Summary: See PHI1210. For certain large inputs, we spend more time than we need to replacing tabs with spaces. Add some fast paths: - When a line only has tabs at the beginning of the line, we don't need to do as much work parsing the rest of the line. - When a line has no unicode characters, we don't need to vectorize it to get the right result. Test Plan: - Added test coverage. - Profiled this, got a ~60x performance increase on a 36,000 line 3MB text file. Reviewers: amckinley Reviewed By: amckinley Differential Revision: https://secure.phabricator.com/D20477