cmd/compile: relax liveness restrictions on ambiguously live

Update gc liveness to remove special conservative treatment
of ambiguously live vars, since there is no longer a need to
protect against GCDEBUG=gcdead.

Change-Id: Id6e2d03218f7d67911e8436d283005a124e6957f
Reviewed-on: https://go-review.googlesource.com/24896Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/plive.go

@@ -1188,17 +1188,18 @@ func livenessepilogue(lv *Liveness) {
 	avarinit := bvalloc(nvars)
 	any := bvalloc(nvars)
 	all := bvalloc(nvars)
-	ambig := bvalloc(localswords())
+	pparamout := bvalloc(localswords())
 
-	// Set ambig bit for the pointers to heap-allocated pparamout variables.
-	// These are implicitly read by post-deferreturn code and thus must be
-	// kept live throughout the function (if there is any defer that recovers).
+	// Record pointers to heap-allocated pparamout variables.  These
+	// are implicitly read by post-deferreturn code and thus must be
+	// kept live throughout the function (if there is any defer that
+	// recovers).
 	if hasdefer {
 		for _, n := range lv.vars {
 			if n.IsOutputParamHeapAddr() {
 				n.Name.Needzero = true
 				xoffset := n.Xoffset + stkptrsize
-				onebitwalktype1(n.Type, &xoffset, ambig)
+				onebitwalktype1(n.Type, &xoffset, pparamout)
 			}
 		}
 	}
@@ -1250,11 +1251,6 @@ func livenessepilogue(lv *Liveness) {
 							if debuglive >= 1 {
 								Warnl(p.Lineno, "%v: %L is ambiguously live", Curfn.Func.Nname, n)
 							}
-
-							// Record in 'ambiguous' bitmap.
-							xoffset := n.Xoffset + stkptrsize
-
-							onebitwalktype1(n.Type, &xoffset, ambig)
 						}
 					}
 				}
@@ -1355,11 +1351,9 @@ func livenessepilogue(lv *Liveness) {
 				locals := lv.livepointers[pos]
 				onebitlivepointermap(lv, liveout, lv.vars, args, locals)
 
-				// Ambiguously live variables are zeroed immediately after
-				// function entry. Mark them live for all the non-entry bitmaps
-				// so that GODEBUG=gcdead=1 mode does not poison them.
+				// Mark pparamout variables (as described above)
 				if p.As == obj.ACALL {
-					bvor(locals, locals, ambig)
+					bvor(locals, locals, pparamout)
 				}
 
 				// Show live pointer bitmaps.

test/live.go

@@ -47,24 +47,25 @@ func f2(b bool) {
 }
 
 func f3(b1, b2 bool) {
-	// Because x and y are ambiguously live, they appear
-	// live throughout the function, to avoid being poisoned
-	// in GODEBUG=gcdead=1 mode.
+	// Here x and y are ambiguously live. In previous go versions they
+	// were marked as live throughout the function to avoid being
+	// poisoned in GODEBUG=gcdead=1 mode; this is now no longer the
+	// case.
 
-	printint(0) // ERROR "live at call to printint: x y$"
+	printint(0)
 	if b1 == false {
-		printint(0) // ERROR "live at call to printint: x y$"
+		printint(0)
 		return
 	}
 
 	if b2 {
 		var x *int
-		printpointer(&x) // ERROR "live at call to printpointer: x y$"
-		printpointer(&x) // ERROR "live at call to printpointer: x y$"
+		printpointer(&x) // ERROR "live at call to printpointer: x$"
+		printpointer(&x) // ERROR "live at call to printpointer: x$"
 	} else {
 		var y *int
-		printpointer(&y) // ERROR "live at call to printpointer: x y$"
-		printpointer(&y) // ERROR "live at call to printpointer: x y$"
+		printpointer(&y) // ERROR "live at call to printpointer: y$"
+		printpointer(&y) // ERROR "live at call to printpointer: y$"
 	}
 	printint(0) // ERROR "f3: x \(type \*int\) is ambiguously live$" "f3: y \(type \*int\) is ambiguously live$" "live at call to printint: x y$"
 }