llvm/docs/BugpointRedesign.md

# Bugpoint Redesign
Author: Diego Treviño (diegotf@google.com)

Date: 2019-06-05

Status: Draft


## Introduction
As use of bugpoint has grown several areas of improvement have been identified
through years of use: confusing to use, slow, it doesn’t always produce high
quality test cases, etc. This document proposes a new approach with a narrower
focus: minimization of IR test cases.


## Proposed New Design


### Narrow focus: test-case reduction
The main focus will be a code reduction strategy to obtain much smaller test
cases that still have the same property as the original one. This will be done
via classic delta debugging and by adding some IR-specific reductions (e.g.
replacing globals, removing unused instructions, etc), similar to what
already exists, but with more in-depth minimization.


Granted, if the community differs on this proposal, the legacy code could still
be present in the tool, but with the caveat of still being documented and
designed towards delta reduction.


### Command-Line Options
We are proposing to reduce the plethora of bugpoint’s options to just two: an
interesting-ness test and the arguments for said test, similar to other delta
reduction tools such as CReduce, Delta, and Lithium; the tool should feel less
 cluttered, and there should also be no uncertainty about how to operate it.


The interesting-ness test that’s going to be run to reduce the code is given
by name:
        `--test=<test_name>`
If a `--test`  option is not given, the program exits; this option is similar
to bugpoint’s current `-compile-custom` option, which lets the user run a
custom script.


The interesting-ness test would be defined as a script that returns 0 when the
IR achieves a user-defined behaviour (e.g. failure to compile on clang) and a
nonzero value when otherwise. Leaving the user the freedom to determine what is
and isn’t interesting to the tool, and thus, streamlining the process of
reducing a test-case.


If the test accepts any arguments (excluding the input ll/bc file), they are
given via the following flag:
        `--test_args=<test_arguments>`
If unspecified, the test is run as given. It’s worth noting that the input file
would be passed as a parameter to the test, similar how `-compile-custom`
currently operates.


### Implementation
The tool would behave similar to CReduce’s functionality in that it would have a
list of passes that try to minimize the given test-case. We should be able to
modularize the tool’s behavior, as well as making it easier to maintain and
expand.


The first version of this redesign would try to:


* Discard functions, instructions and metadata that don’t influence the
  interesting-ness test
* Remove unused parameters from functions
* Eliminate unvisited conditional paths
* Rename variables to more regular ones (such as “a”, “b”, “c”, etc.)


Once these passes are implemented, more meaningful reductions (such as type
reduction) would be added to the tool, to even further reduce IR.


## Background on historical bugpoint issues


### Root Cause Analysis
Presently, bugpoint takes a long time to find the source problem in a given IR
file, mainly due to the fact that it tries to debug the input by running
various strategies to classify the bug, which in turn run multiple optimizer
and compilation passes over the input, taking up a lot of time. Furthermore,
when the IR crashes, it tries to reduce it by performing some sub-optimal
passes (e.g. a lot of unreachable blocks), and sometimes even fails to minimize
at all.


### "Quirky" Interface
Bugpoint’s current interface overwhelms and confuses the user, the help screen
alone ends up confusing rather providing guidance. And, not only are there
numerous features and options, but some of them also work in unexpected ways
and most of the time the user ends up using a custom script. Pruning and
simplifying the interface will be worth considering in order to make the tool
more useful in the general case and easier to maintain.