Re: Writing a C Compiler: lvalues

On 08-05-10 15:34, Andri Wagner wrote:
> What I'm trying to say is: the compiler yields different assembly code
> for when 'x' is a lvalue and when 'x' is not a lvalue.
>
> This gets more confusing when I have expressions such as 'x++'. This
> is simple, since 'x' is obviously a lvalue in this case. In the case
> of the compiler, I can parse 'x' and see that the lookahead points to
> '++', so it's a lvalue.
>
> But what about '(x)++'? In this case, the compiler evaluates the
> subexpression '(x)', and this expression results the value of 'x', not
> the address. Now I have a '++' ahead, so how can I know the address of
> 'x' since all that I have is a value?
>


> All documentation that I found about lvalues were too vague, and
> directed to the programmer, and not to the compiler writer. Are there
> any specific rules for determining if the result of a expression is a
> lvalue?


An lvalue is an expression that evaluates to an address, so it *can* be
used on the left hand side of an assignment. But this is not necessarily
the case. In an expression like (x + 5) x *is* an lvalue, but it isn't
used as such, so it should be compiled as an ordinary rvalue. So in the
expression x = foo, x should be compiled as an lvalue (an address to
which a value is assigned), and in the expression foo = x, x should be
compiled to an rvalue (code that results in the value of x).


As far as I can tell, you're trying syntax-directed translation on a
C-like language. That can be done but, in a grammar like C, you have to
postpone compilation of an identifier until you know how it's used.


If you want to see an example of using immediate (syntax-directed)
compilation of a C-like language, look at the source code of David Betz'
BOB compiler. It compiles to bytecodes, which are interpreted by a
voirtual machine.


The original DrDobbs article:


http://www.drdobbs.com/184409401


The latest sources via:


http://www.xlisp.org/

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