Re: [PATCH] match.pd: Use ranges to optimize some x * y / y to x [PR97997]

[Marc Glisse <marc.glisse@inria.fr>]

On Thu, 26 Nov 2020, Jakub Jelinek via Gcc-patches wrote:

> For signed integers with undefined overflow we already optimize x * y / y
> into x, but for signed integers with -fwrapv or unsigned integers we don't.
> The following patch allows optimizing that into just x if value ranges
> prove that x * y will never overflow.

I've long wanted a helper that checks if VRP thinks an operation could 
overflow, I think at some point it would make sense to move this code to 
some function so that it can be easily reused. Maybe also define a matcher 
so we can write (mult_noovf @0 @1) which would succeed if either overflow 
is undefined or if VRP can prove that no overflow is happening.

Of course that's all ideas for later, refactoring belongs in the second or 
third patch using a feature, not the first one :-)

-- 
Marc Glisse