Add select

[Doppie23]

Description

Introduces a select function that chooses a value based on a condition without branching.
select is similar to cond except it evaluates both branches and picks the result without branching.

cond will automatically be simplified to a select when both branches are considered cheap, based on a simple cost heuristic.

(&&) and (||) are defined using if-then-else to enable short-circuiting. Using this new logic these can now get converted to a select, which then will be turned into a logical and/or.

The accelerate-llvm part: ivogabe/accelerate-llvm#11

How has this been tested?
-

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[ivogabe]

Thanks! I tuned the cond-to-select transformation slightly in 97014c7, and added a transformation that converts a select of a pair to a pair of selects (to enable further optimizations)