cmd/compile/internal/ssa: avoid accidental list ends

Some SSA values don't translate into any instructions. If a function
began with two of them, and both modified the storage of the same
variable, we'd end up with a location list entry that started and ended
at 0. That looks like an end-of-list entry, which would then confuse
downstream tools, particularly the fixup in the linker.

"Fix" this by changing the end of such entries to 1. Should be harmless,
since AFAIK we don't generate any 1-byte instructions. Later CLs will
reduce the frequency of these entries anyway.

Change-Id: I9b7e5e69f914244cc826fb9f4a6acfe2dc695f81
Reviewed-on: https://go-review.googlesource.com/97955
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/debug.go

@@ -947,14 +947,20 @@ func (debugInfo *FuncDebug) PutLocationList(list []byte, ctxt *obj.Link, listSym
 	getPC := debugInfo.GetPC
 	// Re-read list, translating its address from block/value ID to PC.
 	for i := 0; i < len(list); {
-		translate := func() {
-			bv := readPtr(ctxt, list[i:])
-			pc := getPC(decodeValue(ctxt, bv))
-			writePtr(ctxt, list[i:], uint64(pc))
-			i += ctxt.Arch.PtrSize
+		begin := getPC(decodeValue(ctxt, readPtr(ctxt, list[i:])))
+		end := getPC(decodeValue(ctxt, readPtr(ctxt, list[i+ctxt.Arch.PtrSize:])))
+
+		// Horrible hack. If a range contains only zero-width
+		// instructions, e.g. an Arg, and it's at the beginning of the
+		// function, this would be indistinguishable from an
+		// end entry. Fudge it.
+		if begin == 0 && end == 0 {
+			end = 1
 		}
-		translate()
-		translate()
+
+		writePtr(ctxt, list[i:], uint64(begin))
+		writePtr(ctxt, list[i+ctxt.Arch.PtrSize:], uint64(end))
+		i += 2 * ctxt.Arch.PtrSize
 		i += 2 + int(ctxt.Arch.ByteOrder.Uint16(list[i:]))
 	}