7,826 research outputs found
Mechanistic Behavior of Single-Pass Instruction Sequences
Earlier work on program and thread algebra detailed the functional,
observable behavior of programs under execution. In this article we add the
modeling of unobservable, mechanistic processing, in particular processing due
to jump instructions. We model mechanistic processing preceding some further
behavior as a delay of that behavior; we borrow a unary delay operator from
discrete time process algebra. We define a mechanistic improvement ordering on
threads and observe that some threads do not have an optimal implementation.Comment: 12 page
Rapid Visual Categorization is not Guided by Early Salience-Based Selection
The current dominant visual processing paradigm in both human and machine
research is the feedforward, layered hierarchy of neural-like processing
elements. Within this paradigm, visual saliency is seen by many to have a
specific role, namely that of early selection. Early selection is thought to
enable very fast visual performance by limiting processing to only the most
salient candidate portions of an image. This strategy has led to a plethora of
saliency algorithms that have indeed improved processing time efficiency in
machine algorithms, which in turn have strengthened the suggestion that human
vision also employs a similar early selection strategy. However, at least one
set of critical tests of this idea has never been performed with respect to the
role of early selection in human vision. How would the best of the current
saliency models perform on the stimuli used by experimentalists who first
provided evidence for this visual processing paradigm? Would the algorithms
really provide correct candidate sub-images to enable fast categorization on
those same images? Do humans really need this early selection for their
impressive performance? Here, we report on a new series of tests of these
questions whose results suggest that it is quite unlikely that such an early
selection process has any role in human rapid visual categorization.Comment: 22 pages, 9 figure
Indirect jumps improve instruction sequence performance
Instruction sequences with direct and indirect jump instructions are as
expressive as instruction sequences with direct jump instructions only. We show
that, in the case where the number of instructions is not bounded, we are faced
with increases of the maximal internal delays of instruction sequences on
execution that are not bounded by a linear function if we strive for acceptable
increases of the lengths of instruction sequences on elimination of indirect
jump instructions.Comment: 10 pages, definition of maximal internal delay and theorem 1 are
stated more precise; presentation improve
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The elemental mechanism of transcriptional pausing.
Transcriptional pausing underlies regulation of cellular RNA biogenesis. A consensus pause sequence that acts on RNA polymerases (RNAPs) from bacteria to mammals halts RNAP in an elemental paused state from which longer-lived pauses can arise. Although the structural foundations of pauses prolonged by backtracking or nascent RNA hairpins are recognized, the fundamental mechanism of the elemental pause is less well-defined. Here we report a mechanistic dissection that establishes the elemental pause signal (i) is multipartite; (ii) causes a modest conformational shift that puts γ-proteobacterial RNAP in an off-pathway state in which template base loading but not RNA translocation is inhibited; and (iii) allows RNAP to enter pretranslocated and one-base-pair backtracked states easily even though the half-translocated state observed in paused cryo-EM structures rate-limits pause escape. Our findings provide a mechanistic basis for the elemental pause and a framework to understand how pausing is modulated by sequence, cellular conditions, and regulators
Exposing Attention Glitches with Flip-Flop Language Modeling
Why do large language models sometimes output factual inaccuracies and
exhibit erroneous reasoning? The brittleness of these models, particularly when
executing long chains of reasoning, currently seems to be an inevitable price
to pay for their advanced capabilities of coherently synthesizing knowledge,
pragmatics, and abstract thought. Towards making sense of this fundamentally
unsolved problem, this work identifies and analyzes the phenomenon of attention
glitches, in which the Transformer architecture's inductive biases
intermittently fail to capture robust reasoning. To isolate the issue, we
introduce flip-flop language modeling (FFLM), a parametric family of synthetic
benchmarks designed to probe the extrapolative behavior of neural language
models. This simple generative task requires a model to copy binary symbols
over long-range dependencies, ignoring the tokens in between. We find that
Transformer FFLMs suffer from a long tail of sporadic reasoning errors, some of
which we can eliminate using various regularization techniques. Our preliminary
mechanistic analyses show why the remaining errors may be very difficult to
diagnose and resolve. We hypothesize that attention glitches account for (some
of) the closed-domain hallucinations in natural LLMs.Comment: v2: NeurIPS 2023 camera-ready + data releas
Fast simulation techniques for microprocessor design space exploration
Designing a microprocessor is extremely time-consuming. Computer architects heavily rely on architectural simulators, e.g., to drive high-level design decisions during early stage design space exploration. The benefit of architectural simulators is that they yield relatively accurate performance results, are highly parameterizable and are very flexible to use. The downside, however, is that they are at least three or four orders of magnitude slower than real hardware execution. The current trend towards multicore processors exacerbates the problem; as the number of cores on a multicore processor increases, simulation speed has become a major concern in computer architecture research and development.
In this dissertation, we propose and evaluate two simulation techniques that reduce the simulation time significantly: statistical simulation and interval simulation. Statistical simulation speeds up the simulation by reducing the number of dynamically executed instructions. First, we collect a number of program execution characteristics into a statistical profile. From this profile we can generate a synthetic trace that exhibits the same execution behavior but which has a much shorter trace length as compared to the original trace. Simulating this synthetic trace then yields a performance estimate. Interval simulation raises the level of abstraction in architectural simulation; it replaces the core-level cycle-accurate simulation model by a mechanistic analytical model. The analytical model builds on insights from interval analysis: miss events divide the smooth streaming of instructions into so called intervals. The model drives the timing by analyzing the type of the miss events and their latencies, instead of tracking the individual instructions as they propagate through the pipeline stages
About Instruction Sequence Testing
Software testing is presented as a so-called theme within which different
authors and groups have defined different subjects each of these subjects
having a different focus on testing. A uniform concept of software testing is
non-existent and the space of possible coherent perspectives on software
testing, each fitting within the theme, is viewed as being spanned by five
dimensions, each dimension representing two opposite views with a variety of
intermediate views in between.
Instruction sequences are used as a simple theoretical conceptualization of
computer programs. A theory of instruction sequence testing may serve as a
model for a theory of software testing. Instruction sequences testing is
considered a new topic for which definitions may be freely contemplated without
being restricted by existing views on software testing.
The problem of developing a theory of instruction sequence testing is posed.
A survey is given of motivations and scenarios for developing a theory of
instruction sequence testing
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