Re: [PATCH] PR 10011: Introduce gcse_constant_p

[Richard Henderson <rth at redhat dot com>]

On Sun, Mar 30, 2003 at 03:56:05PM -0700, Roger Sayle wrote:
> I have to disagree with you here, moving purge_builtin_constant_p
> is just papering over the problem.  Your fix doesn't address the
> example posted with my original solution.
> 
> void foo()
> {
>   int x = 100;
>   int y = 20000;
>   do {
>     if (x > y)
>       undef ();
>   } while (x == 4);
> }

And what does this have to do with __builtin_constant_p?

> Notice that GCSE is unable to optimize away the call to undef
> on *some* platforms.  If this fundamental problem is fixed, we
> can run purge_builtin_constant_p as late as previously, and even
> take advantage that the argument may be constant due to loop peeling.

I don't believe you.  Please spell out in detail the sequence
of optimizations that ocurrs to make this happen.

> Your change of moving purge_builtin_constant_p just unfairly penalizes
> targets where GCSE can eliminate dead-code, such as x86 Linux.

No it doesn't.  We still run purge_builtin_constant_p *after* gcse.


r~