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# Pass bisection
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This document describes a feature for the bisection tool, which provides
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pass and transformation level bisection for a bad object file.
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Before reading this document, please refer to README.bisect for general usage
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of the bisection tool.
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The benefit of using pass level bisection is:
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When building a bad object file, it can tell you which pass and transformation
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in the compiler caused the error.
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*Notice:* This tool will only work for LLVM/clang, since it is using options
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`-opt-bisect-limit` and `print-debug-counter` that only exist in LLVM.
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## Arguments
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All the required arguments in object-file-level bisection tool are still
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to be provided. In addition, you will need to add the following arguments:
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1. `--pass_bisect`: enables pass level bisection
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2. `--ir_diff`: enables output of IR differences
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Please refer to `--help` or the examples below for details about how to use
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them.
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## HOW TO USE: ChromeOS
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*TODO* - Future work: Currently this only works for Android.
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## HOW TO USE: Android
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1. Prerequisites: A general setup is still needed for Android, which means that
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you need to populate good and bad set of objects with two versions of
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compilers.
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See the documentation in `README.bisect.md` for more detailed instructions.
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1. Pass/Transformation Bisection: If you do not wish to override the other
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arguments, this command should be sufficient to do pass/transformation level
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bisection:
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```
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./run_bisect.py android PATH_TO_ANDROID_HOME_DIR
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--pass_bisect=’android/generate_cmd.sh’
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--prune=False
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--ir_diff
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--verbose
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```
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Where:
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```
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--pass_bisect:
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Enables pass/transformation level bisection and with default
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script to generate the command as ‘android/generate_cmd.sh’.
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--prune:
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For now, prune must be set to False to return only the first
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bad item.
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--ir_diff:
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Optional argument to print out IR differences.
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--verbose:
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To show IR diff, verbose needs to be on.
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```
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Other default arguments:
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```
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--get_initial_items='android/get_initial_items.sh'
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--switch_to_good='android/switch_to_good.sh'
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--switch_to_bad='android/switch_to_bad.sh'
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--test_setup_script='android/test_setup.sh'
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--test_script='android/interactive_test.sh'
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--incremental
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--prune
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--file_args
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```
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You can always override them if needed. See README.bisect for more
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details.
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1. Other features: Features such as resuming, number of jobs, and device id
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remain the same as before. See README.bisect for more details.
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