University of Houston Extending Global Optimizations in the OpenUH Compiler for OpenMP Open64 Workshop, CGO ‘08

University of Houston Goals Exploit the compiler analysis and optimizations for OpenMP programs Enable high level optimizations by taking OpenMP semantics into consideration Build a general framework for OpenMP compiler optimizations 2

University of Houston OpenUH Compiler based on Open64 IPA (Inter Procedural Analyzer) Source code w/ OpenMP directives Source code with runtime library calls Linking CG (code for IA-32, IA-64, Opteron) WOPT (global scalar optimizer) Object files LOWER_MP (Transformation of OpenMP ) A Native Compiler A Native Compiler Executables A Portable OpenMP Runtime library A Portable OpenMP Runtime library FRONTENDS (C/C++, Fortran 90, OpenMP) Open64 Compiler infrastructure LNO (Loop Nest Optimizer) OMP_PRELOWER (Preprocess OpenMP ) WHIRL2C & WHIRL2F (IR-to-source for none-Itanium )

University of Houston OpenUH Compiler based on Open64 IPA (Inter Procedural Analyzer) Source code w/ OpenMP directives Source code with runtime library calls Linking CG (code for IA-32, IA-64, Opteron) WOPT (global scalar optimizer) Object files LOWER_MP (Transformation of OpenMP ) A Native Compiler A Native Compiler Executables A Portable OpenMP Runtime library A Portable OpenMP Runtime library FRONTENDS (C/C++, Fortran 90, OpenMP) Open64 Compiler infrastructure LNO (Loop Nest Optimizer) OMP_PRELOWER (Preprocess OpenMP ) WHIRL2C & WHIRL2F (IR-to-source for none-Itanium )

University of Houston Motivation Compiler flags -O3-O3 –mp3 PRE- example NAS FT NAS UA Why different performance?

University of Houston A PRE Example

University of Houston A PRE Example copy propagation no copy propagation!

University of Houston Parallel Data Flow Analysis Compilers need to further optimize OpenMP codes Most current OpenMP compilers perform optimizations after OpenMP constructs have been lowered to threaded codes –Have to restrict the traditional optimizations inside an OpenMP construct, not crossing synchronizations Need to enable global optimizations –Missed opportunity to perform high-level OpenMP optimizations Such as barrier elimination

University of Houston Solution Method Based on the OpenMP Memory Model –Relaxed Consistency –Flush is the key operation! Design a Parallel Control Flow Graph to represent a OpenMP program

University of Houston Barrier a=1;b=1; Flush(a,b) Else… a=0; b=0; #pragma omp parallel sections { #pragma omp section { a=1; #pragma omp flush(a,b) IF (b == 0){ Critical1; a:= 0; #pragma omp flush(a) }ELSE else1; #pragma omp section { b=1; #pragma omp flush(a,b) IF (a == 0){ Critical2; b= 0; #pragma omp flush(b) }ELSE else2; } A: an OpenMP section example B: The corresponding PCFG Super node: Composite node: Basic Node: Parallel edge: Sequential edge: Entry Conflict edge: If (a ==0) Flush(b) b=0 Else… If (b ==0) Flush(a) a=0

University of Houston CFG HSSA IVR CP DCE CP DCE Emit Input WHIRL tree Output WHIRL tree -Construct CFG -Control Flow Analyses -Flow Free Alias Analysis -Construct HSSA representation -Points-to and Pointer Alias Analysis -Create CODEMAP representation -PREOPT SSA-based optimizations “Flow free copy propagation” -Emit new WHIRL from optimized CFG/SSA PCFG HSSA IVR CP DCE CP DCE Emit Input WHIRL tree Output WHIRL tree -Construct CFG -Control Flow Analyses -Parallel Control Flow Analysis -Flow Free Alias Analysis -Construct HSSA representation -Phi insertion for conflict edges -Points-to and Pointer Alias Analysis -Create CODEMAP representation -SSA-based optimizations “Flow free copy propagation” -Emit new WHIRL from optimized CFG/SSA SSAPRE -Perform PRE on OpenMP code

University of Houston Conclusion Implementing in the OpenUH compiler Improve the scalability of OpenMP programs A framework for conducting more aggressive optimizations for Cluster OpenMP Can be used in conjunction with data race detection tools