Re: What is the right usage of SAVE_EXPR?

[Mike Stump <mrs at apple dot com>]

On Apr 24, 2007, at 8:30 AM, Wolfgang Gellerich wrote:
> What is the policy concerning the usage of SAVE_EXPRs?

Roughly, if you do something like:

  tree foo(a, b)
    return build (a, b);

You don't need any saving.  If you do:

  tree foo(a, b)
    return build (a, build (a, b));

You need to code this as:

  tree foo(a, b)
    a = save_expr (a);
    return build (a, build (a, b));

The person that does the building, does the saving, as they know they  
are going to reuse a value and duplicate evaluations would be wrong.

> I thought the respective language front end were responsible to  
> enclose any expressions with side
> effects this way

No.  The first example above needs no saving.  Only when a value is  
used more than once, does it need to be saved.

> However, also some of the code translating tree nodes into rtxes like
> some functions found in builtins.c worry about the re-evaluation of
> arguments and insert plenty of SAVE_EXPRs. Why is that necessary?

Because they do as in the second example above.