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分类: 嵌入式

2016-09-23 13:09:04

最近发现在keil编译的时候,出现莫名的问题,貌似代码有被优化掉的问题,后来查了下相关的资料,貌似懂了点。
我选择的是默认的default优化方式,上网看了下,默认的是level2级别优化,最后选择level0就没有问题了
下面是网上找的资料,介绍了优化功能介绍

Getting the Best Optimized Code for your Embedded Application 

ARM Compilation Tools 
The ARM Compilation Tools are the only compilation tool s co -developed with the ARM processors, and specifically 
designed to optimally support the ARM architecture. They are a result of 20 years of development, and are recognized as the 
industry -leading C and C++ compilation tools for the ARM, Thumb, and Thumb -2 instructions sets. 
The ARM Compilation tools consist of:  
?  The ARM Compiler, which enables you to compile C and C++ code. It is an optimizing compiler, and features 
command - line options to enable you to control the level of optimization  
?  Linker and Utilities, which assign addresses and lay out sections of code to form a final image  
?  A selection of libraries, including the ISO standard C libraries, and the MicroLIB C library which is optimized for 
embedded applications 
?  Assembler, which generates machine code instructions from  ARM, Thumb or Thumb-2 assembly- level   source code  
 
Compiler Options for Embedded Applications   
The ARM Compilation Tools include a number of compiler optimizations to help you best target your code for your chosen 
microcontroller device and   application area.   
They can be accessed from within ?Vision by clicking on Project –   Options for Target. 
T he options described this document  can be found on the Target   an d  C/C++   tabs  of   the  Options for Targets  dialog.  
 

MDK Compiler Optimizations   

?  Cross- Module  
Optimization  takes information from a prior build and uses it to place UNUSED functions into their 

own ELF section in the corresponding object file. This option is also known as Linker Feedback, and requires you to 
build your application twice to take adv antage of it for reduced code size.   
Cross-Module Optimization has been shown to reduce code size, by removing unused functions from your application. It 
can also improve the performance   of your application, by allowing modules to share inline code.  
 

?  The  M icroLIB   C library
has been optimized to reduce the size of embedded applications. It is a subset of the ISO 

standard C runtime library, and offers a tradeoff between functionality and code size. Some of the standard C library 
functions such as memcpy()   are  slower, while some features of the default library are not supported. Unsupported 
features include:  
o   Operating system functions e.g. abort(), exit(), time(), system(), getenv(),   
o   Wide character and multi-byte support e.g.  mbtowc(), wctomb() 
o   The  stdio   file  I/O function, with the exception of stdin, stdout   and  stderr  
o   Position-independent and thread -safe code  
Use the MicroLIB C library for applications where overall performance can be traded off against the need to reduce code 
size and memory cost.  
 
?  Link- Time Code Generation  instructs the compiler to create objects in an intermediate format so that the linker can 
perform further code optimizations.  This gives the code generator visibility into cross - file dependencies of all objects 
simultaneously, allowing it t o apply a higher level of   optimizations. Link -time code generation  can reduce  code size, and 
allow your application to run  faster.   
 
?  Optimization Levels  can also be adjusted. The different levels of optimization allow you to trade off between  the level 
of debug information  available in the compiled code,  and  the performance of the code. The following  optimization levels 
are available: 
o   - O0  applies minimum optimizations.  
Most optimizations are switched off, and the code generated has the best debug view.  
o   - O1   applies restricted optimization.  
For example, unused inline functions and unused static functions are removed. At this level of optimization, the 
compiler also applies automatic optimizations such as removing redundant code and re -ordering instructions s o 
as to avoid an interlock situation. The code generated is reasonably optimized, with a good debug view.   
o   - O2  applies high optimization (This is the default setting).    
Optimizations applied at this level take advantage of ARM’s in-depth knowledge of the processor architecture, 
to exploit processor -specific behavio r of the given target. It generates well optimized code, but with limited 
debug view.  
o   - O3  applies th e most aggressive optimization.  
The optimization is in accordance with the user’s   – Ospace/- Otime  choice . By default, multi - file compilation is 
enabled, which leads to a longer compile time, but gives the highest levels of optimization.
 

?  The  Optimize for Time checkbox   causes the compiler to optimize  with a greater focus on achieving the best 
performance   when checked  ( - O time) or the smallest code siz e when unchecked ( -O space).  
Unchecking Optimize for Time   selects the  – Ospace option which  instructs the compiler to perform optimizations to 
reduce the image size at the expense of a poss ible increase i n execution time. F or example, using out -of -line function 
calls instead of inline code for large structure copies. This is the default option.   When running the compiler from the 
command line, this option is invoked using ‘ -Ospace’ 
Checking Optimize for Time   selects the  – Otime option which instructs the compiler to optimize the code for the fastest 
execution time, at the risk of an increase in the image size. It is recommended that you compile the time -critical parts of 
your code with – Otime, and the rest  us ing the – Ospace   directive .  
 
?  Split Load and Store Multiples instructs the compiler to split LDM and STM instructions involving a large number of 
registers into a series of loads/stores of fewer multiple registers. This means that an LDM of 16 registers can  be split into 
4 separate LDMs of 4 registers each. This option helps to reduce the interrupt latency on ARM systems which do not 
have a cache or write buffer, and systems which use zero - wait state 32-bit memory.  
For example, the ARM7 and ARM9  processor s t ake can only take   an exception on an instruction boundary. If an 
exception occurs at the start of an LDM of 16 registers in a cacheless ARM7 /ARM9  system, the system will finish 
making 16 accesses to memory before taking the exception. Depending on the memory arbitration system, this can result 
in a very high interrupt latency. Breaking the LDM into 4 individual LDMs for 4 registers means that the processor will 
take the exception after loading a maximum of 4 registers, thereby greatly reducing the interrupt   latency. 
Selecting this option improves the overall performance of the system.  
 
?  The  One ELF Section per Function  option tells the compiler to put all functions into their own individual ELF 
sections. This allows the linker to remove unused functions.    
An  ELF code section typically contains the code for a number of functions.  The linker is normally only able to remove 
unused ELF sections, not unused functions. An ELF section can only be removed if all its contents are unused. 
Therefore, splitting each function into its own ELF section allows the compiler to easily identify which ones are unused, 
and remove them.   
Selecting this option increases the time required to compile your code, but results in improved performance .  
 
 
The combination of options applied  will depend on your optimization goal – whether you are optimizing for smallest code 
size, or best performance. 
The next section illustrates the best optimization options for each of these goals. 

Optimizing for Smallest Code Size 
To optimize your code for the smallest size, the best options to apply are: 
?  The MicroLIB C library  
?  Cross- module optimization 
?  Optimization level 2 ( -O2) 
 
Compile the Measure example without any optimizations  
The Measure example uses analog and digital inputs to simulate a data l ogger.  
 
File  --   Open Project 
C: \Keil \ARM\Boards \Keil \MCBSTM32\Measure\Measure.uv2 
 
Click the  Options for Target button  
                                            
 
In the Target tab:  
?  Uncheck  Cross- Module Optimization 
?  Uncheck  Use MicroLIB 
?  Uncheck  Use  Link- Time Code Generation  
 
In the C/C++ tab: 
?  Set Optimization Level to Zero 
 
Then click OK  to save your changes.  
 
    Project  –  Build target 
      
Without any compiler optimizations applied, the initial code size is 13,656 Bytes. 
MDK Compiler Optimizations  

Optimize the Measur e example for Size  
Apply the compiler optimizations in turn, and re-compile each time to see their effect in reducing the code size for the 
example.   
?  Options for Target –  Target tab:  Use the MicroLIB C library 
?  Options for Target –  Target tab:  Use cross - mod ule optimization  -  Remember to compile twice 
?  Options for Target –   C/C++ tab:  Enable Optimization level 2 ( -O2) 
 
Optimization Applied  Compile Size  Size Reduction  Improvement  
MicroLIB C library  8,960 Bytes   4,696 Bytes   34% smaller 
Cross- Module Compilation  13,500 Bytes  156 Bytes  1.1% smaller 
Optimization level – O2  12,936 Bytes  720 Bytes  5.3% smaller 
All 3 optimization options  8,116 Bytes   5,540 Bytes   40.6% smaller  
 
Applying all the optimizations will reduce the code size down to 8,116 Bytes. 
 
The fully optimized code is 5,540 Bytes smaller, a total code size reduction of 40.6% 
 
MDK Compiler Optimizations  
 
Optimizing for Best Performance 
To optimize your code for performance, the best options to apply are:  
?  Cross- module optimization 
?  Optimization level 3 ( -O3) 
?  Optimize for time 
 
Run the  Dhrystone benchmark without any optimizations 
The Dhrystone benchmark is used to measure and compare the performance of different computers, or the efficiency of the 
code generated for the same computer by different compilers.  
 
    File  –   Open Project 
      C: \Keil \ARM\Examples \DHRY \DHRY.uv2 
 
    Click  the  Options for Target button  
            Turn off optimization settings in the Target and C/C++ tabs , then click  OK                                 
 
    Project –  Build target 
 
    Enter D ebug mode     
 
    View –   Se rial Windows –   UART #1 
                                       Open the  UART #1  window 
 
    View –   Analysis Windows –   Performance Analyzer  
                                       Open the  Performance Analyzer  
 
    Debug –   Run  
                                         Start running the application 
 
    When prompted: 
                             Enter 50000   in the  UART#1   window and press Enter 
 
In the Performance Analyzer window,  note   that   
?  The drhy_1 loop took 2.829s 
?  The dhry_2 took 2.014s 
In the UAR T #1 window, note  that  
?  It took 138.0 ms for 1 run through Dhrystone 
?  The application is executing 7246.4 Dhrystones per second  
 
 
Optimize the Dhrystone example for Performance
  
Re-compile the example with all three of the following optimizations applied:  
?  Options f or Target  –  Target tab: Cross - module optimization –   Remember to compile twice 
?  Options for Target –   C/C++ tab:  Optimization level 3 ( -O3) 
?  Options for Target –   C/C++ tab:  Optimize for Time 
 
Re-run the application, and examine the performance.   
 
Measurement  Without optimizations   With Optimizations  Improvement  
dhry_1  2.829s   1.695s   40.1% faster 
dhry_2  2.014s   1.011s   49.8% faster 
Microseconds for 1 run 
through Dhrystone 
138.0  70  49.3% faster 
Dhrystones per second  7246.4   14,285.7   97.1% more  
 
The fu lly optimize d code achieves approximate ly 2x  the performance  of the un -optimized code. 
Summary 
The ARM Compilation Tools offer a range of options to apply when compiling your code. These options can be combined to 
optimize your code for best performance, for smallest code size, or for any performance point between these two extremes, to 
best suit your targeted microcontroller device and market.    
When optimizing your code, MDK- ARM makes it easy and convenient to measure the effect of the different optimization 
sett ings on your application. The code size is clearly displayed after compilation, and a range of analysis tools such as the 
Performance Analyzer enable you to measure performance.  
The optimization options in the ARM Compilation Tools, together with the easy- to - use analysis tools in MDK - ARM, help 
you to easily optimize your application to meet your specific requirements.

转载:http://blog.csdn.net/yangtalent1206/article/details/8493120

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