[LLVMdev] nsw/nuw for trunc

[John McCall]

On Aug 11, 2011, at 11:03 AM, Chris Lattner wrote:

> 
> 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.

I agree that there's some abstract consistency in providing nsw/nuw
for trunc, even if no current frontends would ever emit it, but I do not
get the impression that Florian actually wants the optimizer making
random transformations based on the assumed undefined behavior
of these truncations.

John.