cmd/compile: avoid double-zeroing

This triggers in 131 functions in std+cmd.
In those functions, it often helps considerably
(2-10% text size reduction).

Noticed while working on #38554.

Change-Id: Id0dbb8e7cb21d469ec08ec3d5be9beb9e8291e9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/229707
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -2418,6 +2418,10 @@
 	&& (disjoint(src, s, dst, s) || isInlinableMemmove(dst, src, s, config))
 	=> (Move {t1} [s] dst src midmem)
 
+// Don't zero the same bits twice.
+(Zero {t} [s] dst1 zero:(Zero {t} [s] dst2 _)) && isSamePtr(dst1, dst2) => zero
+(Zero {t} [s] dst1 vardef:(VarDef (Zero {t} [s] dst2 _))) && isSamePtr(dst1, dst2) => vardef
+
 // Elide self-moves. This only happens rarely (e.g test/fixedbugs/bug277.go).
 // However, this rule is needed to prevent the previous rule from looping forever in such cases.
 (Move dst src mem) && isSamePtr(dst, src) => mem

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

@@ -24204,6 +24204,46 @@ func rewriteValuegeneric_OpZero(v *Value) bool {
 		v.AddArg2(dst1, v0)
 		return true
 	}
+	// match: (Zero {t} [s] dst1 zero:(Zero {t} [s] dst2 _))
+	// cond: isSamePtr(dst1, dst2)
+	// result: zero
+	for {
+		s := auxIntToInt64(v.AuxInt)
+		t := auxToType(v.Aux)
+		dst1 := v_0
+		zero := v_1
+		if zero.Op != OpZero || auxIntToInt64(zero.AuxInt) != s || auxToType(zero.Aux) != t {
+			break
+		}
+		dst2 := zero.Args[0]
+		if !(isSamePtr(dst1, dst2)) {
+			break
+		}
+		v.copyOf(zero)
+		return true
+	}
+	// match: (Zero {t} [s] dst1 vardef:(VarDef (Zero {t} [s] dst2 _)))
+	// cond: isSamePtr(dst1, dst2)
+	// result: vardef
+	for {
+		s := auxIntToInt64(v.AuxInt)
+		t := auxToType(v.Aux)
+		dst1 := v_0
+		vardef := v_1
+		if vardef.Op != OpVarDef {
+			break
+		}
+		vardef_0 := vardef.Args[0]
+		if vardef_0.Op != OpZero || auxIntToInt64(vardef_0.AuxInt) != s || auxToType(vardef_0.Aux) != t {
+			break
+		}
+		dst2 := vardef_0.Args[0]
+		if !(isSamePtr(dst1, dst2)) {
+			break
+		}
+		v.copyOf(vardef)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpZeroExt16to32(v *Value) bool {