108 lines
3.4 KiB
Markdown
108 lines
3.4 KiB
Markdown
# Link Time Optimization(LTO)
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Link time optimization(LTO) improves the optimization effect of GCC, such as reducing binary size, increasing performance, and so on. For more details please refer to related [GCC documents](https://gcc.gnu.org/onlinedocs/gccint/LTO.html).
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## Use
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To use this feature, you need to include the required CMake file in your project's CMakeLists.txt after `project(XXXX)`.
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```cmake
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include($ENV{IDF_PATH}/tools/cmake/project.cmake)
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project(XXXX)
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include(gcc)
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```
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The LTO feature is disabled by default. To use it, you should enable the option `CU_GCC_LTO_ENABLE` in menuconfig. Then specify target components or dependencies to be optimized by LTO after `include(gcc)` as follows:
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```cmake
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include(gcc)
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cu_gcc_lto_set(COMPONENTS component_a component_b
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DEPENDS dependence_a dependence_b)
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cu_gcc_string_1byte_align(COMPONENTS component_c component_d
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DEPENDS dependence_c dependence_d)
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```
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Based on your requirement, set compiling optimization level in the option `COMPILER_OPTIMIZATION`.
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* Note
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```
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1. Reducing firmware size may decrease performance
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2. Increasing performance may increase firmware size
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3. Enable LTO cause compiling time cost increases a lot
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4. Enable LTO may increase task stack cost
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5. Enable string 1-byte align may decrease string process speed
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```
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## Limitation
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At the linking stage, the LTO generates new function indexes instead of the file path as follows:
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- LTO
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```txt
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.text 0x00000000420016f4 0x6 /tmp/ccdjwYMH.ltrans51.ltrans.o
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0x00000000420016f4 app_main
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```
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- Without LTO
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```txt
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.text.app_main 0x00000000420016f4 0x6 esp-idf/main/libmain.a(app_main.c.obj)
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0x00000000420016f4 app_main
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```
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So tools used to relink functions between flash and IRAM can't affect these optimized components and dependencies again. It is recommended that users had better optimize application components and dependencies than kernel and hardware driver ones.
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## Example
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The example applies LTO in `light` of `esp-matter` because its application code is much larger. Add LTO configuration into project script `CMakeLists.txt` as follows:
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```cmake
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project(light)
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include(gcc)
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# Add
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set(app_lto_components main chip esp_matter)
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# Add
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set(idf_lto_components lwip wpa_supplicant nvs_flash)
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# Add
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set(lto_depends mbedcrypto)
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# Add
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cu_gcc_lto_set(COMPONENTS ${app_lto_components} ${idf_lto_components}
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DEPENDS ${lto_depends})
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```
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Configure `ESP32-C2` as the target platform, enable `CU_GCC_LTO_ENABLE` and `CONFIG_COMPILER_OPTIMIZATION_ASSERTIONS_DISABLE`, set `COMPILER_OPTIMIZATION` to be `-Os`.
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Increase the `main` task stack size to `5120` by option `ESP_MAIN_TASK_STACK_SIZE`.
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Compile the project, and then you can see the firmware size decrease a lot:
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Option | Firmware size | Stask cost
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-Os | 1,113,376 | 2508
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-Os + LTO | 1,020,640 | 4204
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Then add `cu_gcc_string_1byte_align` after `cu_gcc_lto_set`:
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```cmake
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# Add
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cu_gcc_lto_set(COMPONENTS ${app_lto_components} ${idf_lto_components}
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DEPENDS ${lto_depends})
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cu_gcc_string_1byte_align(COMPONENTS ${app_lto_components} ${idf_lto_components}
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DEPENDS ${lto_depends})
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```
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Build the project and the firmware size is:
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Option | Firmware size |
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-Os + LTO | 1,020,640 |
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-Os + LTO + string 1-byte align | 1,018,340 |
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