[doc] Add llvm-ifs commandline guide

[haowei]

Summary

This patch adds llvm-ifs commandline guide

[phosek]

llvm/docs/CommandGuide/llvm-ifs.rst

18–21

Consider this.

28

47

I'd format these as code.

73–76

88

These should be formatted as code.

107

I'm not sure if it's guaranteed, but we've confirmed it to be the case.

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This should be formatted as code, the same for other fields below.

[phosek]

LGTM. Can you also file an issue to have this change backported to 14.0.0 release branch?

llvm/docs/CommandGuide/llvm-ifs.rst

92–100

Can you format these as code as well?

[haowei]

Filed https://github.com/llvm/llvm-project/issues/54122

[haowei]

Patch was already landed. Not sure why it is not auto closed. Close it now.

llvm/docs/CommandGuide/index.rst

.. toctree::
    FileCheck
    tblgen
    clang-tblgen
    lldb-tblgen
    llvm-tblgen
    mlir-tblgen
    lit
    llvm-exegesis
+   llvm-ifs
    llvm-locstats
    llvm-pdbutil
    llvm-profgen
    llvm-tli-checker

llvm/docs/CommandGuide/llvm-ifs.rst

+llvm-ifs - shared object stubbing tool
+======================================
+
+.. program:: llvm-ifs
+
+SYNOPSIS
+--------
+
+:program:`llvm-ifs` [*options*] *inputs*
+
+DESCRIPTION
+-----------
+
+:program:`llvm-ifs` is a tool that jointly produces human readable text-based
+stubs (.ifs files) for shared objects and linkable shared object stubs
+(.so files) from either ELF shared objects or text-based stubs. The text-based
+stubs is useful for monitoring ABI changes of the shared object. The linkable
+shared object stubs can be used to avoid unnecessary relinks when the ABI of
+shared libraries does not change.
+
+
+IFS FORMATS
+-----------
+
+Here is an example of the text representation (IFS) of a shared object produced
+by the :program:`llvm-ifs`:
+
+::
+
+  --- !ifs-v1
+  IFSVersion: 3.0
+  SoName: libtest.so /* Optional */
+  Target: x86_64-unknown-linux-gnu   /* Optional, format 1, same format as llvm target triple */
+  Target: { Arch: x86_64, Endianness: little, Bitwidth: 64 } /* Optional, format 2 */
+  NeededLibs:
+    - libc.so.6
+  Symbols:
+    - { Name: sym0, Type: Notype }
+    - { Name: sym1, Type: Object, Size: 0 }
+    - { Name: sym2, Type: Func, Weak: false }
+    - { Name: sym3, Type: TLS }
+    - { Name: sym4, Type: Unknown, Warning: foo }
+  ...
+
+* ``IFSVersion``: Version of the IFS file for reader compatibility.
+
+* ``SoName`` (optional): Name of the shared object file that is being stubbed.
+
+* ``Target`` (optional): The architecture, endianness and bitwise information of
+  this shared object. It can be either in explicit format or in implicit LLVM
+  triple format. It can be optional and can be overridden from command line
+  options.
+
+* ``NeededLibs``: The list of the external shared objects that this library depends on.
+
+* ``Symbols``: A collection of all data needed to link objects for each symbol, sorted by name in ascending order.
+
+  + ``Name``: Symbol name.
+
+  + ``Type``: Whether the symbol is an object, function, no-type, thread local storage, or unknown. Symbol types not explicitly supported are mapped as unknown to improve signal-to-noise ratio.
+
+  + ``Size``: The size of the symbol in question, doesn’t apply to functions, and is optional for NoType symbols.
+
+  + ``Undefined``: Whether or not the symbol is defined in this shared object file.
+
+  + ``Weak``: Whether or not the symbol should be treated as weak.
+
+  + ``Warning`` (optional): Warning text to output when this symbol is linked against.
+
+This YAML based text format contains everything that is needed to generate a
+linkable ELF shared object as well as an Apple TAPI format file. The ordering
+of symbols is sorted, so these files can be easily compared using diff tools.
+If the content of the file changes, it indicates a potentially ABI breaking
+change.
+
+
+ELF STUB FORMAT
+---------------
+
+A minimum ELF file that can be used by linker should have following sections properly populated:
+
+* ELF header.
+
+* Section headers.
+
+* Dynamic symbol table (``.dynsym`` section).
+
+* Dynamic string table (``.dynstr`` section).
+
+* Dynamic table (``.dynamic`` section).
+
+  + DT_SYMTAB entry.
+
+  + DT_STRTAB entry.
+
+  + DT_STRSZ entry.
+
+  + DT_NEEDED entries. (optional)
+
+  + DT_SONAME entry. (optional)
+
+* Section header string table (``.shstrtab`` section)
+
+This ELF file may have compatibility issues with ELF analysis tools that rely on the program headers.
+Linkers like LLD work fine with such a minimum ELF file without errors.
+
+OPTIONS
+-------
+
+.. option:: --input-format=[IFS|ELF|OtherObjectFileFormats]
+
+ Specify input file format. Currently, only text IFS files and ELF shared
+ object files are supported. This flag is optional as the input format can be
+ inferred.
+
+.. option:: --output-elf=<output-filename>
+
+ Specify the output file for ELF shared object stub.
+
+.. option:: --output-ifs=<output-filename>
+
+ Specify the output file for text IFS.
+
+.. option:: --output-tbd=<output-filename>
+
+ Specify the output file for Apple TAPI tbd.
+
+.. option:: --arch=[x86_64|AArch64|...]
+
+ This flag is optional and it should only be used when reading an IFS file
+ which does not define the ``Arch`` (architecture). This flag defines the
+ architecture of the output file, and can be any string supported by ELF
+ 'e_machine' field. If the value is conflicting with the IFS file, an error
+ will be reported and the program will stop.
+
+.. option:: --endianness=[little|big]
+
+ This flag is optional and it should only be used when reading an IFS file
+ which does not define the ``Endianness``. This flag defines the endianness of
+ the output file. If the value is conflicting with the IFS file, an error
+ will be reported and the program will stop.
+
+.. option:: --bitwidth=[32|64]
+
+ This flag is optional and it should only be used when reading an IFS file
+ which does not define the ``BitWidth``. This flag defines the bit width of the
+ output file. If the value is conflicting with the input IFS file, an error
+ will be reported and the program will stop.
+
+.. option:: --target=<target triple>
+
+ This flag is optional and should only be used when reading an IFS file
+ which does not define any target information. This flag defines architecture,
+ endianness and bit width of the output file using llvm target triple.
+ This flag cannot be used simultaneously with other target related flags.
+
+.. option:: --hint-ifs-target=<target triple>
+
+ This flag is optional and should only be used when reading an ELF shared
+ object and generating an IFS file. by default, llvm-ifs will use '``Arch``,
+ ``Endianness`` and ``BitWidth``' fields to reflect the target information from the
+ input object file. Using this flag will tell llvm-ifs the expected target
+ triple in the output IFS file. If the value matches the target information
+ from the object file, this value will be used in the 'Target:' filed in the
+ generated IFS. If it conflicts with the input object file, an error will be
+ reported and the program will stop. 
+
+.. option:: --hint-ifs-target
+
+ This flag is optional and should only be used when outputting an IFS file.
+ This flag strips the ``Arch`` field from the IFS file so it can be overridden
+ later.
+
+.. option:: --strip-ifs-endianness
+
+ This flag is optional and should only be used when outputting an IFS file.
+ This flag strips the ``Endianness`` field from the IFS file so it can be
+ overridden later.
+
+.. option:: --strip-ifs-bitwidth
+
+ This flag is optional and should only be used when outputting an IFS file.
+ This flag strips the ``BitWidth`` field from the IFS file so it can be overridden
+ later.
+
+.. option:: --strip-ifs-target
+
+ This flag is optional and should only be used when outputting an IFS file.
+ This flag strips the ``Target`` field from the IFS file so it can be overridden
+ later.
+
+.. option:: --write-if-changed
+
+ When this flag is set, llvm-ifs will only write the output file if it does not
+ already exist or the content will be different from the existing file.
+
+EXIT STATUS
+-----------
+
+If :program:`llvm-ifs` succeeds, it will exit with 0. Otherwise, if an
+error occurs, it will exit with a non-zero value.