HomePhabricator

[InstCombine] Eliminate casts to optimize ctlz operation

Authored by datta.nagraj on Jun 23 2021, 8:18 AM.

Description

[InstCombine] Eliminate casts to optimize ctlz operation

If a ctlz operation is performed on higher datatype and then
downcasted, then this can be optimized by doing a ctlz operation
on a lower datatype and adding the difference bitsize to the result
of ctlz to provide the same output:

https://alive2.llvm.org/ce/z/8uup9M

The original problem is shown in
https://llvm.org/PR50173

Differential Revision: https://reviews.llvm.org/D103788

Details