4 files changed, 55 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/gen/generic.rules b/src/cmd/compile/internal/ssa/gen/generic.rules
index 328585bbb6..5d64e26358 100644
--- a/src/cmd/compile/internal/ssa/gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/gen/generic.rules
@@ -2020,6 +2020,7 @@
 	=> (Zero {t} [n] dst1 mem)
 (Move {t} [n] dst1 src mem:(VarDef (Zero {t} [n] dst0 _))) && isSamePtr(src, dst0)
 	=> (Zero {t} [n] dst1 mem)
+(Move {t} [n] dst (Addr {sym} (SB)) mem) && symIsROZero(sym) => (Zero {t} [n] dst mem)
 
 // Don't Store to variables that are about to be overwritten by Move/Zero.
 (Zero {t1} [n] p1 store:(Store {t2} (OffPtr [o2] p2) _ mem))
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 0522a955dd..4572c7db20 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -1381,6 +1381,20 @@ func symIsRO(sym interface{}) bool {
 	return lsym.Type == objabi.SRODATA && len(lsym.R) == 0
 }
 
+// symIsROZero reports whether sym is a read-only global whose data contains all zeros.
+func symIsROZero(sym Sym) bool {
+	lsym := sym.(*obj.LSym)
+	if lsym.Type != objabi.SRODATA || len(lsym.R) != 0 {
+		return false
+	}
+	for _, b := range lsym.P {
+		if b != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // read8 reads one byte from the read-only global sym at offset off.
 func read8(sym interface{}, off int64) uint8 {
 	lsym := sym.(*obj.LSym)
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index d0f6363d13..fc2c78eb03 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -12447,6 +12447,31 @@ func rewriteValuegeneric_OpMove(v *Value) bool {
 		v.AddArg2(dst1, mem)
 		return true
 	}
+	// match: (Move {t} [n] dst (Addr {sym} (SB)) mem)
+	// cond: symIsROZero(sym)
+	// result: (Zero {t} [n] dst mem)
+	for {
+		n := auxIntToInt64(v.AuxInt)
+		t := auxToType(v.Aux)
+		dst := v_0
+		if v_1.Op != OpAddr {
+			break
+		}
+		sym := auxToSym(v_1.Aux)
+		v_1_0 := v_1.Args[0]
+		if v_1_0.Op != OpSB {
+			break
+		}
+		mem := v_2
+		if !(symIsROZero(sym)) {
+			break
+		}
+		v.reset(OpZero)
+		v.AuxInt = int64ToAuxInt(n)
+		v.Aux = typeToAux(t)
+		v.AddArg2(dst, mem)
+		return true
+	}
 	// match: (Move {t1} [n] dst1 src1 store:(Store {t2} op:(OffPtr [o2] dst2) _ mem))
 	// cond: isSamePtr(dst1, dst2) && store.Uses == 1 && n >= o2 + t2.Size() && disjoint(src1, n, op, t2.Size()) && clobber(store)
 	// result: (Move {t1} [n] dst1 src1 mem)
diff --git a/test/codegen/issue38554.go b/test/codegen/issue38554.go
new file mode 100644
index 0000000000..84db8473d3
--- /dev/null
+++ b/test/codegen/issue38554.go
@@ -0,0 +1,15 @@
+// asmcheck
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that we are zeroing directly instead of
+// copying a large zero value. Issue 38554.
+
+package codegen
+
+func retlarge() [256]byte {
+	// amd64:-"DUFFCOPY"
+	return [256]byte{}
+}