[dev.ssa] cmd/compile: Use varkill only for non-SSAable vars

For variables which get SSA'd, SSA keeps track of all the def/kill.
It is only for on-stack variables that we need them.

This reduces stack frame sizes significantly because often the
only use of a variable was a varkill, and without that last use
the variable doesn't get allocated in the frame at all.

Fixes #12602

Change-Id: I3f00a768aa5ddd8d7772f375b25f846086a3e689
Reviewed-on: https://go-review.googlesource.com/14758
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -794,7 +794,9 @@ func (s *state) stmt(n *Node) {
 		// We only care about liveness info at call sites, so putting the
 		// varkill in the store chain is enough to keep it correctly ordered
 		// with respect to call ops.
-		s.vars[&memVar] = s.newValue1A(ssa.OpVarKill, ssa.TypeMem, n.Left, s.mem())
+		if !canSSA(n.Left) {
+			s.vars[&memVar] = s.newValue1A(ssa.OpVarKill, ssa.TypeMem, n.Left, s.mem())
+		}
 
 	case OCHECKNIL:
 		p := s.expr(n.Left)

src/cmd/internal/obj/stack.go

@@ -41,7 +41,7 @@ const (
 	STACKSYSTEM = 0
 	StackSystem = STACKSYSTEM
 	StackBig    = 4096
-	StackGuard  = 960*stackGuardMultiplier + StackSystem
+	StackGuard  = 640*stackGuardMultiplier + StackSystem
 	StackSmall  = 128
 	StackLimit  = StackGuard - StackSystem - StackSmall
 )

src/runtime/stack2.go

@@ -86,7 +86,7 @@ const (
 
 	// The stack guard is a pointer this many bytes above the
 	// bottom of the stack.
-	_StackGuard = 960*stackGuardMultiplier + _StackSystem
+	_StackGuard = 640*stackGuardMultiplier + _StackSystem
 
 	// After a stack split check the SP is allowed to be this
 	// many bytes below the stack guard.  This saves an instruction

test/nosplit.go

@@ -285,12 +285,12 @@ TestCases:
 				// Instead of rewriting the test cases above, adjust
 				// the first stack frame to use up the extra bytes.
 				if i == 0 {
-					size += 832 - 128
+					size += 512 - 128
 					// Noopt builds have a larger stackguard.
 					// See ../cmd/dist/buildruntime.go:stackGuardMultiplier
 					for _, s := range strings.Split(os.Getenv("GO_GCFLAGS"), " ") {
 						if s == "-N" {
-							size += 960
+							size += 640
 						}
 					}
 				}