llvm/docs/CommandGuide/llvm-exegesis.rst

llvm-exegesis - LLVM Machine Instruction Benchmark
==================================================

SYNOPSIS
--------

:program:`llvm-exegesis` [*options*]

DESCRIPTION
-----------

:program:`llvm-exegesis` is a benchmarking tool that uses information available
in LLVM to measure host machine instruction characteristics like latency or port
decomposition.

Given an LLVM opcode name and a benchmarking mode, :program:`llvm-exegesis`
generates a code snippet that makes execution as serial (resp. as parallel) as
possible so that we can measure the latency (resp. uop decomposition) of the
instruction.
The code snippet is jitted and executed on the host subtarget. The time taken
(resp. resource usage) is measured using hardware performance counters. The
result is printed out as YAML to the standard output.

The main goal of this tool is to automatically (in)validate the LLVM's TableDef
scheduling models. To that end, we also provide analysis of the results.

EXAMPLES: benchmarking
----------------------

Assume you have an X86-64 machine. To measure the latency of a single
instruction, run:

.. code-block:: bash

    $ llvm-exegesis -mode=latency -opcode-name=ADD64rr

Measuring the uop decomposition of an instruction works similarly:

.. code-block:: bash

    $ llvm-exegesis -mode=uops -opcode-name=ADD64rr

The output is a YAML document (the default is to write to stdout, but you can
redirect the output to a file using `-benchmarks-file`):

.. code-block:: none

  ---
  key:
    opcode_name:     ADD64rr
    mode:            latency
    config:          ''
  cpu_name:        haswell
  llvm_triple:     x86_64-unknown-linux-gnu
  num_repetitions: 10000
  measurements:
    - { key: latency, value: 1.0058, debug_string: '' }
  error:           ''
  info:            'explicit self cycles, selecting one aliasing configuration.
  Snippet:
  ADD64rr R8, R8, R10
  '
  ...

To measure the latency of all instructions for the host architecture, run:

.. code-block:: bash

  #!/bin/bash
  readonly INSTRUCTIONS=$(grep INSTRUCTION_LIST_END build/lib/Target/X86/X86GenInstrInfo.inc | cut -f2 -d=)
  for INSTRUCTION in $(seq 1 ${INSTRUCTIONS});
  do
    ./build/bin/llvm-exegesis -mode=latency -opcode-index=${INSTRUCTION} | sed -n '/---/,$p'
  done

FIXME: Provide an :program:`llvm-exegesis` option to test all instructions.

EXAMPLES: analysis
----------------------

Assuming you have a set of benchmarked instructions (either latency or uops) as
YAML in file `/tmp/benchmarks.yaml`, you can analyze the results using the
following command:

.. code-block:: bash

    $ llvm-exegesis -mode=analysis \
  -benchmarks-file=/tmp/benchmarks.yaml \
  -analysis-clusters-output-file=/tmp/clusters.csv \
  -analysis-inconsistencies-output-file=/tmp/inconsistencies.txt

This will group the instructions into clusters with the same performance
characteristics. The clusters will be written out to `/tmp/clusters.csv` in the
following format:

.. code-block:: none

  cluster_id,opcode_name,config,sched_class
  ...
  2,ADD32ri8_DB,,WriteALU,1.00
  2,ADD32ri_DB,,WriteALU,1.01
  2,ADD32rr,,WriteALU,1.01
  2,ADD32rr_DB,,WriteALU,1.00
  2,ADD32rr_REV,,WriteALU,1.00
  2,ADD64i32,,WriteALU,1.01
  2,ADD64ri32,,WriteALU,1.01
  2,MOVSX64rr32,,BSWAP32r_BSWAP64r_MOVSX64rr32,1.00
  2,VPADDQYrr,,VPADDBYrr_VPADDDYrr_VPADDQYrr_VPADDWYrr_VPSUBBYrr_VPSUBDYrr_VPSUBQYrr_VPSUBWYrr,1.02
  2,VPSUBQYrr,,VPADDBYrr_VPADDDYrr_VPADDQYrr_VPADDWYrr_VPSUBBYrr_VPSUBDYrr_VPSUBQYrr_VPSUBWYrr,1.01
  2,ADD64ri8,,WriteALU,1.00
  2,SETBr,,WriteSETCC,1.01
  ...

:program:`llvm-exegesis` will also analyze the clusters to point out
inconsistencies in the scheduling information. For example,
`/tmp/inconsistencies.txt` will contain messages like:

.. code-block:: none

  Sched Class EXTRACTPSrr_VEXTRACTPSrr contains instructions with distinct performance characteristics, falling into 2 clusters:
  4,EXTRACTPSrr,,3.00
  3,VEXTRACTPSrr,,2.01

  Sched Class WriteCRC32 contains instructions with distinct performance characteristics, falling into 2 clusters:
  4,CRC32r32r16,,3.01
  4,CRC32r32r32,,3.00
  11,CRC32r32r8,,4.01
  4,CRC32r64r64,,3.01
  4,CRC32r64r8,,3.00

Note that the scheduling class names will be resolved only when
:program:`llvm-exegesis` is compiled in debug mode, else only the class id will
be shown. This does not invalidate any of the analysis results though.


OPTIONS
-------

.. option:: -help

 Print a summary of command line options.

.. option:: -opcode-index=<LLVM opcode index>

 Specify the opcode to measure, by index.
 Either `opcode-index` or `opcode-name` must be set.

.. option:: -opcode-name=<LLVM opcode name>

 Specify the opcode to measure, by name.
 Either `opcode-index` or `opcode-name` must be set.

.. option:: -mode=[latency|uops|analysis]

 Specify the run mode.

.. option:: -num-repetitions=<Number of repetition>

 Specify the number of repetitions of the asm snippet.
 Higher values lead to more accurate measurements but lengthen the benchmark.

 .. option:: -benchmarks-file=</path/to/file>

 File to read (`analysis` mode) or write (`latency`/`uops` modes) benchmark
 results. "-" uses stdin/stdout.

.. option:: -analysis-clusters-output-file=</path/to/file>

 If provided, write the analysis clusters as CSV to this file. "-" prints to
 stdout.

.. option:: -analysis-inconsistencies-output-file=</path/to/file>

 If non-empty, write inconsistencies found during analysis to this file. `-`
 prints to stdout.

.. option:: -analysis-numpoints=<dbscan numPoints parameter>

 Specify the numPoints parameters to be used for DBSCAN clustering
 (`analysis` mode).

.. option:: -analysis-espilon=<dbscan epsilon parameter>

 Specify the numPoints parameters to be used for DBSCAN clustering
 (`analysis` mode).


EXIT STATUS
-----------

:program:`llvm-exegesis` returns 0 on success. Otherwise, an error message is
printed to standard error, and the tool returns a non 0 value.