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Motivation for Source Code Optimization Techniques

Survey of the traditional Code Optimization Process

Traditionally, any kind of code optimization is performed by compilers which are specifically designed for dedicated processor architectures. A compiler designed to produce efficient object code includes optimizers. In the following figure, the source code is translated to a medium-level intermediate code, and optimizations that are largely processor-independent are performed on it.



Workflow of an Optimizing Compiler



Examples for such optimizations are:


  • Common subexpression elimination
    A computation is redundant at some point in a program, if that computation has already been performed at an earlier program point. Such redundancies can be eliminated by keeping the result of the first occurrence of the computation in a variable and reusing the variable instead of re-computing the value.
  • Dead code elimination
    Any computation that generates a result which is guaranteed to be never used in the program (so called "dead code") can be eliminated without affecting the program semantics.
  • Loop-invariant code motion
    Computations contained in a loop whose results do not depend on other computations performed in that loop are said to be loop-invariant. Consequently, these computations can be moved outside of the loop code in order to increase performance.
  • Constant folding
    Computations that are guaranteed to result in constants can be moved from program runtime to compile-time. In this case, the compiler "folds" computations on constants to single constants already when generating assembly code.

Hereafter, the improved processor-independent code is translated to a low-level form and further optimizations that are mostly processor-specific are applied to it. These optimizations mainly deal with the efficient mapping of a program to machine-specific assembly instructions. This process which is often called code generation is usually decomposed into several phases for which efficient optimization techniques have been proposed:


  • Code selection
    In this phase, it is decided which assembly instructions will be used to implement code represented by a low-level intermediate representation. The optimization goal is to select a set of assembly instruction instances whose accumulated costs, with respect to a given metric, are minimal.
  • Data routing
    After operations are bound to functional units of the target processor during code selection, data routing deals with transporting data between functional units so as to minimize required transport operations.
  • Register allocation
    The register allocation phase determines where the results of computations are stored, either in memory or in a processor-internal register. Since the number of available physical registers is limited, register allocation decides which program values need to share a register such that the number of registers simultaneously required at any program point does not exceed the physical limit. The optimization goal is to hold as many values as possible in registers in order to avoid expensive memory accesses.
  • Instruction scheduling
    This phase decides when instructions will be executed at program runtime. During this optimization phase, dependencies between instructions as well as limited processor resources and pipelining effects have to be taken into account. The optimization goal can be to minimize the required execution time.


Existing Source Code Optimizations

Using the structure of optimizing compilers presented above, the two lowest levels of optimization depicted in the following figure are implemented.



Abstraction Levels of Code Optimizations



Although being very close to the processor architecture, compilers are often unable to map a source code efficiently to the actual platform. For example, it is very complicated to express a saturated addition in an ANSI-C program:

result = var1 + var2;

if (((var1 ^ var2) & 0x80000000) == 0)
if ((result ^ var1) & 0x80000000)
result = var1 < 0 ? 0x80000000 : 0x7fffffff;

As a consequence, a compiler is unable to recognize such code structures and to generate efficient assembly code, although processors like the TI C6x DSP are able to perform a saturated addition using one machine instruction. Hence, very inefficient control flow dominated machine code is generated during compilation for the given example.



Processor-specific Source Code Optimizations

In order to overcome this problem, it is a common technique that compilers provide the programmer with intrinsics (also known as compiler known functions). Using intrinsics, particular features of a processor can be made visible to the programmer. The compiler maps a call to an intrinsic not to a regular function call, but to a fixed sequence of machine instructions. Thus, intrinsics can be considered as source-level macros without any calling overhead. For example, the complex code shown above implementing a saturated addition can be replaced by the intrinsic result = _sadd(var1, var2); using the TI C6x compiler. This way, the compiler is able to replace the entire C fragment by a single instance of the SADD machine instruction.



Memory Hierarchy Exploitation

The optimization of the global memory accesses of an application is crucial for achieving low power systems, because it has been shown that between 50% and 80% of the power in embedded multimedia systems is consumed by memory accesses. The goal of the so-called Data Transfer and Storage Exploration methodology (DTSE) is on the one hand to minimize the storage requirements of embedded applications so as to reduce the absolute amount of memory needed to execute the application. On the other hand, the locality of data memory accesses is optimized at a very high global level in order to achieve a high utilization of small energy-efficient memories which are close to the processor. The DTSE methodology consists of the following steps:



Data Transfer and Storage Exploration

In the beginning, a memory oriented data flow analysis is performed followed by global data flow and loop transformations in order to reduce the required amount of background memories. Hereafter, data reuse transformations exploit a distributed memory hierarchy. Storage cycle budget distribution is performed in order to determine the bandwidth requirements and the balancing of the available cycle budget over the different memory accesses in the algorithmic specification. The goal of the memory hierarchy layer assignment phase is to produce a netlist of memory units from a memory library as well as an assignment of signals to the available memory units. For multi-processor systems, a task-/data-parallelism exploitation step takes place minimizing the communication and storage overhead induced by the parallel execution of subsystems of an application. Finally, data layout transformations (e.g. in-place mapping) help to increase the cache hit rate and to reduce the memory size by storing signals with non-overlapping lifetimes in the same physical memory location.



Processor-independent Source Code Optimizations

The DTSE steps described above will typically significantly increase the addressing complexity of transformed applications, e.g. by introducing more complex loop bounds, index expressions, conditions etc. Since this computational overhead is neglected by DTSE, the memory optimized system, despite being more power efficient, will suffer a severe performance degradation. Typical examples show that the runtime performance degrades by a factor of five or six after DTSE.

Parts of the additional complexity introduced by DTSE can be removed by source code optimizations for address optimization (ADOPT). Here, an algebraic cost minimization is performed first minimizing the number of operation instances. The goal is to find factorizations of addressing expressions in order to reuse computations as much as possible.

A very high penalty is caused during DTSE by the frequent insertion of non-linear addressing expressions containing modulo or division operators. Since these are usually not supported by the instruction sets of embedded processors, they are realized by calls to software routines of system libraries. Modulo and division can be replaced by variables serving as accumulators in the following way:




int jdiv6=0, jmod6=0;
for (j=0; j
for (j=0; j
a[j/6][j%6] = ...; -> if (jmod6 >= 6) {


jmod6 -= 6; jdiv6++; }


a[jdiv6][jmod6] = ...; }

Nevertheless, the increased amount of addressing arithmetic is not the only kind of overhead introduced by DTSE. Control flow aspects have not yet been targeted at all in the context of data optimization. For this reason, the source code optimization techniques presented on these pages explicitly focus on this area of research.


Goals

The goals of the work presented in the following are:


  • Development of novel optimization techniques which
  • can be automated,
  • are applied at the source code level to embedded applications, and
  • which aim at minimizing execution times and energy dissipation.

The techniques presented on this pages can be classified as processor-independent global source code optimizations. Although motivated by different architectural properties (e.g. instruction pipeline or address generation units), these optimization techniques do not take e.g. different pipeline layouts or the potential support of modulo addressing into account. For this reason, one of the main contributions of these optimizations is their independence of actual processor architectures. The provided experimental results demonstrate that various different processor architectures benefit by comparable orders of magnitude. These analogies between different processors underline the high quality of the optimizations independently of actual architectures.

The optimizations presented in the following bridge the gap between the techniques effectively exploring memory hierarchies and those focusing on a dedicated hardware. For example, the intensive optimization of data transfers and memory hierarchies implicitly leads to a degradation of the control flow of an embedded application. For this reason, another important contribution of this work is its concentration on control flow aspects in data flow dominated embedded software.

In order to meet the goals of automated optimization of runtimes and energy consumption, basic optimization strategies which are rather unconventional in the classical compiler construction are utilized. These include genetic algorithms and geometrical polytope models. Using such powerful techniques allows to solve complex code optimization problems for which good heuristics are out of sight. Although these basic strategies are quite complex, all source code optimizations have proven to be highly efficient so that times elapsed during code optimization are negligible.

As will be shown experimentally, these approaches for source code optimization achieve significant improvements in code quality. Code quality is quantified using numerous existing processors which are available in real hardware. The collection of results bases on measurements of the physical processor hardware. Time consuming and imprecise software simulations were avoided to a large extent. Since another contribution of this work is to provide optimization techniques that work in practice, mostly all optimizations are fully automated and applied to real-life embedded software and processors.