llvm dev - Apr 2015 - [LLVMdev] Why are imm shifts where imm >= width type eliminated entirely?

On 04/20/2015 01:25 PM, David Majnemer wrote:
> These optimizations are not always run on IR that is fed to the backend.
The DAG combiner also performs the undefined shift -> undef though, so 
it should still be OK

-Matt

> The DAG combiner also performs the undefined shift -> undef though, so
it
> should still be OK
DAG combiner doesn't really run to convergence as far as I'm aware, so
you often get extremely difficult to test but necessary checks like
this.

Tim.

There can also be other “problems" like this one:
http://reviews.llvm.org/D6946 <http://reviews.llvm.org/D6946>

- Matthias
> On Apr 20, 2015, at 1:44 PM, Tim Northover <t.p.northover at
gmail.com> wrote:
> 
>> The DAG combiner also performs the undefined shift -> undef though,
so it
>> should still be OK
> 
> DAG combiner doesn't really run to convergence as far as I'm aware,
so
> you often get extremely difficult to test but necessary checks like
> this.
> 
> Tim.
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