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a034ef8713
Introduce ESPTOOLPY_FAST_REFLASHING configuration option. It instructs
ldgen to group entity mappings for libraries deemed mutable (prone to
change) separately from those considered immutable (unlikely to change).
Organizing mutable and immutable libraries separately in the linker
script allows the linker to form larger contiguous blocks of data for
immutable libraries in the application's output sections. These blocks
are likely to stay mostly unchanged between application recompilations,
enabling them to be skipped during reflashing.
Separating mutable and immutable libraries in the linker script to
minimize changes in the output sections is insufficient. Padding is
added after the input sections of mutable libraries in the default data
and text output sections. This creates a buffer for the mutable
libraries, allowing additional changes to be made without altering the
layout of the binary image.
Additionally two optimizations currently in use can still mix data from
these libraries, leading to significant changes even within the grouped
immutable libraries.
1. constant merging
Linker will try to merge input sections that have the MERGE and
STRING flags from different libraries (object files) to perform
optimizations like tail merging. For example, adding a string
literal in a mutable library will also change the addresses of
string literals from immutable libraries in such a merged section,
causing changes in the generated code when those literals are
referenced.
Disabled with COMPILER_DISABLE_MERGE_CONSTANTS(-fno-merge-constants)
2. literal pools on Xtensa
As optimization, the linker may merge literal pools from different
libraries (object files) to improve the generated code size. This
has the same effect as constant merging, and changes in mutable
libraries may cause changes in the generated code for immutable
libraries. To get larger unchanged continuous blocks in the text
output sections for immutable libraries, we need to ensure that the
Xtensa literal pools remain close to their references and are not
merged.
Disabled with CONFIG_COMPILER_ENABLE_TEXT_SECTION_LITERALS(-mtext-section-literals)
When ESPTOOLPY_FAST_REFLASHING is enabled, these two optimizations are
disabled to achieve larger unchaged continuous blocks for the grouped
immutable libraries, even though disabling these optimizations results
in slightly larger code.
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>