1 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Extended Whole Program Paths Sriraman Tallam Rajiv Gupta Xiangyu Zhang University of Arizona

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

3 S. Tallam, R. Gupta, and X. Zhang PACT 2005 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 ?

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

5 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Goals in Designing the eCF  The dependence can be recovered from the Control Flow. X = _ = X *p = _  The dependence can now not be recovered due to possible aliasing.  Additional Control Flow can capture the dependence = X If p==&X 5 6

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

7 S. Tallam, R. Gupta, and X. Zhang PACT 2005 No Cost Capture  All instances of the dependence can be recovered from the control flow trace.

8 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Fixed Cost Capture  A single disambiguation check is sufficient to capture this dependence. Single Check

9 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Variable Cost Capture  The instances of the dependence can be caused by any instance of the definition statement. Multiple Checks

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

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

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

13 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Optimizing Trace Length and Compressibility

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

15 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Register and Memory dependences  A Significant (76 %) of dependences (register) can be recovered from the control flow trace

16 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Uncompressed Trace Sizes  The unified trace is 62 % of the size of Control Flow + Dependence Trace Cont. + Dep. Unified Ratio

17 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Sequitur Compressed Cont. + Dep.UnifiedRatio  The compressed unified trace is 4 % of the size of compressed Control Flow + Dependence Trace

18 S. Tallam, R. Gupta, and X. Zhang PACT 2005 VPC Compressed Ratio UnifiedCont. + Dep.  The compressed unified trace is 21 % of the size of compressed Control Flow + Dependence Trace

19 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Memory Dependence Types  30 % of dependences can be recovered at no cost.

20 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Address Comparisons  Binary Search reduces the address comparisons by 4 orders of magnitude.

21 S. Tallam, R. Gupta, and X. Zhang PACT 2005 Run-time Overhead  There is a 20 % increase in run-time overhead in collecting the unified trace.

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