[llvm-dev] Handling native i16 types in clang and opt

[Friedman, Eli via llvm-dev]

On 5/21/2017 1:22 AM, Alex Susu via llvm-dev wrote:
>   Hello.
>     My target architecture supports natively 16 bit integers (i16).
>
>     Whenever I write in C programs using only short types, clang 
> compiles the program to LLVM and converts the i16 data to i32 to 
> perform arithmetic operations and then truncates the results to i16. 
> Then, the InstructionCombining (INSTCOMBINE or IC) pass removes these 
> conversions back and forth from i16, except for the (s)div LLVM IR 
> operation.

sdiv in particular is special: it has undefined behavior on overflow.  
"sdiv i32 -32768, -1" produces "i32 32768", but "sdiv i16 -32768, -1" is 
undefined.

-Eli