1 s. tallam, r. gupta, and x. zhang pact 2005 extended whole program paths sriraman tallam rajiv...

S. Tallam, R. Gupta, and X. Zhang PACT 2005 1

Extended Whole Program Paths

Sriraman Tallam

Rajiv Gupta Xiangyu Zhang

University of Arizona


Control Flow and Dependence Traces

Control Flow Traces• Sequence of basic blocks.• Identification of hot paths.

Path Sensitive Instruction Scheduling and Optimization. Path Prediction and Instruction Fetching.

Dependence Traces• Capture data dependences.

Flow from a definition to a use.• Data Speculative Optimizations for Itanium.• Computation of Dynamic Slices.


Control Flow and Dependence Traces

Control Flow Traces are smaller than Dependence Traces and can be compressed well.

• Average size for Spec 2K benchmarks is 179 MB.• Compression Factor

Sequitur – 681 VPC – 442

Dependence Traces are large and do not compress as well as Control Flow Traces.

• Average size for Spec 2K benchmarks is 565 MB.• Compression Factor

Sequitur – 1.31 VPC – 5.8

• Is there an alternative trace representation ?


Our Approach

Extended Control Flow Trace – Unified Trace Representation.

• Capture both control flow and dependence information.• The data dependences are embedded as control flow.

The unified trace is smaller than control flow + dependence traces.

Our compressed unified trace is also smaller than the compressed control flow + compressed dependence traces.


Goals in Designing the eCF

The dependence can be recovered from the Control Flow.

X = _

X = _

= X

*p = _

The dependence can now not be recovered due to possible aliasing.

Additional Control Flow can capture the dependence.

1

2 3

4

4= X

If p==&X

5

6


Cost of Capturing Dependences

No-cost capture• For these dependences, no disambiguation checks are

needed.

Fixed cost capture• The number of disambiguation checks needed is a constant.

Variable cost capture.• The number of disambiguation checks varies.


No Cost Capture

All instances of the dependence can be recovered from the control flow trace.


Fixed Cost Capture

A single disambiguation check is sufficient to capture this dependence.

Single Check


Variable Cost Capture

The instances of the dependence can be caused by any instance of the definition statement.

Multiple Checks


Cost of Instrumentation and Trace Compressibility

Reducing the number of checks• Reducing the size of the generated trace.• Reduction in run-time overhead.

Improving the Compressibility• Similar Control Flow Signatures.


Two Phased Approach

Conservative nature of Static Pointer Analysis.• Too many potential dependences per use.

Two phased Approach• Filtering Phase

Find all dependences exercised.• Profiling Phase

Add disambiguation checks only for those dependences exercised.


Binary Search vs. Linear Search

Track the last definition and instance of every write to a memory address.

Search the address array using binary search instead of linear search.


Optimizing Trace Length and Compressibility


Experimental Results

Implementation on the Microsoft Phoenix RDK.

Spec 2K benchmark binaries were rewritten to obtain instrumented versions.

• Easy to implement using Phoenix.

Intermediate representation was low-level x86 instruction set.

• Split dependences into register and memory.• Register dependences are always recoverable from control

flow trace.• Memory dependences were recovered using our approach.


Register and Memory dependences

A Significant (76 %) of dependences (register) can be recovered from the control flow trace


Uncompressed Trace Sizes

The unified trace is 62 % of the size of Control Flow + Dependence Trace

Cont. + Dep. Unified Ratio


Sequitur Compressed

Cont. + Dep. Unified Ratio

The compressed unified trace is 4 % of the size of compressed Control Flow + Dependence Trace


VPC Compressed

RatioUnifiedCont. +

Dep.

The compressed unified trace is 21 % of the size of compressed Control Flow + Dependence Trace


Memory Dependence Types

30 % of dependences can be recovered at no cost.


Address Comparisons

Binary Search reduces the address comparisons by 4 orders of magnitude.


Run-time Overhead

There is a 20 % increase in run-time overhead in collecting the unified trace.


Conclusions

We have designed an extended control flow trace that captures both control flow and data dependence history.

The key to the unified trace is the ability to convert memory data dependences into control flow.

• The resulting unified trace is smaller than the combined control flow + dependence trace.

• The run-time overhead increases by 20 %.

Our Thanks to Hoi Vo of Microsoft Corporation and the Phoenix Compiler Infrastructure Group.

1 s. tallam, r. gupta, and x. zhang pact 2005 extended whole program paths sriraman tallam rajiv...

Documents