diff --git a/src/cmd/compile/internal/ssa/gen/genericOps.go b/src/cmd/compile/internal/ssa/gen/genericOps.go
index 49160ab7c4..c077b0bfcf 100644
--- a/src/cmd/compile/internal/ssa/gen/genericOps.go
+++ b/src/cmd/compile/internal/ssa/gen/genericOps.go
@@ -17,8 +17,6 @@ package main
 // all args take signed inputs, or don't care whether their inputs
 // are signed or unsigned.
 
-// Unused portions of AuxInt are filled by sign-extending the used portion.
-// Users of AuxInt which interpret AuxInt as unsigned (e.g. shifts) must be careful.
 var genericOps = []opData{
 	// 2-input arithmetic
 	// Types must be consistent with Go typing. Add, for example, must take two values
diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go
index cf0118ac3c..d05f6088a5 100644
--- a/src/cmd/compile/internal/ssa/prove.go
+++ b/src/cmd/compile/internal/ssa/prove.go
@@ -234,7 +234,6 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 		r = reverseBits[r]
 	}
 	if v != nil && w.isGenericIntConst() {
-		c := w.AuxInt
 		// Note: all the +1/-1 below could overflow/underflow. Either will
 		// still generate correct results, it will just lead to imprecision.
 		// In fact if there is overflow/underflow, the corresponding
@@ -247,6 +246,7 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 		lim := noLimit
 		switch d {
 		case signed:
+			c := w.AuxInt
 			switch r {
 			case lt:
 				lim.max = c - 1
@@ -279,17 +279,7 @@ func (ft *factsTable) update(parent *Block, v, w *Value, d domain, r relation) {
 				lim.umax = uint64(lim.max)
 			}
 		case unsigned:
-			var uc uint64
-			switch w.Op {
-			case OpConst64:
-				uc = uint64(c)
-			case OpConst32:
-				uc = uint64(uint32(c))
-			case OpConst16:
-				uc = uint64(uint16(c))
-			case OpConst8:
-				uc = uint64(uint8(c))
-			}
+			uc := w.AuxUnsigned()
 			switch r {
 			case lt:
 				lim.umax = uc - 1
diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index ecf7b80115..9a79a99f54 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/src/cmd/compile/internal/ssa/value.go
@@ -31,6 +31,10 @@ type Value struct {
 	// Auxiliary info for this value. The type of this information depends on the opcode and type.
 	// AuxInt is used for integer values, Aux is used for other values.
 	// Floats are stored in AuxInt using math.Float64bits(f).
+	// Unused portions of AuxInt are filled by sign-extending the used portion,
+	// even if the represented value is unsigned.
+	// Users of AuxInt which interpret AuxInt as unsigned (e.g. shifts) must be careful.
+	// Use Value.AuxUnsigned to get the zero-extended value of AuxInt.
 	AuxInt int64
 	Aux    interface{}
 
@@ -86,6 +90,25 @@ func (v *Value) AuxInt32() int32 {
 	return int32(v.AuxInt)
 }
 
+// AuxUnsigned returns v.AuxInt as an unsigned value for OpConst*.
+// v.AuxInt is always sign-extended to 64 bits, even if the
+// represented value is unsigned. This undoes that sign extension.
+func (v *Value) AuxUnsigned() uint64 {
+	c := v.AuxInt
+	switch v.Op {
+	case OpConst64:
+		return uint64(c)
+	case OpConst32:
+		return uint64(uint32(c))
+	case OpConst16:
+		return uint64(uint16(c))
+	case OpConst8:
+		return uint64(uint8(c))
+	}
+	v.Fatalf("op %s isn't OpConst*", v.Op)
+	return 0
+}
+
 func (v *Value) AuxFloat() float64 {
 	if opcodeTable[v.Op].auxType != auxFloat32 && opcodeTable[v.Op].auxType != auxFloat64 {
 		v.Fatalf("op %s doesn't have a float aux field", v.Op)