interp: support some more expressions in const icmp

interp/frame.go

@@ -188,23 +188,8 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 				// Unfortunately, the const propagation in the IR builder
 				// doesn't handle pointer compares of inttoptr values. So we
 				// implement it manually here.
-				isNil := func(v llvm.Value) (result bool, ok bool) {
-					if !v.IsAConstantExpr().IsNil() && v.Opcode() == llvm.IntToPtr {
-						// Whether a constant inttoptr is nil is easy to
-						// determine.
-						operand := v.Operand(0)
-						if operand.IsConstant() {
-							return operand.ZExtValue() == 0, true
-						}
-					}
-					if !v.IsAConstantPointerNull().IsNil() {
-						// A constant pointer null is always null, of course.
-						return true, true
-					}
-					return false, false // not valid
-				}
-				lhsNil, ok1 := isNil(lhs)
-				rhsNil, ok2 := isNil(rhs)
+				lhsNil, ok1 := isPointerNil(lhs)
+				rhsNil, ok2 := isPointerNil(rhs)
 				if ok1 && ok2 {
 					if lhsNil && rhsNil {
 						// Both are nil, so this icmp is always evaluated to true.

interp/utils.go

@@ -94,3 +94,29 @@ func isScalar(t llvm.Type) bool {
 		return false
 	}
 }
+
+// isPointerNil returns whether this is a nil pointer or not. The ok value
+// indicates whether the result is certain: if it is false the result boolean is
+// not valid.
+func isPointerNil(v llvm.Value) (result bool, ok bool) {
+	if !v.IsAConstantExpr().IsNil() {
+		switch v.Opcode() {
+		case llvm.IntToPtr:
+			// Whether a constant inttoptr is nil is easy to
+			// determine.
+			operand := v.Operand(0)
+			if operand.IsConstant() {
+				return operand.ZExtValue() == 0, true
+			}
+		case llvm.BitCast, llvm.GetElementPtr:
+			// These const instructions are just a kind of wrappers for the
+			// underlying pointer.
+			return isPointerNil(v.Operand(0))
+		}
+	}
+	if !v.IsAConstantPointerNull().IsNil() {
+		// A constant pointer null is always null, of course.
+		return true, true
+	}
+	return false, false // not valid
+}