src/cmd/compile/internal/ssa/gen/MIPS64.rules  |  3 ++-
 src/cmd/compile/internal/ssa/gen/RISCV64.rules |  2 +-
 src/cmd/compile/internal/ssa/rewriteMIPS64.go  | 24 ++++++++++++++++++++++--
 src/cmd/compile/internal/ssa/rewriteRISCV64.go | 24 ++++++++++++++++++++++--
 src/runtime/internal/atomic/atomic_test.go     | 30 ++++++++++++++++++++++++++++++

diff --git a/src/cmd/compile/internal/ssa/gen/MIPS64.rules b/src/cmd/compile/internal/ssa/gen/MIPS64.rules
index 292ff2fc7966092fac267f78a39a1d939e221c12..0d6d30fa4c93c9eeeaffca21574a12a1b6fcb335 100644
--- a/src/cmd/compile/internal/ssa/gen/MIPS64.rules
+++ b/src/cmd/compile/internal/ssa/gen/MIPS64.rules
@@ -392,7 +392,8 @@ (AtomicExchange(32|64) ...) => (LoweredAtomicExchange(32|64) ...)
 
 (AtomicAdd(32|64) ...) => (LoweredAtomicAdd(32|64) ...)
 
-(AtomicCompareAndSwap(32|64) ...) => (LoweredAtomicCas(32|64) ...)
+(AtomicCompareAndSwap32 ptr old new mem) => (LoweredAtomicCas32 ptr (SignExt32to64 old) new mem)
+(AtomicCompareAndSwap64 ...) => (LoweredAtomicCas64 ...)
 
 // checks
 (NilCheck ...) => (LoweredNilCheck ...)
diff --git a/src/cmd/compile/internal/ssa/gen/RISCV64.rules b/src/cmd/compile/internal/ssa/gen/RISCV64.rules
index 7aea622c5e81139212726c98f7c72db9ca9be3c3..acef3df3893b9a48eb9ff5bb5819217ed272f41b 100644
--- a/src/cmd/compile/internal/ssa/gen/RISCV64.rules
+++ b/src/cmd/compile/internal/ssa/gen/RISCV64.rules
@@ -568,7 +568,7 @@ 			(SLLI <typ.UInt64> [3] (ANDI <typ.UInt64> [3] ptr)))) mem)
 
 (AtomicAnd32 ...) => (LoweredAtomicAnd32 ...)
 
-(AtomicCompareAndSwap32 ...) => (LoweredAtomicCas32 ...)
+(AtomicCompareAndSwap32 ptr old new mem) => (LoweredAtomicCas32 ptr (SignExt32to64 old) new mem)
 (AtomicCompareAndSwap64 ...) => (LoweredAtomicCas64 ...)
 
 (AtomicExchange32 ...) => (LoweredAtomicExchange32 ...)
diff --git a/src/cmd/compile/internal/ssa/rewriteMIPS64.go b/src/cmd/compile/internal/ssa/rewriteMIPS64.go
index 1fbd556b5cbcd7d32cb9ad6424d5c5d2a14580e7..6a0fd3ad6ee20352bb2eb4b72fc125de7cc8646c 100644
--- a/src/cmd/compile/internal/ssa/rewriteMIPS64.go
+++ b/src/cmd/compile/internal/ssa/rewriteMIPS64.go
@@ -52,8 +52,7 @@ 	case OpAtomicAdd64:
 		v.Op = OpMIPS64LoweredAtomicAdd64
 		return true
 	case OpAtomicCompareAndSwap32:
-		v.Op = OpMIPS64LoweredAtomicCas32
-		return true
+		return rewriteValueMIPS64_OpAtomicCompareAndSwap32(v)
 	case OpAtomicCompareAndSwap64:
 		v.Op = OpMIPS64LoweredAtomicCas64
 		return true
@@ -694,6 +693,27 @@ 		base := v_0
 		v.reset(OpMIPS64MOVVaddr)
 		v.Aux = symToAux(sym)
 		v.AddArg(base)
+		return true
+	}
+}
+func rewriteValueMIPS64_OpAtomicCompareAndSwap32(v *Value) bool {
+	v_3 := v.Args[3]
+	v_2 := v.Args[2]
+	v_1 := v.Args[1]
+	v_0 := v.Args[0]
+	b := v.Block
+	typ := &b.Func.Config.Types
+	// match: (AtomicCompareAndSwap32 ptr old new mem)
+	// result: (LoweredAtomicCas32 ptr (SignExt32to64 old) new mem)
+	for {
+		ptr := v_0
+		old := v_1
+		new := v_2
+		mem := v_3
+		v.reset(OpMIPS64LoweredAtomicCas32)
+		v0 := b.NewValue0(v.Pos, OpSignExt32to64, typ.Int64)
+		v0.AddArg(old)
+		v.AddArg4(ptr, v0, new, mem)
 		return true
 	}
 }
diff --git a/src/cmd/compile/internal/ssa/rewriteRISCV64.go b/src/cmd/compile/internal/ssa/rewriteRISCV64.go
index 6828d97ff8f7df0e26d9360b776110f5aeeb8c7b..b277979061ecc8620b32de15b78e68b8dc6dfaf4 100644
--- a/src/cmd/compile/internal/ssa/rewriteRISCV64.go
+++ b/src/cmd/compile/internal/ssa/rewriteRISCV64.go
@@ -61,8 +61,7 @@ 		return true
 	case OpAtomicAnd8:
 		return rewriteValueRISCV64_OpAtomicAnd8(v)
 	case OpAtomicCompareAndSwap32:
-		v.Op = OpRISCV64LoweredAtomicCas32
-		return true
+		return rewriteValueRISCV64_OpAtomicCompareAndSwap32(v)
 	case OpAtomicCompareAndSwap64:
 		v.Op = OpRISCV64LoweredAtomicCas64
 		return true
@@ -762,6 +761,27 @@ 		v5.AddArg(v6)
 		v2.AddArg2(v3, v5)
 		v1.AddArg(v2)
 		v.AddArg3(v0, v1, mem)
+		return true
+	}
+}
+func rewriteValueRISCV64_OpAtomicCompareAndSwap32(v *Value) bool {
+	v_3 := v.Args[3]
+	v_2 := v.Args[2]
+	v_1 := v.Args[1]
+	v_0 := v.Args[0]
+	b := v.Block
+	typ := &b.Func.Config.Types
+	// match: (AtomicCompareAndSwap32 ptr old new mem)
+	// result: (LoweredAtomicCas32 ptr (SignExt32to64 old) new mem)
+	for {
+		ptr := v_0
+		old := v_1
+		new := v_2
+		mem := v_3
+		v.reset(OpRISCV64LoweredAtomicCas32)
+		v0 := b.NewValue0(v.Pos, OpSignExt32to64, typ.Int64)
+		v0.AddArg(old)
+		v.AddArg4(ptr, v0, new, mem)
 		return true
 	}
 }
diff --git a/src/runtime/internal/atomic/atomic_test.go b/src/runtime/internal/atomic/atomic_test.go
index 2ae60b8507ef5831e2d5345514224696c0b85623..2427bfd211ee9cd84e858783d3a2c63473b7c2fe 100644
--- a/src/runtime/internal/atomic/atomic_test.go
+++ b/src/runtime/internal/atomic/atomic_test.go
@@ -345,6 +345,36 @@ 		}
 	}
 }
 
+func TestCasRel(t *testing.T) {
+	const _magic = 0x5a5aa5a5
+	var x struct {
+		before uint32
+		i      uint32
+		after  uint32
+		o      uint32
+		n      uint32
+	}
+
+	x.before = _magic
+	x.after = _magic
+	for j := 0; j < 32; j += 1 {
+		x.i = (1 << j) + 0
+		x.o = (1 << j) + 0
+		x.n = (1 << j) + 1
+		if !atomic.CasRel(&x.i, x.o, x.n) {
+			t.Fatalf("should have swapped %#x %#x", x.o, x.n)
+		}
+
+		if x.i != x.n {
+			t.Fatalf("wrong x.i after swap: x.i=%#x x.n=%#x", x.i, x.n)
+		}
+
+		if x.before != _magic || x.after != _magic {
+			t.Fatalf("wrong magic: %#x _ %#x != %#x _ %#x", x.before, x.after, _magic, _magic)
+		}
+	}
+}
+
 func TestStorepNoWB(t *testing.T) {
 	var p [2]*int
 	for i := range p {