Provided by: llvm-15_15.0.2-1_amd64 
NAME
llvm-exegesis - LLVM Machine Instruction Benchmark
SYNOPSIS
llvm-exegesis [options]
DESCRIPTION
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, 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.
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:
$ llvm-exegesis -mode=latency -opcode-name=ADD64rr
Measuring the uop decomposition or inverse throughput of an instruction works similarly:
$ 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):
---
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:
$ 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).
$ echo "vzeroupper" | llvm-exegesis -mode=uops -snippets-file=-
Real-life code snippets typically depend on registers or memory. 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. 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”. 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).
# 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:
$ 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:
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
...
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]
Note that the scheduling class names will be resolved only when 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
-help Print a summary of command line options.
-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.
-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.
-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.
-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=.
-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.
-repetition-mode=[duplicate|loop|min]
Specify the repetition mode. duplicate will create a large, straight line basic
block with num-repetitions instructions (repeating the snippet
num-repetitions/snippet size times). loop will, optionally, duplicate the snippet
until the loop body contains at least loop-body-size instructions, and then wrap
the result in a loop which will execute num-repetitions instructions (thus, again,
repeating the snippet num-repetitions/snippet size times). The loop mode,
especially with loop unrolling 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.
-num-repetitions=<Number of repetitions>
Specify the target number of executed instructions. Note that the actual repetition
count of the snippet will be num-repetitions/snippet size. Higher values lead to
more accurate measurements but lengthen the benchmark.
-loop-body-size=<Preferred loop body size>
Only effective for -repetition-mode=[loop|min]. Instead of looping over the
snippet directly, first duplicate it so that the loop body contains at least this
many instructions. This potentially results in loop body being cached in the CPU Op
Cache / Loop Cache, which allows to which may have higher throughput than the CPU
decoders.
-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.
-benchmarks-file=</path/to/file>
File to read (analysis mode) or write (latency/uops/inverse_throughput modes)
benchmark results. “-” uses stdin/stdout.
-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.
-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.
-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).
-analysis-numpoints=<dbscan numPoints parameter>
Specify the numPoints parameters to be used for DBSCAN clustering (analysis mode,
DBSCAN only).
-analysis-clustering-epsilon=<dbscan epsilon parameter>
Specify the epsilon parameter used for clustering of benchmark points (analysis
mode).
-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).
-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.
-ignore-invalid-sched-class=false
If set, ignore instructions that do not have a sched class (class idx = 0).
-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).
--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
llvm-exegesis returns 0 on success. Otherwise, an error message is printed to standard
error, and the tool returns a non 0 value.
AUTHOR
Maintained by the LLVM Team (https://llvm.org/).
COPYRIGHT
2003-2022, LLVM Project