[talks] Talk Thomas Wahl, Fri Dec 4- 12:30pm

Nicole E. Wagenblast nwagenbl at CS.Princeton.EDU
Wed Nov 25 11:32:28 EST 2015

Thomas Wahl, Northeastern University 
Friday, December 4, 12:30 - 1:30 PM 
Computer Science 105 

Behavioral Non-Portability in Scientific Numeric Computing 

The precise semantics of floating-point arithmetic programs depends on the 
execution platform, including the compiler and the target hardware. 
Platform dependencies are particularly pronounced for arithmetic-intensive 
scientific numeric programs and infringe on the highly desirable goal of 
software portability (which is in fact promised by heterogeneous computing 
frameworks like OpenCL): the same program run on the same inputs on 
different platforms can produce different results. So far so bad. 

Serious doubts on the portability of numeric applications arise when these 
differences are behavioral, i.e. when they lead to changes in the control 
flow of a program. In this work I will present an algorithm that takes a 
numeric procedure and determines an input that is likely to lead to 
different decisions depending merely on how the arithmetic in the procedure 
is compiled. Our implementation of the algorithm requires minimal 
intervention by the user. I will illustrate its operation on examples 
characteristic of scientific numeric computing, where control flow 
divergence actually occurs across different execution platforms. 

Time permitting, I will also sketch how to prove the /absence/ of inputs 
that may lead to control flow divergence, i.e. how to prove programs 
/stable/ (in one of the many senses of this word). This is ongoing work. 

Thomas Wahl is an Assistant Professor at Northeastern University. 
This is joint work with Yijia Gu, Mahsa Bayati, and Miriam Leeser at Northeastern. 

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