diff --git a/src/cmd/compile/internal/ssa/gen/ARM.rules b/src/cmd/compile/internal/ssa/gen/ARM.rules
index 9490805f46..aad7236d59 100644
--- a/src/cmd/compile/internal/ssa/gen/ARM.rules
+++ b/src/cmd/compile/internal/ssa/gen/ARM.rules
@@ -1263,8 +1263,8 @@
 (SRLconst (SLLconst x [c]) [d]) && objabi.GOARM==7 && uint64(d)>=uint64(c) && uint64(d)<=31 => (BFXU [(d-c)|(32-d)<<8] x)
 
 // comparison simplification
-(CMP x (RSBconst [0] y)) => (CMN x y)
-(CMN x (RSBconst [0] y)) => (CMP x y)
+((LT|LE|EQ|NE|GE|GT) (CMP x (RSBconst [0] y))) => ((LT|LE|EQ|NE|GE|GT) (CMN x y)) // sense of carry bit not preserved
+((LT|LE|EQ|NE|GE|GT) (CMN x (RSBconst [0] y))) => ((LT|LE|EQ|NE|GE|GT) (CMP x y)) // sense of carry bit not preserved
 (EQ (CMPconst [0] l:(SUB x y)) yes no) && l.Uses==1 => (EQ (CMP x y) yes no)
 (EQ (CMPconst [0] l:(MULS x y a)) yes no) && l.Uses==1 => (EQ (CMP a (MUL <x.Type> x y)) yes no)
 (EQ (CMPconst [0] l:(SUBconst [c] x)) yes no) && l.Uses==1 => (EQ (CMPconst [c] x) yes no)
diff --git a/src/cmd/compile/internal/ssa/rewriteARM.go b/src/cmd/compile/internal/ssa/rewriteARM.go
index 4e44165169..435da688b7 100644
--- a/src/cmd/compile/internal/ssa/rewriteARM.go
+++ b/src/cmd/compile/internal/ssa/rewriteARM.go
@@ -3362,21 +3362,6 @@ func rewriteValueARM_OpARMCMN(v *Value) bool {
 		}
 		break
 	}
-	// match: (CMN x (RSBconst [0] y))
-	// result: (CMP x y)
-	for {
-		for _i0 := 0; _i0 <= 1; _i0, v_0, v_1 = _i0+1, v_1, v_0 {
-			x := v_0
-			if v_1.Op != OpARMRSBconst || auxIntToInt32(v_1.AuxInt) != 0 {
-				continue
-			}
-			y := v_1.Args[0]
-			v.reset(OpARMCMP)
-			v.AddArg2(x, y)
-			return true
-		}
-		break
-	}
 	return false
 }
 func rewriteValueARM_OpARMCMNconst(v *Value) bool {
@@ -3938,18 +3923,6 @@ func rewriteValueARM_OpARMCMP(v *Value) bool {
 		v.AddArg(v0)
 		return true
 	}
-	// match: (CMP x (RSBconst [0] y))
-	// result: (CMN x y)
-	for {
-		x := v_0
-		if v_1.Op != OpARMRSBconst || auxIntToInt32(v_1.AuxInt) != 0 {
-			break
-		}
-		y := v_1.Args[0]
-		v.reset(OpARMCMN)
-		v.AddArg2(x, y)
-		return true
-	}
 	return false
 }
 func rewriteValueARM_OpARMCMPD(v *Value) bool {
@@ -16002,6 +15975,42 @@ func rewriteBlockARM(b *Block) bool {
 			b.resetWithControl(BlockARMEQ, cmp)
 			return true
 		}
+		// match: (EQ (CMP x (RSBconst [0] y)))
+		// result: (EQ (CMN x y))
+		for b.Controls[0].Op == OpARMCMP {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			x := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+				break
+			}
+			y := v_0_1.Args[0]
+			v0 := b.NewValue0(v_0.Pos, OpARMCMN, types.TypeFlags)
+			v0.AddArg2(x, y)
+			b.resetWithControl(BlockARMEQ, v0)
+			return true
+		}
+		// match: (EQ (CMN x (RSBconst [0] y)))
+		// result: (EQ (CMP x y))
+		for b.Controls[0].Op == OpARMCMN {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			v_0_0 := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			for _i0 := 0; _i0 <= 1; _i0, v_0_0, v_0_1 = _i0+1, v_0_1, v_0_0 {
+				x := v_0_0
+				if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+					continue
+				}
+				y := v_0_1.Args[0]
+				v0 := b.NewValue0(v_0.Pos, OpARMCMP, types.TypeFlags)
+				v0.AddArg2(x, y)
+				b.resetWithControl(BlockARMEQ, v0)
+				return true
+			}
+			break
+		}
 		// match: (EQ (CMPconst [0] l:(SUB x y)) yes no)
 		// cond: l.Uses==1
 		// result: (EQ (CMP x y) yes no)
@@ -16848,6 +16857,42 @@ func rewriteBlockARM(b *Block) bool {
 			b.resetWithControl(BlockARMLE, cmp)
 			return true
 		}
+		// match: (GE (CMP x (RSBconst [0] y)))
+		// result: (GE (CMN x y))
+		for b.Controls[0].Op == OpARMCMP {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			x := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+				break
+			}
+			y := v_0_1.Args[0]
+			v0 := b.NewValue0(v_0.Pos, OpARMCMN, types.TypeFlags)
+			v0.AddArg2(x, y)
+			b.resetWithControl(BlockARMGE, v0)
+			return true
+		}
+		// match: (GE (CMN x (RSBconst [0] y)))
+		// result: (GE (CMP x y))
+		for b.Controls[0].Op == OpARMCMN {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			v_0_0 := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			for _i0 := 0; _i0 <= 1; _i0, v_0_0, v_0_1 = _i0+1, v_0_1, v_0_0 {
+				x := v_0_0
+				if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+					continue
+				}
+				y := v_0_1.Args[0]
+				v0 := b.NewValue0(v_0.Pos, OpARMCMP, types.TypeFlags)
+				v0.AddArg2(x, y)
+				b.resetWithControl(BlockARMGE, v0)
+				return true
+			}
+			break
+		}
 		// match: (GE (CMPconst [0] l:(SUB x y)) yes no)
 		// cond: l.Uses==1
 		// result: (GEnoov (CMP x y) yes no)
@@ -17728,6 +17773,42 @@ func rewriteBlockARM(b *Block) bool {
 			b.resetWithControl(BlockARMLT, cmp)
 			return true
 		}
+		// match: (GT (CMP x (RSBconst [0] y)))
+		// result: (GT (CMN x y))
+		for b.Controls[0].Op == OpARMCMP {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			x := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+				break
+			}
+			y := v_0_1.Args[0]
+			v0 := b.NewValue0(v_0.Pos, OpARMCMN, types.TypeFlags)
+			v0.AddArg2(x, y)
+			b.resetWithControl(BlockARMGT, v0)
+			return true
+		}
+		// match: (GT (CMN x (RSBconst [0] y)))
+		// result: (GT (CMP x y))
+		for b.Controls[0].Op == OpARMCMN {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			v_0_0 := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			for _i0 := 0; _i0 <= 1; _i0, v_0_0, v_0_1 = _i0+1, v_0_1, v_0_0 {
+				x := v_0_0
+				if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+					continue
+				}
+				y := v_0_1.Args[0]
+				v0 := b.NewValue0(v_0.Pos, OpARMCMP, types.TypeFlags)
+				v0.AddArg2(x, y)
+				b.resetWithControl(BlockARMGT, v0)
+				return true
+			}
+			break
+		}
 		// match: (GT (CMPconst [0] l:(SUB x y)) yes no)
 		// cond: l.Uses==1
 		// result: (GTnoov (CMP x y) yes no)
@@ -18699,6 +18780,42 @@ func rewriteBlockARM(b *Block) bool {
 			b.resetWithControl(BlockARMGE, cmp)
 			return true
 		}
+		// match: (LE (CMP x (RSBconst [0] y)))
+		// result: (LE (CMN x y))
+		for b.Controls[0].Op == OpARMCMP {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			x := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+				break
+			}
+			y := v_0_1.Args[0]
+			v0 := b.NewValue0(v_0.Pos, OpARMCMN, types.TypeFlags)
+			v0.AddArg2(x, y)
+			b.resetWithControl(BlockARMLE, v0)
+			return true
+		}
+		// match: (LE (CMN x (RSBconst [0] y)))
+		// result: (LE (CMP x y))
+		for b.Controls[0].Op == OpARMCMN {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			v_0_0 := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			for _i0 := 0; _i0 <= 1; _i0, v_0_0, v_0_1 = _i0+1, v_0_1, v_0_0 {
+				x := v_0_0
+				if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+					continue
+				}
+				y := v_0_1.Args[0]
+				v0 := b.NewValue0(v_0.Pos, OpARMCMP, types.TypeFlags)
+				v0.AddArg2(x, y)
+				b.resetWithControl(BlockARMLE, v0)
+				return true
+			}
+			break
+		}
 		// match: (LE (CMPconst [0] l:(SUB x y)) yes no)
 		// cond: l.Uses==1
 		// result: (LEnoov (CMP x y) yes no)
@@ -19579,6 +19696,42 @@ func rewriteBlockARM(b *Block) bool {
 			b.resetWithControl(BlockARMGT, cmp)
 			return true
 		}
+		// match: (LT (CMP x (RSBconst [0] y)))
+		// result: (LT (CMN x y))
+		for b.Controls[0].Op == OpARMCMP {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			x := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+				break
+			}
+			y := v_0_1.Args[0]
+			v0 := b.NewValue0(v_0.Pos, OpARMCMN, types.TypeFlags)
+			v0.AddArg2(x, y)
+			b.resetWithControl(BlockARMLT, v0)
+			return true
+		}
+		// match: (LT (CMN x (RSBconst [0] y)))
+		// result: (LT (CMP x y))
+		for b.Controls[0].Op == OpARMCMN {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			v_0_0 := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			for _i0 := 0; _i0 <= 1; _i0, v_0_0, v_0_1 = _i0+1, v_0_1, v_0_0 {
+				x := v_0_0
+				if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+					continue
+				}
+				y := v_0_1.Args[0]
+				v0 := b.NewValue0(v_0.Pos, OpARMCMP, types.TypeFlags)
+				v0.AddArg2(x, y)
+				b.resetWithControl(BlockARMLT, v0)
+				return true
+			}
+			break
+		}
 		// match: (LT (CMPconst [0] l:(SUB x y)) yes no)
 		// cond: l.Uses==1
 		// result: (LTnoov (CMP x y) yes no)
@@ -20609,6 +20762,42 @@ func rewriteBlockARM(b *Block) bool {
 			b.resetWithControl(BlockARMNE, cmp)
 			return true
 		}
+		// match: (NE (CMP x (RSBconst [0] y)))
+		// result: (NE (CMN x y))
+		for b.Controls[0].Op == OpARMCMP {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			x := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+				break
+			}
+			y := v_0_1.Args[0]
+			v0 := b.NewValue0(v_0.Pos, OpARMCMN, types.TypeFlags)
+			v0.AddArg2(x, y)
+			b.resetWithControl(BlockARMNE, v0)
+			return true
+		}
+		// match: (NE (CMN x (RSBconst [0] y)))
+		// result: (NE (CMP x y))
+		for b.Controls[0].Op == OpARMCMN {
+			v_0 := b.Controls[0]
+			_ = v_0.Args[1]
+			v_0_0 := v_0.Args[0]
+			v_0_1 := v_0.Args[1]
+			for _i0 := 0; _i0 <= 1; _i0, v_0_0, v_0_1 = _i0+1, v_0_1, v_0_0 {
+				x := v_0_0
+				if v_0_1.Op != OpARMRSBconst || auxIntToInt32(v_0_1.AuxInt) != 0 {
+					continue
+				}
+				y := v_0_1.Args[0]
+				v0 := b.NewValue0(v_0.Pos, OpARMCMP, types.TypeFlags)
+				v0.AddArg2(x, y)
+				b.resetWithControl(BlockARMNE, v0)
+				return true
+			}
+			break
+		}
 		// match: (NE (CMPconst [0] l:(SUB x y)) yes no)
 		// cond: l.Uses==1
 		// result: (NE (CMP x y) yes no)
diff --git a/test/fixedbugs/issue41780.go b/test/fixedbugs/issue41780.go
new file mode 100644
index 0000000000..632c144a48
--- /dev/null
+++ b/test/fixedbugs/issue41780.go
@@ -0,0 +1,39 @@
+// run
+
+// 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.
+
+// Checks that conversion of CMP(x,-y) -> CMN(x,y) is only applied in correct context.
+
+package main
+
+type decimal struct {
+	d  [8]byte // digits, big-endian representation
+	dp int     // decimal point
+}
+
+var powtab = []int{1, 3, 6, 9, 13, 16, 19, 23, 26}
+
+//go:noinline
+func foo(d *decimal) int {
+	exp := int(d.d[1])
+	if d.dp < 0 || d.dp == 0 && d.d[0] < '5' {
+		var n int
+		if -d.dp >= len(powtab) {
+			n = 27
+		} else {
+			n = powtab[-d.dp] // incorrect CMP -> CMN substitution causes indexing panic.
+		}
+		exp += n
+	}
+	return exp
+}
+
+func main() {
+	var d decimal
+	d.d[0] = '1'
+	if foo(&d) != 1 {
+		println("FAILURE (though not the one this test was written to catch)")
+	}
+}