Re: New no-undefined-overflow branch

[Paolo Bonzini <bonzini at gnu dot org>]

Joseph S. Myers wrote:
> On Fri, 6 Mar 2009, Paolo Bonzini wrote:
> 
>> I don't think you need to introduce trapping tree codes.  You can
>> introduce them directly in the front-end as
> 
> Multiple front ends want the same thing.  This is why it would be better 
> to introduce the codes in GENERIC and have the language-independent 
> gimplifier contain the code to lower them, even if they don't become part 
> of GIMPLE.

I see your point.  What I'm worried of, is that this codes would be
tested more lightly and, until folding is a middle-end thing only, the
risk of unwanted optimization on -ftrapv code would be high.

You can have common code shared by front-ends.  They could apply it at
GENERICization time (Fortran, Ada) or directly while parsing (C, C++).

>>    (int)((long long) a * (long long) b) == a *nv b ? trap () : a *nv b
> 
> This is not a solution for trapping multiplication in the widest supported 
> type.

There's always range checking, I was pointing out optimization
possibilities; the above one can be optimized like

  (h,l) = a*b
  if (h != l >> 31) trap ();    // signed shift

Paolo