[dev.regabi] cmd/compile: simplify stack temp initialization

This CL simplifies the previous one a little bit further, by combining
reordering stack-temporary initialization and getting rid of an
unneeded temporary variable. (Does not pass toolstash -cmp.)

Change-Id: I17799dfe368484f33a8ddd0ab4f68647d6262147
Reviewed-on: https://go-review.googlesource.com/c/go/+/284225
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

2 files changed, 13 insertions, 23 deletions

diff --git a/src/cmd/compile/internal/walk/complit.go b/src/cmd/compile/internal/walk/complit.go
index a7db453550..97e820238b 100644
--- a/src/cmd/compile/internal/walk/complit.go
+++ b/src/cmd/compile/internal/walk/complit.go
@@ -344,21 +344,18 @@ func slicelit(ctxt initContext, n *ir.CompLitExpr, var_ ir.Node, init *ir.Nodes)
 		if !types.Identical(t, x.Type()) {
 			panic("dotdotdot base type does not match order's assigned type")
 		}
-		a = initStackTemp(init, x, vstat != nil)
+		a = initStackTemp(init, x, vstat)
 	} else if n.Esc() == ir.EscNone {
-		if vstat == nil {
-			// TODO(mdempsky): Remove this useless temporary.
-			// It's only needed to keep toolstash happy.
-			typecheck.Temp(t)
-		}
-		a = initStackTemp(init, typecheck.Temp(t), vstat != nil)
+		a = initStackTemp(init, typecheck.Temp(t), vstat)
 	} else {
 		a = ir.NewUnaryExpr(base.Pos, ir.ONEW, ir.TypeNode(t))
 	}
 	appendWalkStmt(init, ir.NewAssignStmt(base.Pos, vauto, a))
 
-	if vstat != nil {
-		// copy static to heap (4)
+	if vstat != nil && n.Prealloc == nil && n.Esc() != ir.EscNone {
+		// If we allocated on the heap with ONEW, copy the static to the
+		// heap (4). We skip this for stack temporaries, because
+		// initStackTemp already handled the copy.
 		a = ir.NewStarExpr(base.Pos, vauto)
 		appendWalkStmt(init, ir.NewAssignStmt(base.Pos, a, vstat))
 	}
@@ -535,7 +532,7 @@ func anylit(n ir.Node, var_ ir.Node, init *ir.Nodes) {
 		var r ir.Node
 		if n.Prealloc != nil {
 			// n.Prealloc is stack temporary used as backing store.
-			r = initStackTemp(init, n.Prealloc, false)
+			r = initStackTemp(init, n.Prealloc, nil)
 		} else {
 			r = ir.NewUnaryExpr(base.Pos, ir.ONEW, ir.TypeNode(n.X.Type()))
 			r.SetEsc(n.Esc())
diff --git a/src/cmd/compile/internal/walk/temp.go b/src/cmd/compile/internal/walk/temp.go
index 901cb770f3..9879a6c69d 100644
--- a/src/cmd/compile/internal/walk/temp.go
+++ b/src/cmd/compile/internal/walk/temp.go
@@ -12,19 +12,12 @@ import (
 )
 
 // initStackTemp appends statements to init to initialize the given
-// temporary variable, and then returns the expression &tmp. If vardef
-// is true, then the variable is initialized with OVARDEF, and the
-// caller must ensure the variable is later assigned before use;
-// otherwise, it's zero initialized.
-//
-// TODO(mdempsky): Change callers to provide tmp's initial value,
-// rather than just vardef, to make this safer/easier to use.
-func initStackTemp(init *ir.Nodes, tmp *ir.Name, vardef bool) *ir.AddrExpr {
-	if vardef {
-		init.Append(ir.NewUnaryExpr(base.Pos, ir.OVARDEF, tmp))
-	} else {
-		appendWalkStmt(init, ir.NewAssignStmt(base.Pos, tmp, nil))
+// temporary variable to val, and then returns the expression &tmp.
+func initStackTemp(init *ir.Nodes, tmp *ir.Name, val ir.Node) *ir.AddrExpr {
+	if val != nil && !types.Identical(tmp.Type(), val.Type()) {
+		base.Fatalf("bad initial value for %L: %L", tmp, val)
 	}
+	appendWalkStmt(init, ir.NewAssignStmt(base.Pos, tmp, val))
 	return typecheck.Expr(typecheck.NodAddr(tmp)).(*ir.AddrExpr)
 }
 
@@ -32,7 +25,7 @@ func initStackTemp(init *ir.Nodes, tmp *ir.Name, vardef bool) *ir.AddrExpr {
 // allocated temporary variable of the given type. Statements to
 // zero-initialize tmp are appended to init.
 func stackTempAddr(init *ir.Nodes, typ *types.Type) *ir.AddrExpr {
-	return initStackTemp(init, typecheck.Temp(typ), false)
+	return initStackTemp(init, typecheck.Temp(typ), nil)
 }
 
 // stackBufAddr returns thte expression &tmp, where tmp is a newly