llvm/docs/CommandGuide/llvm-exegesis.rst

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

.. program:: llvm-exegesis

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,
throughput, 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. inverse throughput/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.

:program:`llvm-exegesis` can also benchmark arbitrary user-provided code
snippets.

EXAMPLE 1: benchmarking instructions
------------------------------------

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 or inverse throughput of an instruction works similarly:

.. code-block:: bash

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

    $ llvm-exegesis -mode=latency -opcode-index=-1


EXAMPLE 2: benchmarking a custom code snippet
---------------------------------------------

To measure the latency/uops of a custom piece of code, you can specify the
`snippets-file` option (`-` reads from standard input).

.. code-block:: bash

    $ echo "vzeroupper" | llvm-exegesis -mode=uops -snippets-file=-

Real-life code snippets typically depend on registers or memory.
:program:`llvm-exegesis` checks the liveliness of registers (i.e. any register
use has a corresponding def or is a "live in"). If your code depends on the
value of some registers, you have two options:

- Mark the register as requiring a definition. :program:`llvm-exegesis` will
  automatically assign a value to the register. This can be done using the
  directive `LLVM-EXEGESIS-DEFREG <reg name> <hex_value>`, where `<hex_value>`
  is a bit pattern used to fill `<reg_name>`. If `<hex_value>` is smaller than
  the register width, it will be sign-extended.
- Mark the register as a "live in". :program:`llvm-exegesis` will benchmark
  using whatever value was in this registers on entry. This can be done using
  the directive `LLVM-EXEGESIS-LIVEIN <reg name>`.

For example, the following code snippet depends on the values of XMM1 (which
will be set by the tool) and the memory buffer passed in RDI (live in).

.. code-block:: none

  # LLVM-EXEGESIS-LIVEIN RDI
  # LLVM-EXEGESIS-DEFREG XMM1 42
  vmulps	(%rdi), %xmm1, %xmm2
  vhaddps	%xmm2, %xmm2, %xmm3
  addq $0x10, %rdi


EXAMPLE 3: 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.html

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. The output is an html file. For
example, `/tmp/inconsistencies.html` will contain messages like the following :

.. image:: llvm-exegesis-analysis.png
  :align: center

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. Specifying `-1` will result
 in measuring every existing opcode. See example 1 for details.
 Either `opcode-index`, `opcode-name` or `snippets-file` must be set.

.. option:: -opcode-name=<opcode name 1>,<opcode name 2>,...

 Specify the opcode to measure, by name. Several opcodes can be specified as
 a comma-separated list. See example 1 for details.
 Either `opcode-index`, `opcode-name` or `snippets-file` must be set.

.. option:: -snippets-file=<filename>

 Specify the custom code snippet to measure. See example 2 for details.
 Either `opcode-index`, `opcode-name` or `snippets-file` must be set.

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

 Specify the run mode. Note that some modes have additional requirements and options.

 `latency` mode can be  make use of either RDTSC or LBR.
 `latency[LBR]` is only available on X86 (at least `Skylake`).
 To run in `latency` mode, a positive value must be specified for `x86-lbr-sample-period` and `--repetition-mode=loop`.

 In `analysis` mode, you also need to specify at least one of the
 `-analysis-clusters-output-file=` and `-analysis-inconsistencies-output-file=`.

.. option:: -x86-lbr-sample-period=<nBranches/sample>

  Specify the LBR sampling period - how many branches before we take a sample.
  When a positive value is specified for this option and when the mode is `latency`,
  we will use LBRs for measuring.
  On choosing the "right" sampling period, a small value is preferred, but throttling
  could occur if the sampling is too frequent. A prime number should be used to
  avoid consistently skipping certain blocks.
  
.. option:: -repetition-mode=[duplicate|loop|min]

 Specify the repetition mode. `duplicate` will create a large, straight line
 basic block with `num-repetitions` copies of the snippet. `loop` will wrap
 the snippet in a loop which will be run `num-repetitions` times. The `loop`
 mode tends to better hide the effects of the CPU frontend on architectures
 that cache decoded instructions, but consumes a register for counting
 iterations. If performing an analysis over many opcodes, it may be best
 to instead use the `min` mode, which will run each other mode, and produce
 the minimal measured result.

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

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

.. option:: -max-configs-per-opcode=<value>

 Specify the maximum configurations that can be generated for each opcode.
 By default this is `1`, meaning that we assume that a single measurement is
 enough to characterize an opcode. This might not be true of all instructions:
 for example, the performance characteristics of the LEA instruction on X86
 depends on the value of assigned registers and immediates. Setting a value of
 `-max-configs-per-opcode` larger than `1` allows `llvm-exegesis` to explore
 more configurations to discover if some register or immediate assignments
 lead to different performance characteristics.


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

 File to read (`analysis` mode) or write (`latency`/`uops`/`inverse_throughput`
 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. By default, this analysis is not run.

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

 If non-empty, write inconsistencies found during analysis to this file. `-`
 prints to stdout. By default, this analysis is not run.

.. option:: -analysis-clustering=[dbscan,naive]

 Specify the clustering algorithm to use. By default DBSCAN will be used.
 Naive clustering algorithm is better for doing further work on the
 `-analysis-inconsistencies-output-file=` output, it will create one cluster
 per opcode, and check that the cluster is stable (all points are neighbours).

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

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

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

 Specify the epsilon parameter used for clustering of benchmark points
 (`analysis` mode).

.. option:: -analysis-inconsistency-epsilon=<epsilon>

 Specify the epsilon parameter used for detection of when the cluster
 is different from the LLVM schedule profile values (`analysis` mode).

.. option:: -analysis-display-unstable-clusters

 If there is more than one benchmark for an opcode, said benchmarks may end up
 not being clustered into the same cluster if the measured performance
 characteristics are different. by default all such opcodes are filtered out.
 This flag will instead show only such unstable opcodes.

.. option:: -ignore-invalid-sched-class=false

 If set, ignore instructions that do not have a sched class (class idx = 0).

.. option:: -mcpu=<cpu name>

 If set, measure the cpu characteristics using the counters for this CPU. This
 is useful when creating new sched models (the host CPU is unknown to LLVM).

.. option:: --dump-object-to-disk=true

 By default, llvm-exegesis will dump the generated code to a temporary file to
 enable code inspection. You may disable it to speed up the execution and save
 disk space.

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.