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Optimizing CompilersCISC 673

Spring 2009

Feedback Directed Optimization

John CavazosUniversity of Delaware

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Feedback-Directed Optimization (FDO)

Exploit information gathered at run-time to optimize execution “selective optimization”: what to

optimize “FDO” : how to optimize

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Advantages of FDO Can exploit dynamic information

that cannot be inferred statically

System can change and revert decisions when conditions change

Runtime binding allows more flexible systems

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Challenges for automatic online FDO

Compensate for profiling overhead

Compensate for runtime transformation overhead

Account for partial profile available and changing conditions

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Clients of Profiling Information

Inlining, unrolling, method dispatch Dispatch tables, Garbage

Collection Pretching

Misses, Hardware performance monitors [Adl-Tabatabai et al.’04]

Code layout

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Code Layout Motivation Instruction memory critical to program

performance Cache line conflicts are one cause of misses Particularly for direct-mapped caches Page faults

Default code layout for most compilers is bad Instructions placed in source order Procedures placed according to source file

listing Compilers can rearrange code to reduce

misses More performance and less power for free

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Procedure Positioning

“Closest is best” strategy: if procedure calls another frequently, we want two procedures close to one another. Increases chances they will land on the same page, reducing working set.

Construct a weighted call graph Node = procedure Edge from A B means procedure A calls B (perhaps

multiple times) Weight on edge is the total number of dynamic calls

Using the weighted call graph to build the link order for the procedures

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Example Call Graph

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Merging Nodes

1) The heaviest weight edge is one connecting procedures A and D. They are merged. Edge weights are updated.

2) The next edge chosen is the one between C and F.