[llvm-dev] Bugpoint Redesign

[Diego Treviño via llvm-dev]

Hey all,

I wanted to share a proposal
<https://docs.google.com/document/d/171ecPTeXw68fbCghdGw_NPBouWvmvUX8vePlbhhHEdA/edit?usp=sharing>
to revamp the current go-to IR debugging tool: Bugpoint. i'd love to hear
any feedback or general thoughts.

Here's the markdown version of the doc:
---
# Bugpoint Redesign
Author: Diego Treviño (diegotf at google.com)

Date: 2016-06-05

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:


* Split the code into chunks and discard those that fail the given test
* 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, as seen below:

![Bugpoint's help option showcase](
https://lh6.googleusercontent.com/sbpaSVHzpVVZKKAgHL9gvfzTWdgh3ju0KiDYql6WmWZfDYrdauOJMcuo9PP_V1dq8JQfMHOSKTv3lJcSpVytUyU8r5tJ2KTlGB0b2ve7jsZ3nVX8K8ItAbsA0JWkFKw67VJnq99m
)

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.


-- 
Cheers,
Diego
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