cmd/compile: clarify unsigned interpretation of AuxInt

The way Value.AuxInt represents unsigned numbers is currently
documented in genericOps.go, which is not the most obvious place for
it. Move that documentation to Value.AuxInt. Furthermore, to make it
harder to use incorrectly, introduce a Value.AuxUnsigned accessor that
returns the zero-extended value of Value.AuxInt.

Change-Id: I85030c3c68761404058a430e0b1c7464591b2f42
Reviewed-on: https://go-review.googlesource.com/102597
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

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
+			uc := w.AuxUnsigned()
-			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))
-			}
 			switch r {
 			case lt:
 				lim.umax = uc - 1

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)