cmd/compile: don't insert VarDef for already-initialized results

Currently, when we about to emit code that sets the function
results and returns, it emits a VarDef. But in some cases, the
result node is actually live and holding useful data. VarDef
means that we are about to (re)initialize it so all previous
data are dead, but that is not true. Don't insert that.

Also don't add VarDef for register results. We are not going to
store anything (currently it doesn't cause problem, just
unnecessary).

Change-Id: I9dd3b70b4a3f5035af028b143fde8fafa2f11fa0
Reviewed-on: https://go-review.googlesource.com/c/go/+/310589
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/ssagen/ssa.go b/src/cmd/compile/internal/ssagen/ssa.go
index 8f27777cfc..61f23a9c40 100644
--- a/src/cmd/compile/internal/ssagen/ssa.go
+++ b/src/cmd/compile/internal/ssagen/ssa.go
@@ -1941,16 +1941,22 @@ func (s *state) exit() *ssa.Block {
 	// Store SSAable and heap-escaped PPARAMOUT variables back to stack locations.
 	for i, f := range resultFields {
 		n := f.Nname.(*ir.Name)
-		s.vars[memVar] = s.newValue1A(ssa.OpVarDef, types.TypeMem, n, s.mem())
 		if s.canSSA(n) { // result is in some SSA variable
+			if !n.IsOutputParamInRegisters() {
+				// We are about to store to the result slot.
+				s.vars[memVar] = s.newValue1A(ssa.OpVarDef, types.TypeMem, n, s.mem())
+			}
 			results[i] = s.variable(n, n.Type())
 		} else if !n.OnStack() { // result is actually heap allocated
+			// We are about to copy the in-heap result to the result slot.
+			s.vars[memVar] = s.newValue1A(ssa.OpVarDef, types.TypeMem, n, s.mem())
 			ha := s.expr(n.Heapaddr)
 			s.instrumentFields(n.Type(), ha, instrumentRead)
 			results[i] = s.newValue2(ssa.OpDereference, n.Type(), ha, s.mem())
 		} else { // result is not SSA-able; not escaped, so not on heap, but too large for SSA.
 			// Before register ABI this ought to be a self-move, home=dest,
 			// With register ABI, it's still a self-move if parameter is on stack (i.e., too big or overflowed)
+			// No VarDef, as the result slot is already holding live value.
 			results[i] = s.newValue2(ssa.OpDereference, n.Type(), s.addr(n), s.mem())
 		}
 	}