^{2} École Normale Supérieure/French Institute for Research in Computer Science and Automation (INRIA), Paris, France
“Justintime” compilers during program execution are being increasingly used for the execution of dynamic languages such as Perl and Python, and for semidynamic languages, such as Java and
Currently, however, there is one common problem, which arises in the use of the existing approaches of fast justintime compilation. In almost every case, the justintime compiler is very specific to the used object model, to the library running programs, garbage collection and other features of bytecode for the particular system^{[1–3]}. This inevitably leads to a situation where the effort needs to be repeated, and all the useful work that went into creating and developing a fast and high quality justintime compiler for a single virtual machine and a programming language cannot be used a second time for another virtual machine and another programming language.
Justintime compilers are not only useful in the implementation of programming languages. They can also be used in other fields of mathematics and programming. For example, graphical applications can achieve better execution if they compile special means of justintime execution of the program, specialized for “manual” generation of images, more accurately, achieving a better performance than statically generated generalpurpose means. Obviously, such applications do not need object models, garbagecollectors, or large class libraries.
The greatest amount of work during the development of a new justintime compiler relates to the support operations of arithmetics, conversion of data types, reading from memory, writing to memory, creation of loops, analysis of the dataflow graph, algorithm of register allocation, and generation of executable machine code. Only a very small fraction of the work is dependent on the bytecode of the virtual machine and the programming language. The purpose of the libjitlinearscan^{[4, 5]} project consists of providing a necessary and sufficient set of tools that ensure the process of fast justintime compilation of the program, liberating the developer from the work on the features of the programming language and operating system. In those parts where the support of common object models is available, this is done strictly in additional libraries of the operating system, and not in the form of code of the kernel of the execution environment.
The libjitlinearscan library aims to provide developers with more time and opportunity to think about the highlevel design of the virtual machine, without focusing on the details of generation and execution of binary code. At the same time, experts in optimization, design, compiler implementation, and native code generation can focus on more lowlevel problems of binary code execution, instead of working on applied problems.
Super linear complexity algorithms, which are used in compilers, can fail in case of an algorithmic complexity attack. Hence, algorithms that are fast and have a linear complexity of
The contributions of our work are twofold. First, we describe a novel approach for the register allocation based on a virtual execution environment that combines information from static and dynamic program analyses. Second, this paper advances the field of computer science by developing tools for the justintime compilation of program code.
The remainder of this paper is organized as follows. The next section presents an overview of the various approaches for the fast register allocation. Section 3 presents our approach. Section 3.1 presents the principles of register allocation via graph coloring. Section 3.2 presents the formulation of the novel mathematical model. Section 3.3 presents the principles for implementing our approach. Section 4 contains the test results, and it presents and analyzes the obtained experimental data in terms of the performance. Section 5 presents a discussion on the results. The final section concludes the paper.
2 Literature reviewRegister allocation involves four particular problems: the computation of
Altogether, these problems can be effectively solved for chordal graphs. The interference graphs of program code in static single assignment (SSA) form are chordal (e.g., Fig. 1). Therefore, we will solve all these tasks for chordal graphs.
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Fig. 1. An example of a chordal graph 
The compiler optimization of register allocation in machine code causes a significant increase in code execution performance.
The most wellknown approach to global register allocation is via graph coloring^{[2, 9]}, as formalized by Cocke^{[10]}, Ershov^{[11]} and Schwartz^{[12]} and later implemented by Chaitin^{[13, 14]}, Chow and Hennessy^{[15]}. Since the basic graphcoloring problem underlying this approach is nondeterministic polynomial timecomplete (NPcomplete), however, any practical algorithm must simplify the problem using heuristics^{[16–19]}. Many such heuristic variations have been explored, such as linear scan^{[20–22]}, integer linear programming^{[23, 24]}, puzzle solving^{[25, 26]}, repairing^{[27, 28]}, trees^{[29]} and traces^{[30]}. Krause^{[18]} proves that given certain restrictions on the input program (e.g., for C a limit on the use of goto labels) graphcoloring register allocation can be done in polynomial time for a fixed number of registers. The approach is not a practical choice for dynamic compilers since the execution time is exponential in the number of registers. The work of Lozano et al.^{[19]} contains a proof that any optimal graphcoloring register allocation algorithms will have execution time exponential in the number of registers. The latest research on graph coloring based register allocation suggests the CallahanKoblenz algorithm^{[31, 32]} that represents the hierarchical program structure with a tile tree. Such a tree structure isolates the highfrequency and lowfrequency code regions and provides a basis for the allocator′s overall operation and spill placement decisions. While those heuristics yield polynomialtime algorithms, they do not generally run in linear time, with the exception of the Bouchez, Brisk, Hack et al.^{[28, 33–37]} algorithms for programs in static single assignment (SSA) form^{[38–40]}, which partly inspired the present work. We also want to draw attention to useful works^{[41–43]} that develop a similar approach. We make the novel observation that most of the advantages of this lineartime algorithm may be obtained without actually transforming the program to SSA form, since many realistic programs appear to have chordal inference graphs, thereby enabling a much simpler algorithm that can generate code nearly as efficiently.
3 Methods 3.1 Register allocation based on coloringGraph coloring based on constructing an interference graph of variables is the traditional approach to register allocation. The graph is used for mapping the set of dynamic states of variables into the set of their static states during program execution. Vertices in the graph are variables of the program. Thus, the vertices of the graph also represent symbolic and machine registers that are allocated for temporary variables. The method of register allocation based on graph coloring in the general case, without the use of heuristics, has an NPcomplexity. Many approximate solutions of this problem have been studied. However, they are all heuristics. Among the best known is the heuristic of Chaitin based on graph coloring. However, this heuristic works in
If two variables interfere with each other, then an arc is drawn between the vertices of the interference graph. For noninterfering variables, we can use the same register, thereby reducing the number of registers required. On the other hand, the arc between two variables means that we should not assign them both to the same register, because their liveness at a certain point in time of program execution intersects. This graph can be represented as a matrix of lists, depending on which operations are required.
The basic idea of global register allocation (Fig. 2) can be expressed in five steps, which are as follows:
1) During code generation and optimization (whatever phase has preceded allocation of registers) or, as the first stage of register allocation, allocate unique symbolic registers to objects that can be assigned to registers, unique symbolic registers, for example, s1, s2,
2) Determine which objects should be candidates for allocation into registers.
3) Build a socalled interference graph. The interference graph is a graph whose nodes represent the allocated objects and real machine registers of the target machine, and whose arcs (or directed arcs) represent interferences. Then, two allocated objects intersect if they are alive at the same time. An object and a register interfere if the object cannot or should not be allocated to this register (e.g., an integer operand or a register for floatingpoint values).
4) Color the node of the interference graph with R colors, where R is the number of available registers, so that every two adjacent nodes have different colors (this is called coloring with Rcolors).
5) Select for each object a register which is of the same color.
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Fig. 2. A general description of algorithms based on graph coloring 
Fig. 3 shows an example of a program that makes addition of numbers in the variables. All the variables with a common vertex in the interference graph are assigned to different machine registers.
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Fig. 3. Register allocation via graph coloring 
Many problems can be solved more efficiently in the case of chordal graphs. In SSA form,
If all variants of the execution of a program were deterministic and finite, then of course it would be possible to compute the result of the execution of this program using concrete hardware at any time before its execution. However, some programs can achieve an infinite number of side effects from the wholly deterministic area of access for the program, such as reading and writing into the shared memory. Those side effects can be infinite and even incomputable. Therefore, we propose a new mathematical model in order to take this behavior into account. In the mathematical model that we created, there is a possibility to compute the result of the execution of a program using concrete hardware at any time before its execution if all variants of its execution are deterministic and finite. A good machine code generator supports a few mechanisms for its generation and optimization of register allocation. Thus, in order to make the optimal decision, the infinite variants must be analyzed at an appropriate time. This can be done with the use of domainspecific optimization and an analysis of program behavior. For these computations, we use a linear analysis and the mathematical apparatus of probability spaces based on the Kolmogorov axioms. We define the level of abstraction as the probability of making a correct decision about the properties of a program before its actual execution on real hardware, based on the analysis of a limited set of data performed in linear time. Thus, we formalize the hypothesis that more abstract bytecode can be executed more efficiently than less abstract bytecode. Limited resources are to be allocated to the execution of solutions with a maximum level of abstraction, with the goal of maximizing the expectation of success in the selected path of the execution of the program. In other words, we look at the machine code generated by the dynamic compiler as a finite set of random variables. Each of these random variables has its own distribution. Nevertheless, those distributions can be incomputable or be infinitely divisible.
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Fig. 4. A sequence of variables 
We look at a sequence of random variables arranged in some geometrical space (e.g., Fig. 4). First, an input bytecode method is verified and is converted into an intermediate form. Each concrete realization of the binary code is a realization of the random value. We define a function that computes the expected values of a certain target function defined on this binary code.
We expect monotonic properties of the target function from applications of the resources of one class to more resources of the second class. The use of first class resources requires much less execution time for a piece of machine code on a hardware processor, and thus for the entire program. On the other hand, their usage highlights the need to commit secondclass resources at a certain moment in time. Register allocation is simply the easiest example to demonstrate the efficiency of our model for optimization.
Unless we have fully deterministic control of concrete realizations we believe that such realizations can have various distributions of probability according to the concrete values of some input parameters.
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Fig. 5. Markov model 
Thus, we analyze this situation using a simple hidden Markov model with a finite number of linearly ordered nodes (e.g., Fig. 5). In other words, there is a description of the properties of the signal that is analogous to wavelet transforms and thus an analogous relation between the apparatus of characteristic functions and the formulas developed by Kolmogorov, Lévy and Khintchine in the computation of the properties of stochastic processes.
We believe that in the context of uncertainties, nondeterminism and a set of possibilities with equal or variable weights, resources should be directed toward the solution with the maximal probability of obtaining the correct solution based on the aheadoftime prediction of the program in the given domain of optimization.
3.3 ImplementationWe apply abstract interpretation^{[44–46]} in order to predict the properties and behavior of a program before its execution.
We define the goals of register allocation for a given set of properties of a program as, first, the correctness of execution and, second, the minimization of the number of accesses of the local stack memory. We predict the properties of the given program before its execution on real hardware. Each binary property has a certain probability of being obtained. We store each of these binary properties in a bit. Each bit is a degree of freedom. Static analysis creates sets of these bits. Our register allocation algorithms commit the set of bits created by static analysis for optimization and code generation. For a given program there is one deterministic such set of bits.
The probability of predicting the properties of a given program before its execution in a linear time is inversely proportional to the size of the analyzed program. We also describe below the required transformation for the computation of that probability and its properties. We call this probability the level of abstraction. Because it is practically difficult to build a simple deterministic process for the computation of all possible variants of behavior for a defined program, we also look at the defined properties of the generated machine code. Each piece of machine code has an expected weight given by an approximation which depends linearly on the computed level of abstraction.
Properties are defined on a certain domain of optimization. For example, as mentioned above, they can be the number of accesses of the memory, the security of the machine code and/or the size of the generated machine code. In all cases, there should be an interval of machine code for achieving some set (or singleton) of the defined strategies of optimization. Thus, we look at the process of register allocation for justintime compilers as the result of the defined process of static analysis. For that defined process, each managed method is mapped onto a set of random variables in a defined probability space. That random variable vector is a set of possible machine codes as well as a set of data for which the source code is compiled. In other words, each compiled object is a realization of an ordered set of random variables. We consider their distributions below.
We define this weight as the difference between the expected optimistic gains and the expected pessimistic losses. In other words, we look at the expected best variant and the expected worst variant. We then define a function of wins and losses, which are defined on primitive chains of events
$\begin{aligned}& { E}(losses( \left\langle X \right\rangle )) = \\& \quad \int_{t = 0}^{t{{ = }}T} {abstraction} ( \left\langle X(t) \right\rangle ){ E}(losses( \left\langle X(t) \right\rangle )){{d}}t\\& { E}(wins( \left\langle X \right\rangle )) = \\& \quad \int_{t = 0}^{t{{ = }}T} {abstraction} ( \left\langle X(t) \right\rangle ){ E}(wins( \left\langle X(t) \right\rangle )){{d}}t\\& { E}(weight( \left\langle X \right\rangle )) = { E}(wins( \left\langle X \right\rangle )losses( \left\langle X \right\rangle )) = \\& \quad { E}(wins( \left\langle X \right\rangle ))  { E}(losses( \left\langle X \right\rangle )) = \\& \quad \int_{t = 0}^{t{{ = }}T} {abstraction} ( \left\langle X(t) \right\rangle ){ E}(weight( \left\langle X(t) \right\rangle )){{d}}t.\end{aligned}$ 
E(
We represent the intermediate code by independent sets, in which there are variables. Each variable is assigned a virtual register for all ranges of its life. There is a limited set of resources for the allocation of hardware registers. We optimize the use of this limited resource of hardware registers for register allocation. Below, we show the expected weight that a variable should be assigned a hardware register.
Let us look at the step in which there is already information about the ranges of a live variable. Suppose the distribution of properties of the intermediate code in a certain continuous region that contains the properties for optimization is uniform and the level of abstraction is constant. This is a valid assumption because a general distribution can usually be approximated by various uniform distributions in different regions, and we may even let the number of regions become infinite. Our motivation for this assumption is that the probability of finding properties for a sequence of bits with only a limited number of bits in finite time is inversely proportional to the total number of bits in that sequence and proportional to the total number of bits that can be analyzed in linear time. Of course, it is possible to have an unknown distribution. Nevertheless, we believe that a uniform distribution is a good approximation at this scale. Thus,
$abstraction( \left\langle X \right\rangle ) = \frac{1}{{Power( \left\langle X \right\rangle )}}.$ 
The power function returns the number of bits that can be used in the representation of the set of all possible variants of the behavior of the machine code and data. Each block of bits can either use a variable or ignore it. We introduce the function Δ(
${ E}(weight( \left\langle X \right\rangle )) \!=\!\! \int_A^B {\frac{1}{{Power( \left\langle X \right\rangle )}}} \Delta (t){{d}}t \!=\! \frac{U}{{B A}}.$ 
Here, U is the total number of usages of the variable. Each variable has a physical geometrical space in the program representation where it is live. A is the coordinate of the appearance of the firstused live range of the variable, and B is the coordinate of its lastused live range. When U is constant for all the variables and there is a high intensity of appearance of new variables and A is much smaller than B, we get the formula for the weights of the original heuristic linear scan.
${ E}(weight( \left\langle X \right\rangle )) = \frac{1}{B}.$ 
This section discusses the experimental results of the use of the developed library. Five levels of optimization are supported. Optimization level 0 uses the default calling convention, global register allocation based on the number of uses, and floatingpoint operations are using floating point unit (FPU). Optimization level 1 uses only local register allocation in each block of instructions. Optimization level 2 uses linear scan, and fast analysis of the liveness of variables is using the graph of control flow. At optimization level 2, only one interval of liveness of variables for each virtual register is present. Optimization level 3 uses the algorithm of secondchance binpacking based on multiple intervals of liveness of variables for each virtual register. Optimization level 4 includes deadcode elimination. In addition, the new generator of machine code uses single instruction, multiple data (SIMD) extensions and streaming SIMD extension registers (XMM) registers to represent floatingpoint operations.
PnetMark^{[47, 48]} was created on the basis of CaffeineMark benchmarks for the Java virtual machine. Conversion of test was made using the transformation of benchmarks from descriptions in Java language into
Each benchmark was executed 80 times for each variant of the execution environment, shown in Table 1, and the algorithm of register allocation. This number was selected as a result of consultations with the specialists, who develop Portable.NET.
On the Yaxis in Figs. 6–12 is the average number of invocations of the function in a given unit of time. The increase of performance in the presented benchmarks was computed as the number, which is determined relatively to the performance of PnetMark benchmark for the mode LSO4 internal and direct unrolled.
Analogically, the performance values for the SciMark2 and Linpack benchmarks are also computed below.
Fig. 6 determines the time of passing all regression tests of the library of classes.
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Fig. 6. Time of passing tests of the library of classes of Portable.NET (in seconds) 
Figs. 7 and 8 present the results for the PnetMark, SciMark2, and Linpack benchmarks.
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Fig. 7. PnetMark, SciMark2, Linpack benchmarks 
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Fig. 8. PnetMark, SciMark2, Linpack benchmarks 
Higher values mean better performance of the runtime in benchmarks. The increase of performance in PnetMark is 116%, in SciMark2 it is 239%, and in Linpack it is 136%.
As seen from the estimates of standard deviation, the variance of the results in our environment is substantially less than other variants of the execution environment of .NET from competitors and the previous version of the justintime compiler.
Let us look at two examples of functions that we call power, and methods for computation of the expected benefit. The first definition uses the number of operational codes included in the set of intervals of liveness, and the number of times the variable was used. This register allocator is designated as abstract linear scan (ALS). The second definition uses the size of the machine code and the number of times the memory in the generated machine code was accessed. In other words, each operational code has two elements with two offsets, wherein the machine code starts with the current operational code and ends with it. Computations are performed for the number of accesses to the primitive code generator i486_membase_emit, to which a counter based on a global variable is added. In other words, we computed the number of expected memory accesses in the generated machine code, excluding loops. This register allocator is denoted as an abstract linear scan based on the heuristics of machine code. The allocators of registers are compared to the heuristics of simple linear scan, which saves the register with the farthest end of time of liveness. We denote this register allocator as linear scan (LS). Moreover, another heuristic of register allocation is used, which is based on the number of intervals of liveness of each register. The results of the application of these heuristics are presented in Fig. 8. An analysis of the results demonstrates that the improvement in code speed results in smaller size of machine code.
The graphs in Figs. 9–12 show the estimation of standard deviation for various performance benchmarks. It is worth noting that the numerical expression of estimations of dispersion for these random values has a largely demonstrative and relative character. This is because the obtained results of its numerical expression vary depending on the hardware platform and a variety of other factors. Each point on the graphs took 2 hours of computation and required 80 runs of the benchmark. It has determining value for the analysis of the research results. It tells that objective unbiased statistical studies over a considerable period of time were conducted, a significant sample from realizations of random variables was taken. These results are objective for this hardware platform and operating system. They show the possibility of conducting repeated measurements and are analogous in quality to the results for other platforms and operating systems.
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Fig. 9. Estimation of deviation of values of PnetMark, SciMark2 and Linpack benchmarks 
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Fig. 10. Estimation of deviation of values of PnetMark benchmark 
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Fig. 11. Estimation of deviation of values of SciMark2 bench mark 
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Fig. 12. Estimation of deviation of values of Linpack bench mark 
5 Discussions
As follows from the data in Figs. 6–12, the new heuristics of register allocation show improvements in performance benchmarks in comparison with the use of the simple heuristic of linear scan. When using register allocation based on machine code, analysis of the assembly language code shows the inefficiency of the algorithm of local allocation of registers, applied to the chosen path of optimization using the algorithm of global allocation of registers. The algorithm of local allocation of registers is the next object for further research and optimization. When using the heuristic with machine code, there is an improvement of the PnetMark benchmark. However, the performance of SciMark2 and Linpack decreases. Studies show that this appears to be due to the inefficiency of using only two points of optimization. This methodology and further possible improvements are similar to the approach of applying the simplex method for finding the optimal result. In this case, the global algorithm of allocation of registers yields impressive results, both the number of virtual registers that use the local stack and the size of the local stack used for global variables are smaller. However, the local algorithm of allocation of registers adds many accesses to the memory stack. This can be solved by using a larger number of experimental points and a greater number of optimization passes. The experiment confirms that most of the variables in the intermediate representation, which is obtained from the compilation of common intermediate language (CIL), have only one interval of liveness. Most of the intervals of liveness of virtual registers are short and have a length not greater than three operational codes if deadcode elimination is not applied previously. Most intervals of liveness of virtual registers are not above four operational codes if deadcode elimination was applied. Most of these variables are temporary. Thus, the use of the heuristic of saving into memory virtual registers with the largest number of intervals of liveness is justified. Fig. 13 shows the sample distribution function of the lengths of liveness of variables.
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Fig. 13. Sample distribution function of the length of the interval 
The number of intervals of liveness with optimization level 2 is always equal to 1. The distribution of the number of intervals of liveness with optimization levels 3 and 4 is shown in Fig. 14.
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Fig. 14. Sample distribution function of the number of intervals 
The improvement in performance benchmarks is the result of the analysis of variables′ liveness space and deadcode elimination. The most productive algorithm variant is the one that uses the intermediate representation for the computation of variables′ weights. In our opinion, the heuristic based on the candidates of machine code has greater potential. It gives results that are close to the heuristic described above with the use of two points. In addition, it requires more computational time, because it can use more experimental points for finding the optimal solution. It can be used to advantage, because not all machine code properties are clearly defined in intermediate code.
It should be noted that the original implementation of the algorithm of linear scan, which uses only one interval of liveness for compilation, worked two times faster than the algorithm of register allocation, based on the number of uses of variables. In order to implement the use of multiple intervals of liveness, it was necessary to implement the more general algorithm. This algorithm slowed down compilation time.
The increased performance of the new algorithms of register allocation happens at all levels, but it is particularly significant on low levels of optimization. In addition, it is much more significant after the use of information provided by static analysis of the space of liveness of variables and after deadcode elimination.
Compared with the interpretation mode, fast compilation with libjitlinearscan does not increase the time of execution of the virtual machine or the time of compilation, and thus the quality of the machine code improves. The increased performance of the abstract linear scan compared with simple linear scan at optimization level 2 in PnetMark is 5.30%, in SciMark2 – 4.00%, and in Linpack – 12.59%. In addition, improvements were noted in all intermediate benchmarks in PnetMark, SciMark2 and Linpack.
New algorithms do not have any unnecessary computational costs, and additionally use only the information about the number of uses of variables. It should be noted that when simple linear scan is enabled, this information is computed anyway in invocations of the API.
6 ConclusionsThe paper studied a new class of rapid register allocation algorithms and presented experimental data on their behavior. Thus, the authors found that the increase of runtime performance for PnetMark is 116%, for SciMark2 – 239%, and 136% for Linpack. As seen from the estimates of standard deviation, the variance of the results in the environment is substantially less than other variants of .NET execution environment of competitors and the previous version of the justintime compiler. The results of heuristics application for PnetMark is 105%, 155% for SciMark2, and 93% for Linpack. These results are objective for this hardware platform and operating system. They show the possibility of performing repeated measurements and are similar in quality to the results for other platforms and operating systems. The paper provides the estimation of value deviation of PnetMark, SciMark2, and Linpack benchmarks. The maximum value deviation of PnetMark benchmark is for internal LSO2. The maximum value deviation of SciMark2 and Linpack benchmarks is for mono 2.0. Thus, the new heuristics of register allocation show improvements in benchmarks performance in comparison with the use of simple heuristics of linear scan. The researchers also found that the performance of SciMark2 and Linpack decreases and this appears to be due to the inefficiency of using only two points of optimization. The conducted experiment confirmed that most of the variables in the intermediate representation of libjitlinearscan, obtained from the compilation of CIL, have only one interval of liveness.
The results of the performed research may be useful for the development of rapid justintime compilers, methods of automatic neutralization of vulnerabilities, converters of bytecodes of virtual machines, and compilers for executing the programs of virtual machines such as Java, Python, Perl, Ruby, LLVM, and Parrot.
We have outlined a theoretical possibility and have demonstrated the possibility of the practical applicability of the proposed approach. We have tested various options and have specified the optimal algorithm implementations, based on the practical use purposes. It is worth noting that the proposed approach is open to improvement, and although we have demonstrated the applicability of the principle and the possibility of the algorithm, the proposed approach at the momentin a great measure a theoretical possibility. Further research directions could be concerned with various possibilities of dynamic compilation and the dynamic neutralization of data leakages. For example, it is possible to create new domainoriented algorithms for the optimization of the performance of code and neutralization of threats that target the narrow specifics of the application. The given algorithms can be specialized and optimized for the application in a variety of areas where embedded systems or mainframes can be or are being used. These areas include, for instance, power consumption or various aspects of the secure execution of programs in operating systems for general and specialized purposes.
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