pw_bloat#

Pigweed AI summary: The bloat module in Pigweed provides tools and helpers for using Bloaty McBloatface, including generating size report cards for output binaries through Pigweed's GN build system. These size report cards track the memory usage of a system over time and provide a breakdown of which parts of the code have the largest size impact. The module also includes a CLI command, pw_bloat, which can run size reports on ELF binaries. However, this CLI plugin is still experimental and only supports a small subset

The bloat module provides tools and helpers around using Bloaty McBloatface including generating size report cards for output binaries through Pigweed’s GN build system.

Bloat report cards allow tracking the memory usage of a system over time as code changes are made and provide a breakdown of which parts of the code have the largest size impact.

pw bloat CLI command#

Pigweed AI summary: The "pw bloat" CLI command includes a plugin for running size reports on ELF binaries. However, the bloat CLI plugin is still experimental and only supports a small subset of pw_bloat's capabilities. It currently only runs on binaries that define memory region symbols. The article provides basic usage instructions for running a size report on a single executable and running a size report diff.

pw_bloat includes a plugin for the Pigweed command line capable of running size reports on ELF binaries.

Note

The bloat CLI plugin is still experimental and only supports a small subset of pw_bloat’s capabilities. Notably, it currently only runs on binaries which define memory region symbols; refer to the memoryregions documentation for details.

Basic usage#

Pigweed AI summary: The paragraph describes how to run a size report on a single executable and how to run a size report diff. The code snippets show the commands to be used for each process and the resulting output. The output includes information on memory regions, symbols, and sizes, with differences highlighted in the size report diff.

Running a size report on a single executable

$ pw bloat out/docs/obj/pw_result/size_report/bin/ladder_and_then.elf

▒█████▄   █▓  ▄███▒  ▒█    ▒█ ░▓████▒ ░▓████▒ ▒▓████▄
 ▒█░  █░ ░█▒ ██▒ ▀█▒ ▒█░ █ ▒█  ▒█   ▀  ▒█   ▀  ▒█  ▀█▌
 ▒█▄▄▄█░ ░█▒ █▓░ ▄▄░ ▒█░ █ ▒█  ▒███    ▒███    ░█   █▌
 ▒█▀     ░█░ ▓█   █▓ ░█░ █ ▒█  ▒█   ▄  ▒█   ▄  ░█  ▄█▌
 ▒█      ░█░ ░▓███▀   ▒█▓▀▓█░ ░▓████▒ ░▓████▒ ▒▓████▀

+----------------------+---------+
|     memoryregions    |  sizes  |
+======================+=========+
|FLASH                 |1,048,064|
|RAM                   |  196,608|
|VECTOR_TABLE          |      512|
+======================+=========+
|Total                 |1,245,184|
+----------------------+---------+

Running a size report diff

$ pw bloat out/docs/obj/pw_metric/size_report/bin/one_metric.elf \
      --diff out/docs/obj/pw_metric/size_report/bin/base.elf \
      -d symbols

▒█████▄   █▓  ▄███▒  ▒█    ▒█ ░▓████▒ ░▓████▒ ▒▓████▄
 ▒█░  █░ ░█▒ ██▒ ▀█▒ ▒█░ █ ▒█  ▒█   ▀  ▒█   ▀  ▒█  ▀█▌
 ▒█▄▄▄█░ ░█▒ █▓░ ▄▄░ ▒█░ █ ▒█  ▒███    ▒███    ░█   █▌
 ▒█▀     ░█░ ▓█   █▓ ░█░ █ ▒█  ▒█   ▄  ▒█   ▄  ░█  ▄█▌
 ▒█      ░█░ ░▓███▀   ▒█▓▀▓█░ ░▓████▒ ░▓████▒ ▒▓████▀

+-----------------------------------------------------------------------------------+
|                                                                                   |
+-----------------------------------------------------------------------------------+
| diff|     memoryregions    |                    symbols                    | sizes|
+=====+======================+===============================================+======+
|     |FLASH                 |                                               |    -4|
|     |                      |[section .FLASH.unused_space]                  |  -408|
|     |                      |main                                           |   +60|
|     |                      |__sf_fake_stdout                               |    +4|
|     |                      |pw_boot_PreStaticMemoryInit                    |    -2|
|     |                      |_isatty                                        |    -2|
|  NEW|                      |_GLOBAL__sub_I_group_foo                       |   +84|
|  NEW|                      |pw::metric::Group::~Group()                    |   +34|
|  NEW|                      |pw::intrusive_list_impl::List::insert_after()  |   +32|
|  NEW|                      |pw::metric::Metric::Increment()                |   +32|
|  NEW|                      |__cxa_atexit                                   |   +28|
|  NEW|                      |pw::metric::Metric::Metric()                   |   +28|
|  NEW|                      |pw::metric::Metric::as_int()                   |   +28|
|  NEW|                      |pw::intrusive_list_impl::List::Item::unlist()  |   +20|
|  NEW|                      |pw::metric::Group::Group()                     |   +18|
|  NEW|                      |pw::intrusive_list_impl::List::Item::previous()|   +14|
|  NEW|                      |pw::metric::TypedMetric<>::~TypedMetric()      |   +14|
|  NEW|                      |__aeabi_atexit                                 |   +12|
+-----+----------------------+-----------------------------------------------+------+
|     |RAM                   |                                               |     0|
|     |                      |[section .stack]                               |   -32|
|  NEW|                      |group_foo                                      |   +16|
|  NEW|                      |metric_x                                       |   +12|
|  NEW|                      |[section .static_init_ram]                     |    +4|
+=====+======================+===============================================+======+
|Total|                      |                                               |    -4|
+-----+----------------------+-----------------------------------------------+------+

Defining size reports in GN#

Pigweed AI summary: This document describes how to define size reports in GN using the templates pw_size_diff and pw_size_report. The pw_size_diff template requires at least two executable targets to perform a size diff, and allows for global or individual base targets, source filters, and data sources. The pw_size_report template provides a size report for a single binary and allows for optional data sources and source filters. Size reports are typically included in ReST documentation and can also be printed in the build output. The pw_size_report

Diff Size Reports#

Pigweed AI summary: This section discusses how size reports can be defined using the GN template "pw_size_diff", which requires at least two executable targets to perform a size diff. The base for the size diff can be specified globally or individually per-binary within the "binaries" list. The section also provides a code example and mentions that a sample size report table can be found in the module docs for "pw_checksum".

Size reports can be defined using the GN template pw_size_diff. The template requires at least two executable targets on which to perform a size diff. The base for the size diff can be specified either globally through the top-level base argument, or individually per-binary within the binaries list.

Arguments

  • base: Optional default base target for all listed binaries.

  • source_filter: Optional global regex to filter labels in the diff output.

  • data_sources: Optional global list of datasources from bloaty config file

  • binaries: List of binaries to size diff. Each binary specifies a target, a label for the diff, and optionally a base target, source filter, and data sources that override the global ones (if specified).

import("$dir_pw_bloat/bloat.gni")

executable("empty_base") {
  sources = [ "empty_main.cc" ]
}

executable("hello_world_printf") {
  sources = [ "hello_printf.cc" ]
}

executable("hello_world_iostream") {
  sources = [ "hello_iostream.cc" ]
}

pw_size_diff("my_size_report") {
  base = ":empty_base"
  data_sources = "symbols,segments"
  binaries = [
    {
      target = ":hello_world_printf"
      label = "Hello world using printf"
    },
    {
      target = ":hello_world_iostream"
      label = "Hello world using iostream"
      data_sources = "symbols"
    },
  ]
}

A sample pw_size_diff ReST size report table can be found within module docs. For example, see the Size report section of the pw_checksum module for more detail.

Single Binary Size Reports#

Pigweed AI summary: This section describes how to generate a size report for a single binary using the <literal>pw_size_report</literal> template in GN build. The report includes information on segment names, symbols, and sizes. The section also provides arguments for the template, such as the target binary and optional data sources and source filter. The generated report can be included in ReST documentation and printed in the build output if desired.

Size reports can also be defined using pw_size_report, which provides a size report for a single binary. The template requires a target binary.

Arguments

  • target: Binary target to run size report on.

  • data_sources: Optional list of data sources to organize outputs.

  • source_filter: Optional regex to filter labels in the output.

import("$dir_pw_bloat/bloat.gni")

executable("hello_world_iostream") {
  sources = [ "hello_iostream.cc" ]
}

pw_size_report("hello_world_iostream_size_report") {
  target = ":hello_iostream"
  data_sources = "segments,symbols"
  source_filter = "pw::hello"
}

Sample Single Binary ASCII Table Generated

┌─────────────┬──────────────────────────────────────────────────┬──────┐
│segment_names│                      symbols                     │ sizes│
├═════════════┼══════════════════════════════════════════════════┼══════┤
│FLASH        │                                                  │12,072│
│             │pw::kvs::KeyValueStore::InitializeMetadata()      │   684│
│             │pw::kvs::KeyValueStore::Init()                    │   456│
│             │pw::kvs::internal::EntryCache::Find()             │   444│
│             │pw::kvs::FakeFlashMemory::Write()                 │   240│
│             │pw::kvs::internal::Entry::VerifyChecksumInFlash() │   228│
│             │pw::kvs::KeyValueStore::GarbageCollectSector()    │   220│
│             │pw::kvs::KeyValueStore::RemoveDeletedKeyEntries() │   220│
│             │pw::kvs::KeyValueStore::AppendEntry()             │   204│
│             │pw::kvs::KeyValueStore::Get()                     │   194│
│             │pw::kvs::internal::Entry::Read()                  │   188│
│             │pw::kvs::ChecksumAlgorithm::Finish()              │    26│
│             │pw::kvs::internal::Entry::ReadKey()               │    26│
│             │pw::kvs::internal::Sectors::BaseAddress()         │    24│
│             │pw::kvs::ChecksumAlgorithm::Update()              │    20│
│             │pw::kvs::FlashTestPartition()                     │     8│
│             │pw::kvs::FakeFlashMemory::Disable()               │     6│
│             │pw::kvs::FakeFlashMemory::Enable()                │     6│
│             │pw::kvs::FlashMemory::SelfTest()                  │     6│
│             │pw::kvs::FlashPartition::Init()                   │     6│
│             │pw::kvs::FlashPartition::sector_size_bytes()      │     6│
│             │pw::kvs::FakeFlashMemory::IsEnabled()             │     4│
├─────────────┼──────────────────────────────────────────────────┼──────┤
│RAM          │                                                  │ 1,424│
│             │test_kvs                                          │   992│
│             │pw::kvs::(anonymous namespace)::test_flash        │   384│
│             │pw::kvs::(anonymous namespace)::test_partition    │    24│
│             │pw::kvs::FakeFlashMemory::no_errors_              │    12│
│             │borrowable_kvs                                    │     8│
│             │kvs_entry_count                                   │     4│
├═════════════┼══════════════════════════════════════════════════┼══════┤
│Total        │                                                  │13,496│
└─────────────┴──────────────────────────────────────────────────┴──────┘

Size reports are typically included in ReST documentation, as described in Documentation integration. Size reports may also be printed in the build output if desired. To enable this in the GN build (pigweed/pw_bloat/bloat.gni), set the pw_bloat_SHOW_SIZE_REPORTS build arg to true.

Collecting size report data#

Pigweed AI summary: This section explains how to collect size report data from multiple images into a single JSON file using the pw_size_report_aggregation template. The JSON file contains the sizes of all top-level labels in the binary, and can be used to display per-commit size deltas through CI. The template requires a list of pw_size_report targets and the path to the output JSON file. An example code snippet is provided.

Each pw_size_report target outputs a JSON file containing the sizes of all top-level labels in the binary. (By default, this represents “segments”, i.e. ELF program headers.) If a build produces multiple images, it may be useful to collect all of their sizes into a single file to provide a snapshot of sizes at some point in time — for example, to display per-commit size deltas through CI.

The pw_size_report_aggregation template is provided to collect multiple size reports’ data into a single JSON file.

Arguments

  • deps: List of pw_size_report targets whose data to collect.

  • output: Path to the output JSON file.

import("$dir_pw_bloat/bloat.gni")

pw_size_report_aggregation("image_sizes") {
   deps = [
     ":app_image_size_report",
     ":bootloader_image_size_report",
   ]
   output = "$root_gen_dir/artifacts/image_sizes.json"
}

Documentation integration#

Pigweed AI summary: The documentation integration feature allows for easy addition of bloat reports to documentation files. The pw_size_diff and pw_size_report targets output a file with a tabular report card that can be directly imported into a ReST documentation file using the include directive. The example size reports under //pw_bloat/examples are imported into the file using this method, resulting in a table for each example showing the label, segment, and delta.

Bloat reports are easy to add to documentation files. All pw_size_diff and pw_size_report targets output a file containing a tabular report card. This file can be imported directly into a ReST documentation file using the include directive.

For example, the simple_bloat_loop and simple_bloat_function size reports under //pw_bloat/examples are imported into this file as follows:

Simple bloat loop example
^^^^^^^^^^^^^^^^^^^^^^^^^
.. include:: examples/simple_bloat_loop

Simple bloat function example
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. include:: examples/simple_bloat_function

Resulting in this output:

Simple bloat loop example#

Pigweed AI summary: This is a simple example of a bloat loop, with a table showing the label, segment, and delta. The loop is called "Simple bloat loop" and the segment is "main". The delta is +32. There is also a dropdown menu for "FLASH" with a delta of +32 and a segment of "main".

Label

Segment

Delta

Simple bloat loop
FLASH

+32

main

+32

Simple bloat function example#

Pigweed AI summary: This section provides an example of a simple bloat function, with a table showing the label, segment, and delta values. The function is labeled "Simple bloat function" and has a dropdown menu for the "FLASH" segment with a value of +16.

Label

Segment

Delta

Simple bloat function
FLASH

+16

main

+16

Additional Bloaty data sources#

Pigweed AI summary: The Pigweed Python tool, pw_bloat.bloaty_config, generates bloaty configuration files to help embedded developers understand how much space is left in their ELF files. The tool works by tweaking linker scripts to expose extra information and enable additional data sources. The utilization data source reports used and free space, while the memoryregions data source maps segments to memory regions defined in the linker script. The preprocessor macros PW_BLOAT_MEMORY_REGION and PW_BLOAT_MEMORY_REGION_MAP define symbols for memory regions

Bloaty McBloatface by itself cannot help answer some questions which embedded developers frequently face such as understanding how much space is left. To address this, Pigweed provides Python tooling (pw_bloat.bloaty_config) to generate bloaty configuration files based on the final ELF files through small tweaks in the linker scripts to expose extra information.

See the sections below on how to enable the additional data sections through modifications in your linker script(s).

As an example to generate the helper configuration which enables additional data sources for example.elf if you’ve updated your linker script(s) accordingly, simply run python -m pw_bloaty.bloaty_config example.elf > example.bloaty. The example.bloaty can then be used with bloaty using the -c flag, for example bloaty -c example.bloaty example.elf --domain vm -d memoryregions,utilization which may return something like:

84.2%  1023Ki    FLASH
  94.2%   963Ki    Free space
   5.8%  59.6Ki    Used space
15.8%   192Ki    RAM
 100.0%   192Ki    Used space
 0.0%     512    VECTOR_TABLE
  96.9%     496    Free space
   3.1%      16    Used space
 0.0%       0    Not resident in memory
   NAN%       0    Used space

utilization data source#

Pigweed AI summary: The article discusses the utilization data source for embedded developers using bloaty. To accurately determine the space used and remaining, section sizes must be used to account for section alignment requirements. The utilization data source works with any ELF file, reporting used space for the sum of virtual memory size of all sections. To report free space, linker scripts must include properly aligned sections that span the unused remaining space for the relevant memory region with the unused_space string anywhere in their name. The preprocessor macro PW_BLOAT

The most common question many embedded developers face when using bloaty is how much space you are using and how much space is left. To correctly answer this, section sizes must be used in order to correctly account for section alignment requirements.

The generated utilization data source will work with any ELF file, where Used Space is reported for the sum of virtual memory size of all sections.

In order for Free Space to be reported, your linker scripts must include properly aligned sections which span the unused remaining space for the relevant memory region with the unused_space string anywhere in their name. This typically means creating a trailing section which is pinned to span to the end of the memory region.

For example imagine this partial example GNU LD linker script:

MEMORY
{
  FLASH(rx) : \
    ORIGIN = PW_BOOT_FLASH_BEGIN, \
    LENGTH = PW_BOOT_FLASH_SIZE
  RAM(rwx) : \
    ORIGIN = PW_BOOT_RAM_BEGIN, \
    LENGTH = PW_BOOT_RAM_SIZE
}

SECTIONS
{
  /* Main executable code. */
  .code : ALIGN(4)
  {
    /* Application code. */
    *(.text)
    *(.text*)
    KEEP(*(.init))
    KEEP(*(.fini))

    . = ALIGN(4);
    /* Constants.*/
    *(.rodata)
    *(.rodata*)
  } >FLASH

  /* Explicitly initialized global and static data. (.data)*/
  .static_init_ram : ALIGN(4)
  {
    *(.data)
    *(.data*)
    . = ALIGN(4);
  } >RAM AT> FLASH

  /* Zero initialized global/static data. (.bss) */
  .zero_init_ram (NOLOAD) : ALIGN(4)
  {
    *(.bss)
    *(.bss*)
    *(COMMON)
    . = ALIGN(4);
  } >RAM
}

Could be modified as follows enable Free Space reporting:

MEMORY
{
  FLASH(rx) : ORIGIN = PW_BOOT_FLASH_BEGIN, LENGTH = PW_BOOT_FLASH_SIZE
  RAM(rwx) : ORIGIN = PW_BOOT_RAM_BEGIN, LENGTH = PW_BOOT_RAM_SIZE

  /* Each memory region above has an associated .*.unused_space section that
   * overlays the unused space at the end of the memory segment. These
   * segments are used by pw_bloat.bloaty_config to create the utilization
   * data source for bloaty size reports.
   *
   * These sections MUST be located immediately after the last section that is
   * placed in the respective memory region or lld will issue a warning like:
   *
   *   warning: ignoring memory region assignment for non-allocatable section
   *      '.VECTOR_TABLE.unused_space'
   *
   * If this warning occurs, it's also likely that LLD will have created quite
   * large padded regions in the ELF file due to bad cursor operations. This
   * can cause ELF files to balloon from hundreds of kilobytes to hundreds of
   * megabytes.
   *
   * Attempting to add sections to the memory region AFTER the unused_space
   * section will cause the region to overflow.
   */
}

SECTIONS
{
  /* Main executable code. */
  .code : ALIGN(4)
  {
    /* Application code. */
    *(.text)
    *(.text*)
    KEEP(*(.init))
    KEEP(*(.fini))

    . = ALIGN(4);
    /* Constants.*/
    *(.rodata)
    *(.rodata*)
  } >FLASH

  /* Explicitly initialized global and static data. (.data)*/
  .static_init_ram : ALIGN(4)
  {
    *(.data)
    *(.data*)
    . = ALIGN(4);
  } >RAM AT> FLASH

  /* Defines a section representing the unused space in the FLASH segment.
   * This MUST be the last section assigned to the FLASH region.
   */
  PW_BLOAT_UNUSED_SPACE(FLASH)

  /* Zero initialized global/static data. (.bss). */
  .zero_init_ram (NOLOAD) : ALIGN(4)
  {
    *(.bss)
    *(.bss*)
    *(COMMON)
    . = ALIGN(4);
  } >RAM

  /* Defines a section representing the unused space in the RAM segment. This
   * MUST be the last section assigned to the RAM region.
   */
  PW_BLOAT_UNUSED_SPACE(RAM)
}

The preprocessor macro PW_BLOAT_UNUSED_SPACE is defined in pw_bloat/bloat_macros.ld. To use these macros include this file in your pw_linker_script as follows:

pw_linker_script("my_linker_script") {
  includes = [ "$dir_pw_bloat/bloat_macros.ld" ]
  linker_script = "my_project_linker_script.ld"
}

Note that linker scripts are not natively supported by GN and can’t be provided through deps, the bloat_macros.ld must be passed in the includes list.

memoryregions data source#

Pigweed AI summary: This section discusses the problem of attributing symbols and sections to memory regions defined in a linker script. The ELF format does not include original memory regions, making attribution difficult. Instead of parsing *.map files, the pw_bloat.bloaty_config consumes symbols defined in the linker script with a special format to extract information from the ELF file. These symbols are defined by preprocessor macros PW_BLOAT_MEMORY_REGION and PW_BLOAT_MEMORY_REGION_MAP. To use these macros, include pw_bloat/bloat

Understanding how symbols, sections, and other data sources can be attributed back to the memory regions defined in your linker script is another common problem area. Unfortunately the ELF format does not include the original memory regions, meaning bloaty can not do this today by itself. In addition, it’s relatively common that there are multiple memory regions which alias to the same memory but through different buses which could make attribution difficult.

Instead of taking the less portable and brittle approach to parse *.map files, pw_bloat.bloaty_config consumes symbols which are defined in the linker script with a special format to extract this information from the ELF file: pw_bloat_config_memory_region_NAME_{start,end}{_N,}.

These symbols are defined by the preprocessor macros PW_BLOAT_MEMORY_REGION and PW_BLOAT_MEMORY_REGION_MAP with the right address and size for the regions. To use these macros include the pw_bloat/bloat_macros.ld in your pw_linker_script as follows:

pw_linker_script("my_linker_script") {
  includes = [ "$dir_pw_bloat/bloat_macros.ld" ]
  linker_script = "my_project_linker_script.ld"
}

These symbols are then used to determine how to map segments to these memory regions. Note that segments must be used in order to account for inter-section padding which are not attributed against any sections.

As an example, if you have a single view in the single memory region named FLASH, then you should include the following macro in your linker script to generate the symbols needed for the that region:

PW_BLOAT_MEMORY_REGION(FLASH)

As another example, if you have two aliased memory regions (DCTM and ITCM) into the same effective memory named you’d like to call RAM, then you should produce the following four symbols in your linker script:

PW_BLOAT_MEMORY_REGION_MAP(RAM, ITCM)
PW_BLOAT_MEMORY_REGION_MAP(RAM, DTCM)