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feat: add compiler config for not merging const sections

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Probably GCC-13.x and on-wards uses "-fmerge-constants" to merge
the const section (string/floating-point) across compilation units.
This makes it difficult to properly analyze the size output of rodata
section across libraries, the merged section (big in size) is showed
across a single library.

The config option added here can help to disable this compiler behavior
and help to provide better size analysis. It can be used during
development phase only as it increases rodata section size.
This commit is contained in:
Mahavir Jain 2024-07-12 11:50:17 +05:30
parent 1fd11f6e0c
commit 3dc80527ab
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GPG Key ID: 99324EF4A00734E0
4 changed files with 15 additions and 1 deletions
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4
CMakeLists.txt
View File

@ -136,6 +136,10 @@ if(CONFIG_COMPILER_OPTIMIZATION_ASSERTIONS_DISABLE)
list(APPEND compile_definitions "-DNDEBUG")
endif()
if(CONFIG_COMPILER_NO_MERGE_CONSTANTS)
list(APPEND compile_options "-fno-merge-constants")
endif()
if(CONFIG_COMPILER_STACK_CHECK_MODE_NORM)
list(APPEND compile_options "-fstack-protector")
elseif(CONFIG_COMPILER_STACK_CHECK_MODE_STRONG)

9
Kconfig
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@ -514,6 +514,15 @@ mainmenu "Espressif IoT Development Framework Configuration"
help
Stack smashing protection.
config COMPILER_NO_MERGE_CONSTANTS
bool "Disable merging const sections"
depends on IDF_TOOLCHAIN_GCC
help
Disable merging identical constants (string/floating-point) across compilation units.
This helps in better size analysis of the application binary as the rodata section
distribution is more uniform across libraries. On downside, it may increase
the binary size and hence should be used during development phase only.
config COMPILER_WARN_WRITE_STRINGS
bool "Enable -Wwrite-strings warning flag"
default "n"

2
docs/en/api-guides/tools/idf-size.rst
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@ -136,7 +136,7 @@ The table is sorted in descending order of the total contribution of the static
.. note::
The size of the ``.rodata`` section in the ``Flash Data`` memory type may appear very large for a single archive. This occurs due to linker relaxations. The linker may attempt to combine object file sections with ``MERGE`` and ``STRINGS`` flags from all archives into one to perform tail string optimization. Consequently, one archive may end up with a very large ``.rodata`` section, containing string literals from other archives. This is evident in the ``.rodata`` section of the ``libesp_app_format.a`` archive.
The size of the ``.rodata`` section in the ``Flash Data`` memory type may appear very large for a single archive. This occurs due to linker relaxations. The linker may attempt to combine object file sections with ``MERGE`` and ``STRINGS`` flags from all archives into one to perform tail string optimization. Consequently, one archive may end up with a very large ``.rodata`` section, containing string literals from other archives. This is evident in the ``.rodata`` section of the ``libesp_app_format.a`` archive. The specific compiler behavior here can be turned off by enabling :ref:`CONFIG_COMPILER_NO_MERGE_CONSTANTS` option (only for GCC toolchain), please read help for more details.
Source File Usage Summary ``idf.py size-files``

1
tools/test_apps/system/build_test/sdkconfig.ci.no_merge_constants Normal file
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@ -0,0 +1 @@
CONFIG_COMPILER_NO_MERGE_CONSTANTS=y