+				zeroEnd = dataLen
+			} else {
 				for {
 					// look for beginning of zeroes
 					for segmentEnd < dataLen && data[segmentEnd] != 0 {
@@ -457,6 +464,7 @@ func writeDataSec(ctxt *ld.Link) {
 					}
 					segmentEnd = zeroEnd
 				}
+			}
 
 			segments = append(segments, &dataSegment{
 				offset: offset,