[RFC] Implementing detection of saturation and rounding arithmetic

[Joseph Myers]

On Tue, 11 May 2021, David Brown wrote:

> It is also worth noting that gcc already has support for saturating
> types on some targets:
> 
> <https://gcc.gnu.org/onlinedocs/gcc/Fixed-Point.html>
> 
> My testing of these (quite a long time ago) left me with a feeling that
> it was not a feature anyone had worked hard to optimise - certainly it

The implementation isn't well-integrated with any optimizations for 
arithmetic on ordinary integer types / modes, because it has its own 
completely separate machine modes and operations on those.  I still think 
it would be better to have a GIMPLE pass that lowers from fixed-point 
types to saturating etc. operations on ordinary integer types, as I said 
in <https://gcc.gnu.org/legacy-ml/gcc-patches/2011-05/msg00846.html>.

Note however that such lowering should be more or less independent of 
what's being discussed in this thread - this thread is about better 
optimization of such operations on ordinary types (with or without 
built-in functions of some kind in addition to recognition of such 
operations written in generic C), which you can do independently of 
what's done with fixed-point types.

-- 
Joseph S. Myers
joseph@codesourcery.com