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Loop Inversion
In computer science, loop inversion is a compiler optimization and loop transformation in which a while loop is replaced by an if block containing a do..while loop. When used correctly, it may improve performance due to instruction pipelining. Example in C int i, a 00 i = 0; while (i = 100 goto L2 Let us assume that ''i'' had been initialized to some value less than 100. Now let us look at the instructions executed at the moment after ''i'' has been incremented to 99 in the loop: goto L1 if i = 100 goto L2 Now, let's look at the optimized version: i := 0 if i >= 100 goto L2 L1: a := 0 i := i + 1 if i = 100 goto L2 We didn't waste any cycles compared to the original version. Now consider the case where ''i'' has been incremented to 99: if i < 100 goto L1 a := 0 i := i + 1 if i < 100 < [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computer Science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical disciplines (including the design and implementation of Computer architecture, hardware and Computer programming, software). Computer science is generally considered an area of research, academic research and distinct from computer programming. Algorithms and data structures are central to computer science. The theory of computation concerns abstract models of computation and general classes of computational problem, problems that can be solved using them. The fields of cryptography and computer security involve studying the means for secure communication and for preventing Vulnerability (computing), security vulnerabilities. Computer graphics (computer science), Computer graphics and computational geometry address the generation of images. Progr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Compiler Optimization
In computing, an optimizing compiler is a compiler that tries to minimize or maximize some attributes of an executable computer program. Common requirements are to minimize a program's execution time, memory footprint, storage size, and power consumption (the last three being popular for portable computers). Compiler optimization is generally implemented using a sequence of ''optimizing transformations'', algorithms which take a program and transform it to produce a semantically equivalent output program that uses fewer resources or executes faster. It has been shown that some code optimization problems are NP-complete, or even undecidable. In practice, factors such as the programmer's willingness to wait for the compiler to complete its task place upper limits on the optimizations that a compiler might provide. Optimization is generally a very CPU- and memory-intensive process. In the past, computer memory limitations were also a major factor in limiting which optimizations co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Loop Transformation
In compiler theory, loop optimization is the process of increasing execution speed and reducing the overheads associated with loops. It plays an important role in improving cache performance and making effective use of parallel processing capabilities. Most execution time of a scientific program is spent on loops; as such, many compiler optimization techniques have been developed to make them faster. Representation of computation and transformations Since instructions inside loops can be executed repeatedly, it is frequently not possible to give a bound on the number of instruction executions that will be impacted by a loop optimization. This presents challenges when reasoning about the correctness and benefits of a loop optimization, specifically the representations of the computation being optimized and the optimization(s) being performed.In the book Reasoning About Program Transformations', Jean-Francois Collard discusses in depth the general question of representing execu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
While Loop
In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The ''while'' loop can be thought of as a repeating if statement. Overview The ''while'' construct consists of a block of code and a condition/expression. The condition/expression is evaluated, and if the condition/expression is ''true'', the code within all of their following in the block is executed. This repeats until the condition/expression becomes false. Because the ''while'' loop checks the condition/expression before the block is executed, the control structure is often also known as a pre-test loop. Compare this with the ''do while'' loop, which tests the condition/expression ''after'' the loop has executed. For example, in the C programming language (as well as Java, C#, Objective-C, and C++, which use the same syntax in this case), the code fragment int x = 0; while (x 0 loop Factorial := Fac ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
If Statement
In computer science, conditionals (that is, conditional statements, conditional expressions and conditional constructs,) are programming language commands for handling decisions. Specifically, conditionals perform different computations or actions depending on whether a programmer-defined boolean ''condition'' evaluates to true or false. In terms of control flow, the decision is always achieved by selectively altering the control flow based on some condition (apart from the case of branch predication). Although dynamic dispatch is not usually classified as a conditional construct, it is another way to select between alternatives at runtime. Terminology In imperative programming languages, the term "conditional statement" is usually used, whereas in functional programming, the terms "conditional expression" or "conditional construct" are preferred, because these terms all have distinct meanings. If–then(–else) The if–then construct (sometimes called if–then–els ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Do While Loop
In most computer programming languages a do while loop is a control flow statement that executes a block of code and then either repeats the block or exits the loop depending on a given boolean condition. The ''do while'' construct consists of a process symbol and a condition. First the code within the block is executed. Then the condition is evaluated. If the condition is true the code within the block is executed again. This repeats until the condition becomes false. Do while loops check the condition after the block of code is executed. This control structure can be known as a post-test loop. This means the do-while loop is an exit-condition loop. However a while loop will test the condition before the code within the block is executed. This means that the code is always executed first and then the expression or test condition is evaluated. This process is repeated as long as the expression evaluates to true. If the expression is false the loop terminates. A while loop ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Instruction Pipelining
In computer engineering, instruction pipelining or ILP is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous "pipeline") performed by different processor units with different parts of instructions processed in parallel. Concept and motivation In a pipelined computer, instructions flow through the central processing unit (CPU) in stages. For example, it might have one stage for each step of the von Neumann cycle: Fetch the instruction, fetch the operands, do the instruction, write the results. A pipelined computer usually has "pipeline registers" after each stage. These store information from the instruction and calculations so that the logic gates of the next stage can do the next step. This arrangement lets the CPU complete an instruction on each clock cycle. It is common for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Central Processing Unit
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions in the program. This contrasts with external components such as main memory and I/O circuitry, and specialized processors such as graphics processing units (GPUs). The form, design, and implementation of CPUs have changed over time, but their fundamental operation remains almost unchanged. Principal components of a CPU include the arithmetic–logic unit (ALU) that performs arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching (from memory), decoding and execution (of instructions) by directing the coordinated operations of the ALU, registers and other co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Instruction Pipeline
In computer engineering, instruction pipelining or ILP is a technique for implementing instruction-level parallelism within a single processor. Pipelining attempts to keep every part of the processor busy with some instruction by dividing incoming instructions into a series of sequential steps (the eponymous "pipeline") performed by different processor units with different parts of instructions processed in parallel. Concept and motivation In a pipelined computer, instructions flow through the central processing unit (CPU) in stages. For example, it might have one stage for each step of the von Neumann cycle: Fetch the instruction, fetch the operands, do the instruction, write the results. A pipelined computer usually has "pipeline registers" after each stage. These store information from the instruction and calculations so that the logic gates of the next stage can do the next step. This arrangement lets the CPU complete an instruction on each clock cycle. It is common for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pipeline Stall
In the design of pipelined computer processors, a pipeline stall is a delay in execution of an instruction in order to resolve a hazard. Details In a standard five-stage pipeline, during the decoding stage, the control unit will determine whether the decoded instruction reads from a register to which the currently executed instruction writes. If this condition holds, the control unit will stall the instruction by one clock cycle. It also stalls the instruction in the fetch stage, to prevent the instruction in that stage from being overwritten by the next instruction in the program. In a Von Neumann architecture which uses the program counter (PC) register to determine the current instruction being fetched in the pipeline, to prevent new instructions from being fetched when an instruction in the decoding stage has been stalled, the value in the PC register and the instruction in the fetch stage are preserved to prevent changes. The values are preserved until the instruction c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Loop-invariant Code Motion
In computer programming, loop-invariant code consists of statements or expressions (in an imperative programming language) that can be moved outside the body of a loop without affecting the semantics of the program. Loop-invariant code motion (also called hoisting or scalar promotion) is a compiler optimization that performs this movement automatically. Example In the following code sample, two optimizations can be applied. int i = 0; while (i < n) Although the calculation x = y + z and x * x is loop-invariant, precautions must be taken before moving the code outside the loop. It is possible that the loop condition is false (for example, if n holds a negative value), and in such case, the loop body should not be executed at all. One way of guaranteeing correct behaviour is using a conditional branch outside of the loop. Evaluating the loop condition can have
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