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Remarks

Introduction to the LLVM remark diagnostics

LLVM is able to emit diagnostics from passes describing whether an optimization has been performed or missed for a particular reason, which should give more insight to users about what the compiler did during the compilation pipeline.

There are three main remark types:

Passed

Remarks that describe a successful optimization performed by the compiler.

Example:
foo inlined into bar with (cost=always): always inline attribute

Missed

Remarks that describe an attempt to an optimization by the compiler that could not be performed.

Example:
foo not inlined into bar because it should never be inlined
(cost=never): noinline function attribute

Analysis

Remarks that describe the result of an analysis, that can bring more information to the user regarding the generated code.

Example:
16 stack bytes in function
10 instructions in function

Enabling optimization remarks

There are two modes that are supported for enabling optimization remarks in LLVM: through remark diagnostics, or through serialized remarks.

Remark diagnostics

Optimization remarks can be emitted as diagnostics. These diagnostics will be propagated to front-ends if desired, or emitted by tools like :doc:`llc <CommandGuide/llc>` or :doc:`opt <CommandGuide/opt>`.

Serialized remarks

While diagnostics are useful during development, it is often more useful to refer to optimization remarks post-compilation, typically during performance analysis.

For that, LLVM can serialize the remarks produced for each compilation unit to a file that can be consumed later.

By default, the format of the serialized remarks is :ref:`YAML <yamlremarks>`, and it can be accompanied by a :ref:`section <remarkssection>` in the object files to easily retrieve it.

:doc:`llc <CommandGuide/llc>` and :doc:`opt <CommandGuide/opt>` support the following options:

Basic options

Content configuration

Other tools that support remarks:

:program:`llvm-lto`

:program:`gold-plugin` and :program:`lld`

Serialization modes

There are two modes available for serializing remarks:

Separate

In this mode, the remarks and the metadata are serialized separately. The client is responsible for parsing the metadata first, then use the metadata to correctly parse the remarks.

Standalone

In this mode, the remarks and the metadata are serialized to the same stream. The metadata will always come before the remarks.

The compiler does not support emitting standalone remarks. This mode is more suited for post-processing tools like linkers, that can merge the remarks for one whole project.

YAML remarks

A typical remark serialized to YAML looks like this:

--- !<TYPE>
Pass: <pass>
Name: <name>
DebugLoc: { File: <file>, Line: <line>, Column: <column> }
Function: <function>
Hotness: <hotness>
Args:
  - <key>: <value>
    DebugLoc: { File: <arg-file>, Line: <arg-line>, Column: <arg-column> }

The following entries are mandatory:

  • <TYPE>: can be Passed, Missed, Analysis, AnalysisFPCommute, AnalysisAliasing, Failure.
  • <pass>: the name of the pass that emitted this remark.
  • <name>: the name of the remark coming from <pass>.
  • <function>: the mangled name of the function.

If a DebugLoc entry is specified, the following fields are required:

  • <file>
  • <line>
  • <column>

If an arg entry is specified, the following fields are required:

  • <key>
  • <value>

If a DebugLoc entry is specified within an arg entry, the following fields are required:

  • <arg-file>
  • <arg-line>
  • <arg-column>

YAML with a string table

The YAML serialization supports the usage of a string table by using the yaml-strtab format.

This format replaces strings in the YAML output with integers representing the index in the string table that can be provided separately through metadata.

The following entries can take advantage of the string table while respecting YAML rules:

  • <pass>
  • <name>
  • <function>
  • <file>
  • <value>
  • <arg-file>

Currently, none of the tools in :ref:`the opt-viewer directory <optviewer>` support this format.

YAML metadata

The metadata used together with the YAML format is:

  • a magic number: "REMARKS\0"
  • the version number: a little-endian uint64_t
  • the total size of the string table (the size itself excluded): little-endian uint64_t
  • a list of null-terminated strings

Optional:

  • the absolute file path to the serialized remark diagnostics: a null-terminated string.

When the metadata is serialized separately from the remarks, the file path should be present and point to the file where the remarks are serialized to.

In case the metadata only acts as a header to the remarks, the file path can be omitted.

LLVM bitstream remarks

This format is using :doc:`LLVM bitstream <BitCodeFormat>` to serialize remarks and their associated metadata.

A bitstream remark stream can be identified by the magic number "RMRK" that is placed at the very beginning.

The format for serializing remarks is composed of two different block types:

META_BLOCK

The block providing information about the rest of the content in the stream.

Exactly one block is expected. Having multiple metadata blocks is an error.

This block can contain the following records:

RECORD_META_CONTAINER_INFO

The container version and type.

Version: u32

Type: u2

RECORD_META_REMARK_VERSION

The version of the remark entries. This can change independently from the container version.

Version: u32

RECORD_META_STRTAB

The string table used by the remark entries. The format of the string table is a sequence of strings separated by \0.

RECORD_META_EXTERNAL_FILE

The external remark file path that contains the remark blocks associated with this metadata. This is an absolute path.

REMARK_BLOCK

The block describing a remark entry.

0 or more blocks per file are allowed. Each block will depend on the :ref:`META_BLOCK <bitstreamremarksmetablock>` in order to be parsed correctly.

This block can contain the following records:

RECORD_REMARK_HEADER

The header of the remark. This contains all the mandatory information about a remark.

Type u3
Remark name VBR6 (string table index)
Pass name VBR6 (string table index)
Function name VBR6 (string table index)

RECORD_REMARK_DEBUG_LOC

The source location for the corresponding remark. This record is optional.

File VBR7 (string table index)
Line u32
Column u32

RECORD_REMARK_HOTNESS

The hotness of the remark. This record is optional.

Hotness | VBR8 (string table index)

RECORD_REMARK_ARG_WITH_DEBUGLOC

A remark argument with an associated debug location.

Key VBR7 (string table index)
Value VBR7 (string table index)
File VBR7 (string table index)
Line u32
Column u32

RECORD_REMARK_ARG_WITHOUT_DEBUGLOC

A remark argument with an associated debug location.

Key VBR7 (string table index)
Value VBR7 (string table index)

The remark container

Bitstream remarks are designed to be used in two different modes:

The separate mode

The separate mode is the mode that is typically used during compilation. It provides a way to serialize the remark entries to a stream while some metadata is kept in memory to be emitted in the product of the compilation (typically, an object file).

The standalone mode

The standalone mode is typically stored and used after the distribution of a program. It contains all the information that allows the parsing of all the remarks without having any external dependencies.

In order to support multiple modes, the format introduces the concept of a bitstream remark container type.

SeparateRemarksMeta: the metadata emitted separately

This container type expects only a :ref:`META_BLOCK <bitstreamremarksmetablock>` containing only:

Typically, this is emitted in a section in the object files, allowing clients to retrieve remarks and their associated metadata directly from intermediate products.

SeparateRemarksFile: the remark entries emitted separately

This container type expects only a :ref:`META_BLOCK <bitstreamremarksmetablock>` containing only:

This container type expects 0 or more :ref:`REMARK_BLOCK <bitstreamremarksremarkblock>`.

Typically, this is emitted in a side-file alongside an object file, and is made to be able to stream to without increasing the memory consumption of the compiler. This is referenced by the :ref:`RECORD_META_EXTERNAL_FILE <bitstreamremarksrecordmetaexternalfile>` entry in the :ref:`SeparateRemarksMeta <bitstreamremarksseparateremarksmeta>` container.

When the parser tries to parse a container that contains the metadata for the separate remarks, it should parse the version and type, then keep the string table in memory while opening the external file, validating its metadata and parsing the remark entries.

The container versions from the separate container should match in order to have a well-formed file.

Standalone: the metadata and the remark entries emitted together

This container type expects only a :ref:`META_BLOCK <bitstreamremarksmetablock>` containing only:

This container type expects 0 or more :ref:`REMARK_BLOCK <bitstreamremarksremarkblock>`.

A complete output of :program:`llvm-bcanalyzer` on the different container types:

SeparateRemarksMeta

SeparateRemarksFile

Standalone

opt-viewer

The opt-viewer directory contains a collection of tools that visualize and summarize serialized remarks.

The tools only support the yaml format.

opt-viewer.py

Output a HTML page which gives visual feedback on compiler interactions with your program.

Examples:
$ opt-viewer.py my_yaml_file.opt.yaml
$ opt-viewer.py my_build_dir/

opt-stats.py

Output statistics about the optimization remarks in the input set.

Example:
$ opt-stats.py my_yaml_file.opt.yaml

Total number of remarks           3


Top 10 remarks by pass:
  inline                         33%
  asm-printer                    33%
  prologepilog                   33%

Top 10 remarks:
  asm-printer/InstructionCount   33%
  inline/NoDefinition            33%
  prologepilog/StackSize         33%

opt-diff.py

Produce a new YAML file which contains all of the changes in optimizations between two YAML files.

Typically, this tool should be used to do diffs between:

  • new compiler + fixed source vs old compiler + fixed source
  • fixed compiler + new source vs fixed compiler + old source

This diff file can be displayed using :ref:`opt-viewer.py <optviewerpy>`.

Example:
$ opt-diff.py my_opt_yaml1.opt.yaml my_opt_yaml2.opt.yaml -o my_opt_diff.opt.yaml
$ opt-viewer.py my_opt_diff.opt.yaml

Emitting remark diagnostics in the object file

A section containing metadata on remark diagnostics will be emitted when -remarks-section is passed. The section contains the metadata associated to the format used to serialize the remarks.

The section is named:

  • __LLVM,__remarks (MachO)
  • .remarks (ELF)

C API

LLVM provides a library that can be used to parse remarks through a shared library named libRemarks.

The typical usage through the C API is like the following:

LLVMRemarkParserRef Parser = LLVMRemarkParserCreateYAML(Buf, Size);
LLVMRemarkEntryRef Remark = NULL;
while ((Remark = LLVMRemarkParserGetNext(Parser))) {
   // use Remark
   LLVMRemarkEntryDispose(Remark); // Release memory.
}
bool HasError = LLVMRemarkParserHasError(Parser);
LLVMRemarkParserDispose(Parser);