Re: [PATCH] c++: Avoid incorrect shortening of divisions [PR108365]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jan 12, 2023 at 02:37:13PM -0500, Jason Merrill wrote:
> > But the C++ FE was checking if op0 is a NOP_EXPR from TYPE_UNSIGNED.
> > First of all, not sure if the operand of NOP_EXPR couldn't be non-integral
> > type where TYPE_UNSIGNED wouldn't be meaningful, but more importantly,
> > even if it is a cast from unsigned integral type, we only know it can't be
> > minimum signed value if it is a widening cast, if it is same precision or
> > narrowing cast, we know nothing.
> 
> Curious, this divergence goes back to 1994, when the C++ front-end was
> merged and tege changed the condition in the C front-end.

And it was changed to match the modulo condition adjusted by rms in 1993.

> > So, the following patch for the NOP_EXPR cases checks just in case that
> > it is from integral type and more importantly checks it is a widening
> > conversion, and then next to it also allows op0 to be just unsigned,
> > promoted or not, as that is what the C FE will do for those cases too
> > and I believe it must work - either the division/modulo common type
> > will be that unsigned type, then we can shorten and don't need to worry
> > about UB, or it will be some wider signed type but then it can't be most
> > negative value of the wider type.
> 
> Why not use the same condition in C and C++?

I can test that.  Do you mean change the C FE to match the patched C++
or change C++ FE to just test TYPE_UNSIGNED (orig_op0)?
I think both should work, though what I wrote perhaps can shorten in more
cases.  Can try to construct testcases where it differs...

	Jakub