Re: [PATCH] c, c++, v2: Avoid incorrect shortening of divisions [PR108365]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jan 12, 2023 at 09:31:23PM +0100, Jakub Jelinek via Gcc-patches wrote:
> On Thu, Jan 12, 2023 at 08:55:32PM +0100, Jakub Jelinek via Gcc-patches wrote:
> > > > 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...
> 
> E.g.
> int f1 (int x, int y) { return (unsigned) x / y; }
> unsigned short f2 (unsigned short x, unsigned short y) { return (unsigned) x / y; }
> unsigned int f3 (unsigned int x, unsigned int y) { return (long long) x / y; }
> C++ FE before and after the patch shortens the division in f2 and f3,
> C FE shortens only in f2.  So using the C FE condition would be a regression
> for C++.
> 
> Therefore I'm going to test following patch:

Bootstrapped/regtested successfully on x86_64-linux and i686-linux.

	Jakub