[LLVMdev] nsw/nuw for trunc

[Chris Lattner]

On Aug 11, 2011, at 5:17 AM, Florian Merz wrote:

> Hi everyone,
> 
> we'd like to be able to check for loss of information in trunc operations in 
> our LLVM-based bounded model checker [1]. For this it is important if the 
> trunc was on a signed or unsigned integer, so we need nsw and nuw flags for 
> this. Would you accept a patch that adds these flags to LLVM (and possibly 
> clang)?

In contrast to the other folks, I think that this makes perfect sense, and NSW/NUW are the right thing to use.

It would be valid for instcombine to delete "sext(trunc_nsw x)" when the source and dest types are the same, similarly "zext(trunc_nuw x)".

The semantics are very similar to the left shift operator and many others.

-Chris