AN720 データシートの表示（PDF） - Silicon Laboratories

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AN720

PRECISION32 OPTIMIZATIONCONSIDERATIONS FOR CODESIZE ANDSPEED

Silicon Laboratories 

AN720

5.6. Unused Code Removal

Each file in a project becomes an object that is included. In other words, if any functions in a file are used, then the

entire file is included by default. This can become an issue for a project using the si32HAL and only a few functions

from each module.

Removed (unused) functions can be viewed in the map files for the projects.

For Precision32, the -ffunction-sections and -fdata-sections optimization flags place each function and data item

into separate sections in the file before linking them into the project. This means the compiler can optimize out any

unused functions. These flags are present in Example and AppBuilder projects by default and should be configured

on a file-by-file basis. To add or remove these options to a file:

1. Right-click on the file_name in the Project Explorer view.

2. Select Properties.

3. In the C/C++ BuildSettingsTool Settings tabMCU C CompilerMiscellaneous options, add or

remove the -ffunction-sections and -fdata-sections flags after the -fno-builtin flag to the Other flags

text box.

Figure 7. Modifying the Remove Unused Code Compiler Flags in the Precision32 IDE

These flags must be compiled with the --gc-sections linker command, which is enabled by default in the

Precision32 IDE. It is recommended that this linker command always remain enabled. These flags only have a

benefit in some cases, and may cause larger code size and slower execution in some cases.

Using the sim3u1xx_Blinky and demo_si32UsbAudio default examples in the si32HAL 1.0.1 software package,

Table 10 and Table 11 show the relative Debug build sizes with different unused code removal settings. For no

unused code removal, the projects were compiled without -ffunction-sections and-fdata-sections and with --gc-

sections. For the examples with unused code removal, the projects were compiled with -ffunction-sections, -

fdata-sections, and --gc-sections. Table 12 shows the CoreMark build sizes, and Table 13 shows the CoreMark

scores for the different unused code removal settings.

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