[llvm] [X86] Combine LRINT/LLRINT and TRUNC when TRUNC has nsw flag (PR #126217)

[Craig Topper via llvm-commits]

topperc wrote:

> > @phoebewang have you looked into compiling `(int)rintf(x)` to `cvtss2si`?
> 
> Do you mean the FE proposes I posted before? No, it's not in my priority list. But I don't think we need to wait for it, there are many nsw/nuw trunc transformations if you search in the tests.

(int)rintf(x) compiles to llvm.rintf+fptosi. The fptosi has undefined behavior if the fp value doesn't fit into an integer. This idiom is something that could be optimized without needing any new frontend support. Maybe it's an alternative sequence we could propose to users instead of trying to make this lrint+trunc work?

https://github.com/llvm/llvm-project/pull/126217