[LLVMdev] Why are imm shifts where imm >= width type eliminated entirely?

[Stephen Canon]

> On Apr 20, 2015, at 4:00 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> 
> For example:
> 
> unsigned int x, y;
> 
> void foo()
> {
>      y = x >> 129;
> }
> 
> Where int is a 16bit type, the .ll is producing only 'ret void' at O3. At O0 the .ll looks fine but then llc gets rid of it an simply returns.
> 
> I'm just curious what the reasoning is for this? It isn't trying to set y to anything at all.
> 
> Thanks.

Shifts amounts equal to or larger than the width of the promoted type being shifted are undefined behavior.

– Steve