Improving performance by branch reordering

Yang, Menglong; Uh, Gang-Ryung; Whalley, David

doi:10.1145/277652.277711

Cited by 6 publications

(5 citation statements)

References 8 publications

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“…This approach may be reasonable for a small sequence length (e.g. n ≤ 7), which seem to handle most branch sequences with a common successor [Yang 1998]. Consider the original and reordered sequences of range conditions in Figure 3.…”

Section: Discussionmentioning

confidence: 99%

Efficient and effective branch reordering using profile data

Yang

Whalley

2002

ACM Trans. Program. Lang. Syst.

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The conditional branch has long been considered an expensive operation. The relative cost of conditional branches has increased as recently designed machines are now relying on deeper pipelines and higher multiple issue. Reducing the number of conditional branches executed often results in a substantial performance benefit. This paper describes a code-improving transformation to reorder sequences of conditional branches that compare a common variable to constants. The goal is to obtain an ordering where the fewest average number of branches in the sequence will be executed. First, sequences of branches that can be reordered are detected in the control flow. Second, profiling information is collected to predict the probability that each branch will transfer control out of the sequence. Third, the cost of performing each conditional branch is estimated. Fourth, the most beneficial ordering of the branches based on the estimated probability and cost is selected. The most beneficial ordering often includes the insertion of additional conditional branches that did not previously exist in the sequence. Finally, the control flow is restructured to reflect the new ordering. The results of applying the transformation are on average reductions of about 8% fewer instructions executed and 13% branches performed, as well as about a 4% decrease in execution time.

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Section: Discussionmentioning

confidence: 99%

Efficient and effective branch reordering using profile data

Yang

Whalley

2002

ACM Trans. Program. Lang. Syst.

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“…Conditional branches have been coalesced together into an indirect jump from a jump table [6]. Sequences of branches have been reordered to allow the sequence to be exited earlier, which reduces the number of branches executed [7,8]. There has been recent work on eliminating branches using ILP architectural features.…”

Section: Related Workmentioning

confidence: 99%

Branch Elimination via Multi-variable Condition Merging

Kreahling

Whalley

Bailey

et al. 2003

Euro-Par 2003 Parallel Processing

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Conditional branches are expensive. Branches require a significant percentage of execution cycles since they occur frequently and cause pipeline flushes when mispredicted. In addition, branches result in forks in the control flow, which can prevent other code-improving transformations from being applied. In this paper we describe profile-based techniques for replacing the execution of a set of two or more branches with a single branch on a conventional scalar processor. First, we gather profile information to detect the frequently executed paths in a program. Second, we detect sets of conditions in frequently executed paths that can be merged into a single condition. Third, we estimate the benefit of merging each set of conditions. Finally, we restructure the control flow to merge the sets that are deemed beneficial. The results show that eliminating branches by merging conditions can significantly reduce the number of conditional branches performed in non-numerical applications.

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“…This approach is generalized and extended to the program level in [20]. In the second category of control flow optimization, branch reordering, the order in which branches are evaluated is changed to reduce the average depth traversed through a network of branches [22]. The final category of control flow optimization research focuses on the reduction of control dependence height.…”

Section: Related Workmentioning

confidence: 99%

Program decision logic optimization using predication and control speculation

2001

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Abstract-The mainstream arrival of predication, a means other than branching of selecting instructions for execution, has required compiler architects to reformulate fundamental analyses and transformations. Traditionally, the compiler has generated branches straightforwardly to implement control flow designed by the programmer and has then performed sophisticated "global" optimizations to move and optimize code around them. In this model, the inherent tie between the control state of the program and the location of the single instruction pointer serialized runtime evaluation of control and limited the extent to which the compiler could optimize the control structure of the program (without extensive code replication). Predication provides a means of control independent of branches and instruction fetch location, freeing both compiler and architecture from these restrictions; effective compilation of predicated code, however, requires sophisticated understanding of the program's control structure. This article explores a representational technique which, through direct code analysis, maps the program's control component into a canonical database, a reduced ordered binary decision diagram (ROBDD), which fully enables the compiler to utilize and manipulate predication. This abstraction is then applied to optimize the program's control component, transforming it into a form more amenable to instruction-level parallel (ILP) execution.

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Improving performance by branch reordering

Cited by 6 publications

References 8 publications

Efficient and effective branch reordering using profile data

Efficient and effective branch reordering using profile data

Branch Elimination via Multi-variable Condition Merging

Program decision logic optimization using predication and control speculation

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