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Monday, April 8 • 3:10pm - 3:40pm
Lightning Talks

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Swinging Modulo Scheduling together with Register Allocation   
Lama Saba
VLIW architectures rely heavily on Modulo Scheduling to optimize ILP in loops. Modulo Scheduling can be achieved today in LLVM using the MachinePipeliner pass, which implements a Swing Modulo Scheduler prior to register allocation [1]. For some VLIW architectures, such as those lacking hardware interlocks or the ability to spill registers onto a stack, the MachinePipeliner's decisions become crucial for the success of the register allocation phase, since they affect the latter’s decisions to generate splits or spills, which in turn can result in an inefficient or even an unsuccessful resource allocation.
Nevertheless, even though the MachinePipeliner aims to schedule with a minimal Initiation Interval, it is structured in a way that facilities trying larger Initiation Intervals or a different ordering, this structure lends itself to alternative, possibly less aggressive scheduling retries, after more aggressive attempts have failed in register allocation.
This talk introduces this issue and explores how we can achieve successful modulo scheduling and register allocation for such architectures in LLVM by introducing a repetitive rollback-and-retry mechanism for altering scheduling decisions based on the register allocator’s outcome, and how we can leverage such an approach to improve the scheduling of VLIW architectures in general.

[1] An Implementation of Swing Modulo Scheduling in a Production Compiler - Brendon Cahoon - http://llvm.org/devmtg/2015-10/slides/Cahoon-SwingModuloScheduling.pdf

LLVM for the Apollo Guidance Computer 
Lewis Revill
Nearly 50 years ago on the 20th of July 1969 humans set foot on the
moon for the first time. Among the many extraordinary engineering feats
that made this possible was the Apollo Guidance Computer, an innovative
processor for its time with an instruction set that was thought up well
before the advent of C. So 50 years later, why not implement support
for it in a modern compiler such as LLVM?

This talk will give a brief overview of some of the architectural
features of the Apollo Guidance Computer followed by an account of my
implementation of an LLVM target so far. The shortcomings of LLVM when
it comes to implementing such an unusual architecture will be
discussed along with the workarounds used to overcome them.

Catch dangling inner pointers with the Clang Static Analyzer 
Réka Kovács
C++ container classes provide methods that return a raw pointer to the container's inner buffer. When the container is destroyed, the inner buffer is deallocated. A common bug is to use such a raw pointer after deallocation, which may lead to crashes or other unexpected behavior.

This lightning talk will present a new Clang Static Analyzer checker designed to address the above described problems, implemented last year as a Google Summer of Code project. The checker has found serious problems in popular open source projects with a negligible false positive rate. Future plans include adding support for view-like constructs and non-STL containers.

Cross translation unit test case reduction
Réka Kovács
C-Reduce, released by Regehr et al. in 2012, is an excellent tool designed to generate a minimal test case from a C/C++ file that has some specific property (e.g. triggers a bug). One of the most interesting parts of C-Reduce is Clang Delta, which is a set of compiler-like transformations implemented using Clang libraries. Clang Delta includes transformations like changing a function parameter to a global variable etc.

With the introduction of the experimental cross translation unit analysis feature in the Clang Static Analyzer, there arose a need to investigate crashes, bugs, or false positive reports that spread across different translation units. Unfortunately, C-Reduce was designed to minimize one translation unit at a time, and some of the Clang Delta transformations cannot be applied to multiple TUs in their original form.

This talk/poster is a status report about a work in progress that aims to make it possible to use C-Reduce for cross translation unit test case reduction.


Reka Kovacs

PhD student, Eotvos Lorand University

Lama Saba

Intel Israel

Monday April 8, 2019 3:10pm - 3:40pm CEST