Linker Script Generation

Overview

There are several memory regions where code and data can be placed. Usually, code and read-only data are placed in flash regions, writable data in RAM, etc. A common action is changing where code/data are mapped by default, say placing critical code/rodata in RAM for performance reasons or placing code/data/rodata in RTC memory for use in a wake stub or the ULP coprocessor.

IDF provides the ability for defining these placements at the component level using the linker script generation mechanism. The component presents how it would like to map the input sections of its object files (or even functions/data) through linker fragment files. During app build, the linker fragment files are collected, parsed and processed; and the linker script template is augmented with information generated from the fragment files to produce the final linker script. This linker script is then used for the linking the final app binary.

Quick Start

This section presents a guide for quickly placing code/data to RAM and RTC memory; as well as demonstrating how to make these placements dependent on project configuration values. In a true quick start fashion, this section glosses over terms and concepts that will be discussed at a later part of the document. However, whenever it does so, it provides a link to the relevant section on the first mention.

Preparation

Make

Create a linker fragment file inside the component directory, which is just a text file with a .lf extension. In order for the build system to collect your fragment file, add an entry to it from the component, set the variable COMPONENT_ADD_LDFRAGMENTS to your linker file/s before the register_component call.

# file paths relative to component Makefile
COMPONENT_ADD_LDFRAGMENTS += "path/to/linker_fragment_file.lf" "path/to/another_linker_fragment_file.lf"

CMake

For CMake set the variable COMPONENT_ADD_LDFRAGMENTS to your linker file/s before the register_component call.

# file paths relative to CMakeLists.txt
set(COMPONENT_ADD_LDFRAGMENTS "path/to/linker_fragment_file.lf" "path/to/another_linker_fragment_file.lf")

register_component()

Specifying placements

This mechanism allows specifying placement of the following entities:

  • one or multiple object files within the component
  • one or multiple function/variable using their names
  • the entire component library

For the following text, suppose we have the following:

  • a component named component that is archived as library libcomponent.a during build
  • three object files archived under the library, object1.o, object2.o and object3.o
  • under object1.o, the function function1 is defined; under object2.o, the function function2 is defined
  • there exists configuration PERFORMANCE_MODE and PERFORMANCE_LEVEL in one of the IDF KConfig files, with the set value indicated by entries CONFIG_PERFORMANCE_MODE and CONFIG_PERFORMANCE_LEVEL in the project sdkconfig

In the created linker fragment file, we write:

[mapping]
archive: libcomponent.a
entries:

This creates an empty mapping fragment, which doesn’t do anything yet. During linking the default placements will still be used for libcomponent.a, unless the entries key is populated.

Placing object files

Suppose the entirety of object1.o is performance-critical, so it is desirable to place it in RAM. On the other hand, all of object2.o contains things to be executed coming out of deep sleep, so it needs to be put under RTC memory. We can write:

[mapping]
archive: libcomponent.a
entries:
    object1 (noflash)     # places all code / read-only data under IRAM/ DRAM
    object2 (rtc)         # places all code/ data and read-only data under RTC fast memory/ RTC slow memory

What happens to object3.o? Since it is not specified, default placements are used for object3.o.

Placing functions/data using their names

Continuing our example, suppose that among functions defined under object1.o, only function1 is performance-critical; and under object2.o, only function2 needs to execute after the chip comes out of deep sleep. This could be accomplished by writing:

[mapping]
archive: libcomponent.a
entries:
    object1:function1 (noflash)
    object2:function2 (rtc)

The default placements are used for the rest of the functions in object1.o and object2.o and the entire object3.o. Something similar can be achieved for placing data by writing the variable name instead of the function name after :.

Warning

There are limitations in placing code/data using their symbol names. In order to ensure proper placements, an alternative would be to group relevant code and data into source files, and use object file placement.

Placing entire component

In this example, suppose that the entire component needs to be placed in RAM. This can be written as:

[mapping]
archive: libcomponent.a
entries:
    * (noflash)

Similarly, this places the entire component in RTC memory:

[mapping]
archive: libcomponent.a
entries:
    * (rtc)

Configuration-dependent placements

Suppose that the entire component library should only be placed when CONFIG_PERFORMANCE_MODE == y in the sdkconfig. This could be written as:

[mapping]
archive: libcomponent.a
entries:
    : PERFORMANCE_MODE = y
    * (noflash)

In pseudocode, this translates to:

if PERFORMANCE_MODE = y
    place entire libcomponent.a in RAM
else
    use default placements

It is also possible to have multiple conditions to test. Suppose the following requirements: when CONFIG_PERFORMANCE_LEVEL == 1, only object1.o is put in RAM; when CONFIG_PERFORMANCE_LEVEL == 2, object1.o and object2.o; and when CONFIG_PERFORMANCE_LEVEL == 3 all object files under the archive are to be put into RAM. When these three are false however, put entire library in RTC memory. This scenario is a bit contrived, but, it can be written as:

[mapping]
archive: libcomponent.a
entries:
    : PERFORMANCE_LEVEL = 3
    * (noflash)
    : PERFORMANCE_LEVEL = 2
    object1 (noflash)
    object2 (noflash)
    : PERFORMANCE_LEVEL = 1
    object1 (noflash)
    : default
    * (rtc)

Which reads:

if CONFIG_PERFORMANCE_LEVEL == 3
    place entire libcomponent.a in RAM
else if CONFIG_PERFORMANCE_LEVEL == 2
    only place object1.o and object2.o in RAM
else if CONFIG_PERFORMANCE_LEVEL == 1
    only place object1.o in RAM
else
    place entire libcomponent.a in RTC memory

The conditions test support other operations.

The ‘default’ placements

Up until this point, the term ‘default placements’ has been mentioned as fallback placements for when the placement rules rtc and noflash are not specified. The tokens noflash or rtc are not merely keywords known by the mechanism, but are actually objects called scheme fragments that are specified by the user. Due to the commonness of these placement use cases, they are pre-defined in IDF.

Similarly, there exists a default scheme fragment which defines what the default placement rules should be, which is discussed here.

Note

For an example of an IDF component using this feature, see freertos/CMakeLists.txt. The freertos component uses this mechanism to place all code, literal and rodata of all of its object files to the instruction RAM memory region for performance reasons.

This marks the end of the quick start guide. The following text discusses this mechanism in a little bit more detail, such its components, essential concepts, the syntax, how it is integrated with the build system, etc. The following sections should be helpful in creating custom mappings or modifying default behavior.

Components

Linker Fragment Files

The fragment files contain objects called ‘fragments’. These fragments contain pieces of information which, when put together, form placement rules that tell where to place sections of object files in the output binary.

Another way of putting it is that processing linker fragment files aims to create the section placement rules inside GNU LD SECTIONS command. Where to collect and put these section placement rules is represented internally as a target token.

The three types of fragments are discussed below.

Note

Fragments have a name property (except mapping fragments) and are known globally. Fragment naming follows C variable naming rules, i.e. case sensitive, must begin with a letter or underscore, alphanumeric/underscore after initial characters are allowed, no spaces/special characters. Each type of fragment has its own namespace. In cases where multiple fragments of the same type and name are encountered, an exception is thrown.

I. Sections

Sections fragments defines a list of object file sections that the GCC compiler emits. It may be a default section (e.g. .text, .data) or it may be user defined section through the __attribute__ keyword.

The use of an optional ‘+’ indicates the inclusion of the section in the list, as well as sections that start with it. This is the preferred method over listing both explicitly.

Syntax

[sections:name]
entries:
    .section+
    .section
    ...

Example

# Non-preferred
[sections:text]
entries:
    .text
    .text.*
    .literal
    .literal.*

# Preferred, equivalent to the one above
[sections:text]
entries:
    .text+              # means .text and .text.*
    .literal+           # means .literal and .literal.*

II. Scheme

Scheme fragments define what target a sections fragment is assigned to.

Syntax

[scheme:name]
entries:
    sections -> target
    sections -> target
    ...

Example

[scheme:noflash]
entries:
    text -> iram0_text          # the entries under the sections fragment named text will go to iram0_text
    rodata -> dram0_data        # the entries under the sections fragment named rodata will go to dram0_data

The default scheme

There exists a special scheme with the name default. This scheme is special because catch-all placement rules are generated from its entries. This means that, if one of its entries is text -> flash_text, the placement rule

*(.literal .literal.* .text .text.*)

will be generated for the target flash_text.

These catch-all rules then effectively serve as fallback rules for those whose mappings were not specified.

Note

The default scheme is defined in esp32/ld/esp32_fragments.lf. The noflash and rtc scheme fragments which are built-in schemes referenced in the quick start guide are also defined in this file.

III. Mapping

Mapping fragments define what scheme fragment to use for mappable entities, i.e. object files, function names, variable names. There are two types of entries for this fragment: mapping entries and condition entries.

Note

Mapping fragments have no explicit name property. Internally, the name is constructed from the value of the archive entry.

Syntax

[mapping]
archive: archive                # output archive file name, as built (i.e. libxxx.a)
entries:
    : condition                 # condition entry, non-default
    object:symbol (scheme)      # mapping entry, Type I
    object (scheme)             # mapping entry, Type II
    * (scheme)                  # mapping entry, Type III

    # optional separation/comments, for readability

    : default                   # condition entry, default
    * (scheme)                  # mapping entry, Type III

Mapping Entries

There are three types of mapping entries:

Type I
The object file name and symbol name are specified. The symbol name can be a function name or a variable name.
Type II
Only the object file name is specified.
Type III
* is specified, which is a short-hand for all the object files under the archive.

To know what a mapping entry means, let us expand a Type II entry. Originally:

object (scheme)

Then expanding the scheme fragment from its entries definitions, we have:

object (sections -> target,
        sections -> target,
        ...)

Expanding the sections fragment with its entries definition:

object (.section,      # given this object file
        .section,      # put its sections listed here at this
        ... -> target, # target

        .section,
        .section,      # same should be done for these sections
        ... -> target,

        ...)           # and so on

On Type I Mapping Entries

Type I mapping entry is possible due to compiler flags -ffunction-sections and -ffdata-sections. If the user opts to remove these flags, then the Type I mapping will not work. Furthermore, even if the user does not opt to compile without these flags, there are still limitations as the implementation is dependent on the emitted output sections.

For example, with -ffunction-sections, separate sections are emitted for each function; with section names predictably constructed i.e. .text.{func_name} and .literal.{func_name}. This is not the case for string literals within the function, as they go to pooled or generated section names.

With -fdata-sections, for global scope data the compiler predictably emits either .data.{var_name}, .rodata.{var_name} or .bss.{var_name}; and so Type I mapping entry works for these. However, this is not the case for static data declared in function scope, as the generated section name is a result of mangling the variable name with some other information.

Condition Entries

Condition entries enable the linker script generation to be configuration-aware. Depending on whether expressions involving configuration values are true or not, a particular set of mapping entries can be used. The evaluation uses eval_string from :idf_file:`tools/kconfig_new/kconfiglib.py` and adheres to its required syntax and limitations.

All mapping entries defined after a condition entry until the next one or the end of the mapping fragment belongs to that condition entry. During processing conditions are tested sequentially, and the mapping entries under the first condition that evaluates to TRUE are used.

A default condition can be defined (though every mapping contains an implicit, empty one), whose mapping entries get used in the event no conditions evaluates to TRUE.

Example

[scheme:noflash]
entries:
    text -> iram0_text
    rodata -> dram0_data

[mapping:lwip]
archive: liblwip.a
entries:
    : LWIP_IRAM_OPTIMIZATION = y         # if CONFIG_LWIP_IRAM_OPTIMIZATION is set to 'y' in sdkconfig
    ip4:ip4_route_src_hook (noflash)     # map ip4.o:ip4_route_src_hook, ip4.o:ip4_route_src and
    ip4:ip4_route_src (noflash)          # ip4.o:ip4_route using the noflash scheme, which puts
    ip4:ip4_route (noflash)              # them in RAM

    : default                            # else no special mapping rules apply

Linker Script Template

The linker script template is the skeleton in which the generated placement rules are put into. It is an otherwise ordinary linker script, with a specific marker syntax that indicates where the generated placement rules are placed.

Syntax

To reference the placement rules collected under a target token, the following syntax is used:

mapping[target]

Example

The example below is an excerpt from a possible linker script template. It defines an output section .iram0.text, and inside is a marker referencing the target iram0_text.

.iram0.text :
{
    /* Code marked as runnning out of IRAM */
    _iram_text_start = ABSOLUTE(.);

    /* Marker referencing iram0_text */
    mapping[iram0_text]

    INCLUDE esp32.spiram.rom-functions-iram.ld
    _iram_text_end = ABSOLUTE(.);
} > iram0_0_seg

Suppose the generator collected the fragment definitions below:

[sections:text]
    .text+
    .literal+

[sections:iram]
    .iram1+

[scheme:default]
entries:
    text -> flash_text
    iram -> iram0_text

[scheme:noflash]
entries:
    text -> iram0_text

[mapping:freertos]
archive: libfreertos.a
entries:
    * (noflash)

Then the corresponding excerpt from the generated linker script will be as follows:

.iram0.text :
{
    /* Code marked as runnning out of IRAM */
    _iram_text_start = ABSOLUTE(.);

    /* Placement rules generated from the processed fragments, placed where the marker was in the template */
    *(.iram1 .iram1.*)
    *libfreertos.a:(.literal .text .literal.* .text.*)

    INCLUDE esp32.spiram.rom-functions-iram.ld
    _iram_text_end = ABSOLUTE(.);
} > iram0_0_seg

*libfreertos.a:(.literal .text .literal.* .text.*)

Rule generated from the entry * (noflash) of the freertos mapping fragment. All text sections of all object files under the archive libfreertos.a will be collected under the target iram0_text (as per the noflash scheme) and placed wherever in the template iram0_text is referenced by a marker.

*(.iram1 .iram1.*)

Rule generated from the default scheme entry iram -> iram0_text. Since the default scheme specifies an iram -> iram0_text entry, it too is placed wherever iram0_text is referenced by a marker. Since it is a rule generated from the default scheme, it comes first among all other rules collected under the same target name.

Integration with Build System

The linker script generation occurs during application build, before the final output binary is linked. The tool that implements the mechanism lives under $(IDF_PATH)/tools/ldgen.

Linker Script Template

Currently, the linker script template used is esp32/ld/esp32.common.ld.in, and is used only for the app build. The generated output script is put under the build directory of the same component. Modifying this linker script template triggers a re-link of the app binary.

Linker Fragment File

Any component can add a fragment file to the build. In order to add a fragment file to process, use the command ldgen_add_fragment_file as mentioned here. Modifying any fragment file presented to the build system triggers a re-link of the app binary.