cmd/compile: remove zeroing after newobject

The Zero op right after newobject has been removed. But this rule
does not cover Store of constant zero (for SSA-able types). Add
rules to cover Store op as well.

Updates #19027.

Change-Id: I5d2b62eeca0aa9ce8dc7205b264b779de01c660b
Reviewed-on: https://go-review.googlesource.com/36836
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1390,6 +1390,18 @@
 	&& isSameSym(mem.Aux, "runtime.newobject")
 	&& c == config.ctxt.FixedFrameSize() + config.RegSize // offset of return value
 	-> mem
+(Store (Load (OffPtr [c] (SP)) mem) x mem)
+	&& isConstZero(x)
+	&& mem.Op == OpStaticCall
+	&& isSameSym(mem.Aux, "runtime.newobject")
+	&& c == config.ctxt.FixedFrameSize() + config.RegSize // offset of return value
+	-> mem
+(Store (OffPtr (Load (OffPtr [c] (SP)) mem)) x mem)
+	&& isConstZero(x)
+	&& mem.Op == OpStaticCall
+	&& isSameSym(mem.Aux, "runtime.newobject")
+	&& c == config.ctxt.FixedFrameSize() + config.RegSize // offset of return value
+	-> mem
 // nil checks just need to rewrite to something useless.
 // they will be deadcode eliminated soon afterwards.
 (NilCheck (Load (OffPtr [c] (SP)) mem) mem)

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

@@ -541,3 +541,13 @@ func experiment(f *Func) bool {
 	}
 	return r
 }
+
+func isConstZero(v *Value) bool {
+	switch v.Op {
+	case OpConstNil:
+		return true
+	case OpConst64, OpConst32, OpConst16, OpConst8, OpConstBool, OpConst32F, OpConst64F:
+		return v.AuxInt == 0
+	}
+	return false
+}

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

@@ -14668,6 +14668,70 @@ func rewriteValuegeneric_OpStore(v *Value, config *Config) bool {
 		v.AddArg(mem)
 		return true
 	}
+	// match: (Store (Load (OffPtr [c] (SP)) mem) x mem)
+	// cond: isConstZero(x) 	&& mem.Op == OpStaticCall 	&& isSameSym(mem.Aux, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize
+	// result: mem
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpLoad {
+			break
+		}
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpOffPtr {
+			break
+		}
+		c := v_0_0.AuxInt
+		v_0_0_0 := v_0_0.Args[0]
+		if v_0_0_0.Op != OpSP {
+			break
+		}
+		mem := v_0.Args[1]
+		x := v.Args[1]
+		if mem != v.Args[2] {
+			break
+		}
+		if !(isConstZero(x) && mem.Op == OpStaticCall && isSameSym(mem.Aux, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = mem.Type
+		v.AddArg(mem)
+		return true
+	}
+	// match: (Store (OffPtr (Load (OffPtr [c] (SP)) mem)) x mem)
+	// cond: isConstZero(x) 	&& mem.Op == OpStaticCall 	&& isSameSym(mem.Aux, "runtime.newobject") 	&& c == config.ctxt.FixedFrameSize() + config.RegSize
+	// result: mem
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpOffPtr {
+			break
+		}
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpLoad {
+			break
+		}
+		v_0_0_0 := v_0_0.Args[0]
+		if v_0_0_0.Op != OpOffPtr {
+			break
+		}
+		c := v_0_0_0.AuxInt
+		v_0_0_0_0 := v_0_0_0.Args[0]
+		if v_0_0_0_0.Op != OpSP {
+			break
+		}
+		mem := v_0_0.Args[1]
+		x := v.Args[1]
+		if mem != v.Args[2] {
+			break
+		}
+		if !(isConstZero(x) && mem.Op == OpStaticCall && isSameSym(mem.Aux, "runtime.newobject") && c == config.ctxt.FixedFrameSize()+config.RegSize) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = mem.Type
+		v.AddArg(mem)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpStringLen(v *Value, config *Config) bool {