[dev.ssa] cmd/compile/internal/ssa: large object load/store vs. vardef

VarDef declarations are getting in the way of rewriting load/store
pairs into moves.  This change fixes that, albeit in a really hacky way.
Better options would be appreciated.

Increases coverage during make.bash from 67% to 71%.

Change-Id: I336e967687e2238c7d0d64e3b37132a731ad15c3
Reviewed-on: https://go-review.googlesource.com/14347Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -179,6 +179,7 @@
 
 // big-object moves (TODO: remove?)
 (Store [size] dst (Load src mem) mem) && size > config.IntSize -> (Move [size] dst src mem)
+(Store [size] dst (Load src mem) (VarDef {x} mem)) && size > config.IntSize -> (Move [size] dst src (VarDef {x} mem))
 
 (If (IsNonNil (GetG)) yes no) -> (First nil yes no)
 

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -362,8 +362,8 @@ var genericOps = []opData{
 	// Used during ssa construction.  Like Copy, but the arg has not been specified yet.
 	{name: "FwdRef"},
 
-	{name: "VarDef"},  // aux is a *gc.Node of a variable that is about to be initialized.  arg0=mem, returns mem
-	{name: "VarKill"}, // aux is a *gc.Node of a variable that is known to be dead.  arg0=mem, returns mem
+	{name: "VarDef", typ: "Mem"}, // aux is a *gc.Node of a variable that is about to be initialized.  arg0=mem, returns mem
+	{name: "VarKill"},            // aux is a *gc.Node of a variable that is known to be dead.  arg0=mem, returns mem
 }
 
 //     kind           control    successors

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

@@ -1434,6 +1434,44 @@ func rewriteValuegeneric(v *Value, config *Config) bool {
 		goto enda18a7163888e2f4fca9f38bae56cef42
 	enda18a7163888e2f4fca9f38bae56cef42:
 		;
+		// match: (Store [size] dst (Load src mem) (VarDef {x} mem))
+		// cond: size > config.IntSize
+		// result: (Move [size] dst src (VarDef {x} mem))
+		{
+			size := v.AuxInt
+			dst := v.Args[0]
+			if v.Args[1].Op != OpLoad {
+				goto endc671c9b1be99e3125fe81e29018bc0e6
+			}
+			src := v.Args[1].Args[0]
+			mem := v.Args[1].Args[1]
+			if v.Args[2].Op != OpVarDef {
+				goto endc671c9b1be99e3125fe81e29018bc0e6
+			}
+			x := v.Args[2].Aux
+			if v.Args[2].Args[0] != mem {
+				goto endc671c9b1be99e3125fe81e29018bc0e6
+			}
+			if !(size > config.IntSize) {
+				goto endc671c9b1be99e3125fe81e29018bc0e6
+			}
+			v.Op = OpMove
+			v.AuxInt = 0
+			v.Aux = nil
+			v.resetArgs()
+			v.AuxInt = size
+			v.AddArg(dst)
+			v.AddArg(src)
+			v0 := b.NewValue0(v.Line, OpVarDef, TypeInvalid)
+			v0.Aux = x
+			v0.AddArg(mem)
+			v0.Type = TypeMem
+			v.AddArg(v0)
+			return true
+		}
+		goto endc671c9b1be99e3125fe81e29018bc0e6
+	endc671c9b1be99e3125fe81e29018bc0e6:
+		;
 	case OpStringLen:
 		// match: (StringLen (StringMake _ len))
 		// cond: