[Bug tree-optimization/21982] GCC should combine adjacent stdio calls

[dnovillo at redhat dot com]

------- Additional Comments From dnovillo at redhat dot com  2005-06-09 19:03 -------
Subject: Re:  GCC should combine adjacent stdio calls

On Thu, Jun 09, 2005 at 05:02:28PM -0000, ghazi at gcc dot gnu dot org wrote:

>   int i=0, j=2;
>   printf("%d", i);
>   j++;
>   printf("%d", j);
> 
> Pushing the first printf further down, this could be reordered as:
>   int i=0, j=2;
>   j++;
>   printf("%d", i);
>   printf("%d", j);
> 
> which would expose another combination possibility.
> 
> Paul seemed to think this wasn't hard with the existing infrastructure, and 
> that was two years ago.  
>
Oh, absolutely.  The algorithm I'm using will naturally do this.
This is a purely local transformation, we obviously cannot merge
builtins in different control flow paths, so the transformation
goes like this: when we get to a builtin, we try to merge it with
a previously found builtin.  The only time we reset the concept
of "previously found builtin" is when we find a CALL_EXPR or an
ASM_EXPR which are the only ones that may have side-effects
affecting the output of the program.

If the program manipulates the same FILE * that is being used by
the stdio calls, then we'd lose.  But I think that's fair game,
right?


Diego.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21982