speed up clear_cfg
speed up clear_cfg

Summary:
Before: clear cfg is ~22% of total (inclusive) cycles
{F123878493}

After: clear_cfg is ~2% of total (inclusive) cycles. It's the small box on the right side that's barely visible :D
{F123878494}

Reviewed By: int3, minjang, arnaudvenet

Differential Revision: D7665219

fbshipit-source-id: 8176fdcc5cbb159c13260abc013e878994fa73f4

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