diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index bfc0004ff8..34a2ee8d13 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -1295,7 +1295,9 @@ func zeroUpper32Bits(x *Value, depth int) bool {
 		OpARM64MADDW, OpARM64MSUBW, OpARM64RORW, OpARM64RORWconst:
 		return true
 	case OpArg: // note: but not ArgIntReg
-		return x.Type.Size() == 4
+		// amd64 always loads args from the stack unsigned.
+		// most other architectures load them sign/zero extended based on the type.
+		return x.Type.Size() == 4 && (x.Type.IsUnsigned() || x.Block.Func.Config.arch == "amd64")
 	case OpPhi, OpSelect0, OpSelect1:
 		// Phis can use each-other as an arguments, instead of tracking visited values,
 		// just limit recursion depth.
@@ -1319,7 +1321,7 @@ func zeroUpper48Bits(x *Value, depth int) bool {
 	case OpAMD64MOVWQZX, OpAMD64MOVWload, OpAMD64MOVWloadidx1, OpAMD64MOVWloadidx2:
 		return true
 	case OpArg: // note: but not ArgIntReg
-		return x.Type.Size() == 2
+		return x.Type.Size() == 2 && (x.Type.IsUnsigned() || x.Block.Func.Config.arch == "amd64")
 	case OpPhi, OpSelect0, OpSelect1:
 		// Phis can use each-other as an arguments, instead of tracking visited values,
 		// just limit recursion depth.
@@ -1343,7 +1345,7 @@ func zeroUpper56Bits(x *Value, depth int) bool {
 	case OpAMD64MOVBQZX, OpAMD64MOVBload, OpAMD64MOVBloadidx1:
 		return true
 	case OpArg: // note: but not ArgIntReg
-		return x.Type.Size() == 1
+		return x.Type.Size() == 1 && (x.Type.IsUnsigned() || x.Block.Func.Config.arch == "amd64")
 	case OpPhi, OpSelect0, OpSelect1:
 		// Phis can use each-other as an arguments, instead of tracking visited values,
 		// just limit recursion depth.
diff --git a/test/fixedbugs/issue66066b.go b/test/fixedbugs/issue66066b.go
new file mode 100644
index 0000000000..7540a85293
--- /dev/null
+++ b/test/fixedbugs/issue66066b.go
@@ -0,0 +1,58 @@
+// run
+
+// Copyright 2024 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.
+
+package main
+
+//go:noinline
+func f32(_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, x int32) uint64 {
+	return uint64(uint32(x))
+}
+
+//go:noinline
+func f16(_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, x int16) uint64 {
+	return uint64(uint16(x))
+}
+
+//go:noinline
+func f8(_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, x int8) uint64 {
+	return uint64(uint8(x))
+}
+
+//go:noinline
+func g32(_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, x uint32) int64 {
+	return int64(int32(x))
+}
+
+//go:noinline
+func g16(_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, x uint16) int64 {
+	return int64(int16(x))
+}
+
+//go:noinline
+func g8(_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, x uint8) int64 {
+	return int64(int8(x))
+}
+
+func main() {
+	if got := f32(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1); got != 0xffffffff {
+		println("bad f32", got)
+	}
+	if got := f16(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1); got != 0xffff {
+		println("bad f16", got)
+	}
+	if got := f8(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1); got != 0xff {
+		println("bad f8", got)
+	}
+	if got := g32(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xffffffff); got != -1 {
+		println("bad g32", got)
+	}
+	if got := g16(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xffff); got != -1 {
+		println("bad g16", got)
+	}
+	if got := g8(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff); got != -1 {
+		println("bad g8", got)
+	}
+}