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A diagram depicting an optimizing compiler removing a potentially useless call to assembly instruction "b" by sinking it to its point of use. Code Sinking, also known as lazy code motion, is a term for a technique that reduces wasted instructions by moving instructions to branches in which they are used: [1] If an operation is executed before a branch, and only one of the branch paths use the ...
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 .
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 ...
The assumption is that the loop must be normalized – written so that the loop index/variable starts at 1 and gets incremented by 1 in every iteration. For example, in the following loop, a=2, b=3, c=2, d=0 and GCD(a,c)=2 and (d-b) is -3. Since 2 does not divide -3, no dependence is possible.
This can improve locality of reference, both of the data being accessed in the loop and the code in the loop's body. Fusion or combining – this combines the bodies of two adjacent loops that would iterate the same number of times (whether or not that number is known at compile time), as long as they make no reference to each other's data.
In the C programming language, Duff's device is a way of manually implementing loop unrolling by interleaving two syntactic constructs of C: the do-while loop and a switch statement. Its discovery is credited to Tom Duff in November 1983, when Duff was working for Lucasfilm and used it to speed up a real-time animation program.
Post-pass optimizers usually work on the assembly language or machine code level (in contrast with compilers that optimize intermediate representations of programs). One such example is the Portable C Compiler (PCC) of the 1980s, which had an optional pass that would perform post-optimizations on the generated assembly code. [3]: 736
Trace scheduling was originally developed for Very Long Instruction Word, or VLIW machines, and is a form of global code motion. It works by converting a loop to long straight-line code sequence using loop unrolling and static branch prediction. This process separates out "unlikely" code and adds handlers for exits from trace.