Kristof Beyls, Erik D’Hollander, Frederik Vandeputte ICCS 2005 – May 23 RDVIS: A Tool That Visualizes the Causes of Low Locality and Hints Program Optimizations

Overview 1. Motivation: cache bottleneck 2. Some theoretical background: reuses 3. View 1: cache-missing reuses 4. View 2: reuse pair clusters corresponding to program optimizations 5. Experimental results 6. Implementation details 7. Conclusion

1. Motivation Many programs incur large cache bottlenecks. Mainly caused by poor locality (temporal or spatial) Temporal locality is hard to optimize automatically in a compiler Therefore: need to help programmer to pin-point sources of low temporal locality.

2. Theoretical background Stream of memory accesses: accesses:abcaab references:r1r1r2r1r1r1 basic block:bb1bb1bb2bb1bb1bb1 Reuses / Reuse Distance Reference pair / Reference pair histogram Basic Block Vector of Intermediately executed code Cache miss  reuse distance ≥ cache size

2. Theoretical background Stream of memory accesses: accesses:abcaab references:r1r1r2r1r1r1 basic block:bb1bb1bb2bb1bb1bb1 Reuses / Reuse Distance Reference pair / Reference pair histogram Basic Block Vector of Intermediately executed code. Reference pair r1-r1 Reuse distance

3. RDVIS by example: matrix multiplication Reuses between a[i*N+k] at distance 2^9 Reuses between b[k*N+j] at distance 2^17 How to bring reuses of b[k*N+j] closer together? What separates reuses? What code is executed between reuses?

3. RDVIS by example: matrix multiplication Reuses occur between iterations of i-loop Solution: bring iterations of i-loop inwards

3. RDVIS by example: matrix multiplication Next to optimize: reuses of A[i*N+k]

3. Matrix multiplication: final result L1 cache L2 cache Main memory Exec. Time on P4: Orig: 0.740s Opt.: 0.223s Speedup: 3.3

4. Cluster Analysis In more complex programs, there can be many arrows. Many arrows can often by optimized by the same program transformation. Key idea: “When the same code is executed between use and reuse, probably the same program transformation is needed.”

4. Cluster Analysis by example: equake Many different arrows contribute to long- distance reuse

2(bis). Theoretical background Stream of memory accesses: accesses:abcaab references:r1r1r2r1r1r1 basic block:bb1bb1bb2bb1bb1bb1 Reuses / Reuse Distance Reference pair / Reference pair histogram Basic Block Vector of Intermediately executed code of a reference pair. BBV(Reference pair r1-r1).66 % exec. betw. reuses bb2bb1Basic block

4. Cluster Analysis by example: equake LOOP FUSION!

5. Experimental Results

6. Some Implementation Details Instrumentation added to GCC 4: –Exact source location info added to all abstract syntax tree nodes. –Source location info is added in language-specific front-end (currently only C, Fortran is being added). –Instrumentation occurs in language-independent middle-end. Inserts function call for each memory reference Inserts function call at begin of each basic block Writes out source location info for memory references and basic blocks

7. Conclusion Visualization indicates reuses at a long distance, and the code that is executed between those reuses. Clustering of intermediately executed code leads to reference pairs that are optimizable with the same program transformation. Give RDVIS a try:

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