This document discusses compiler optimization techniques. It describes Intel's guided optimization path which involves using general optimization options, identifying performance hotspots, and using inter-procedural optimization (IPO), profile-guided optimization (PGO), and parallel performance options. IPO allows optimizations across procedures, PGO improves performance based on profiling, and high-level optimizations exploit properties of high-level languages. The document provides examples of optimizations performed by each technique.

1. Performance Optimization
by Compilers
Luo Bu
2010-12-18
中科信软高级技术培训中心－www.info-soft.cn
Compiler Optimization Introduction
 IPO (Inter-Procedural Optimization)
 PGO (Profile-Guided Optimization)
 HLO (High-Level Optimization)
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2. Intel guided optimization path
• Use the General Optimization Options
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• Use processor-specific options
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• Identify performance hotspots
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• Use IPO and PGO
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• Use parallel performance options for Multi-core, Multi-
5 processor, or HT enabled system
• Use thread profiler to maximize performance
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IPO (Inter-Procedural Optimization)
Interprocedural Optimization (IPO) allows the compiler to analyze your
code to determine where you can benefit from specific optimizations.
When you use IPO with the -x or -ax (Linux* OS) options, or the /Qx or
/Qax (Windows* OS) options, you may see additional optimizations for
Intel microprocessors than for non-Intel microprocessors.
 inlining  unreferenced variable removal
 constant propagation  whole program analysis
 mod/ref analysis  array dimension padding
 alias analysis  common block splitting
 forward substitution  stack frame alignment
 routine key-attribute propagation  structure splitting and field reordering
 address-taken analysis  formal parameter alignment analysis
 partial dead call elimination  C++ class hierarchy analysis
 symbol table data promotion  indirect call conversion
 common block variable coalescing  specialization
 dead function elimination
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3. PGO (Profiled-Guided Optimization)
Profile-guided
Optimization (PGO)
improves application
performance by
reorganizing code layout
to reduce instruction-
cache problems,
shrinking code size, and
reducing branch
mispredictions. PGO
provides information to
the compiler about areas
of an application that are
most frequently executed.
By knowing these areas,
the compiler is able to be
more selective and
specific in optimizing the
application.
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HLO (High-Level Optimization)
High-level Optimizations (HLO) exploits the properties of source code
constructs in applications developed in high-level programming
languages. While the default optimization level, -O2 option, performs
some high-level optimizations, specifying -O3 provides the best chance
for performing loop transformations to optimize memory accesses.
 Loop Permutation or Interchange  Predicate Optimization
 Loop Distribution  Loop Reversal
 Loop Fusion  Profile-Guided Loop Unrolling
 Loop Unrolling  Loop Peeling
 Data Prefetching  Data Transformation: Malloc
 Scalar Replacement Combining and Memset Combining
 Unroll and Jam  Loop Rerolling
 Loop Blocking or Tiling  Memset and Memcpy Recognition
 Partial-Sum Optimization  Statement Sinking for Creating
Perfect Loopnests
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4. Truth from Graph
Benchmark Tuning Curve with gcc/icc, on various optimization
flags
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Compiler optimization
 GCC Optimization
• http://www.linuxjournal.com/article/7269
• http://www.network-theory.co.uk/docs/gccintro/gccintro_49.html
• http://gcc.gnu.org/wiki/LinkTimeOptimization
• http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
 Intel C++ Optimization(aka icc)
• http://software.intel.com/sites/products/collateral/hpc/compilers/compiler_qrg
12.pdf
• http://software.intel.com/file/6282
• http://software.intel.com/sites/products/documentation/hpc/composerxe/en-
us/cpp/lin/index.htm
• http://software.intel.com/en-us/articles/performance-tools-for-software-
developers-intel-compiler-options-for-sse-generation-and-processor-specific-
optimizations/
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5. Q
&A
QUESTIONS
ANSWERS
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