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acr_why_no_split_stmts.md

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'Redemption' Automated Code Repair Tool Copyright 2023, 2024 Carnegie Mellon University. NO WARRANTY. THIS CARNEGIE MELLON UNIVERSITY AND SOFTWARE ENGINEERING INSTITUTE MATERIAL IS FURNISHED ON AN 'AS-IS' BASIS. CARNEGIE MELLON UNIVERSITY MAKES NO WARRANTIES OF ANY KIND, EITHER EXPRESSED OR IMPLIED, AS TO ANY MATTER INCLUDING, BUT NOT LIMITED TO, WARRANTY OF FITNESS FOR PURPOSE OR MERCHANTABILITY, EXCLUSIVITY, OR RESULTS OBTAINED FROM USE OF THE MATERIAL. CARNEGIE MELLON UNIVERSITY DOES NOT MAKE ANY WARRANTY OF ANY KIND WITH RESPECT TO FREEDOM FROM PATENT, TRADEMARK, OR COPYRIGHT INFRINGEMENT. Licensed under a MIT (SEI)-style license, please see License.txt or contact [email protected] for full terms. [DISTRIBUTION STATEMENT A] This material has been approved for public release and unlimited distribution. Please see Copyright notice for non-US Government use and distribution. This Software includes and/or makes use of Third-Party Software each subject to its own license. DM23-2165

For the FY18-20 ACR line project, we designed the repairs so that we never need to split an expression into multiple statements. The reason for this was to avoid complications that arise when modifying the text of the source code at the character level (rather than at the AST level).

As an example, consider the following snippet from coreutils regexec.c (https://github.com/coreutils/gnulib/blob/769c6d033c94ec54cb72c7db2ddcf7d587b5ab37/lib/regexec.c), lines 3549--3554:

    #ifdef RE_ENABLE_I18N
            if (dfa->mb_cur_max > 1)
                bitset_merge (accepts, dfa->sb_char);
            else
    #endif
                bitset_set_all (accepts);

Repairing the bitset_set_all (accepts) line in a way that splits it into two statements would require insertion of curly brackets. And for it to work with both possible values of RE_ENABLE_I18N, the opening curly brace would need to appear after the #endif, not on the same line as the else statement.

As another example, consider the following snippet:

    for (int x = foo(); x < bar(); x++) {
        baz();
    }
    int x = 42;

Needing to split any of the expressions on the for line into multiple statements would require relocating them elsewhere (either inside the body of the loop or outside the loop).

The above examples aren't impossible to split into multiple statements, but splitting does introduce extra complexity that can be avoided by introducing auxiliary functions so that the expressions can be replaced with other expressions instead of splitting into multiple statements.