[PATCH] Check for all known bits on ret in InstCombine

[Chandler Carruth]

On Tue, Jul 22, 2014 at 12:18 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> > And I think
> > that we should clearly DCE *completely dead* sub-graphs that are
> > being kept alive solely due to the assumption.
>
> I suppose the real question is: what does completely dead mean? If I have:
>
> int a;
> void foo() {
>   __builtin_assume(a == 5);
> }
>
> is this completely dead? I would say no so long as foo() could be inlined
> anywhere.
>

Only dead if 'a' has no other users.

I'm defining dead as an SSA subgraph where the only sinks (uses which have
"side-effects" or otherwise can't be removed) are the assumptions. I think
this is sufficiently conservative to remove assumptions from true dead code
regions and prevent them from keeping control flow alive without
sacrificing optimization opportunities.

-Chandler
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