Provided by: llvm-18_18.1.3-1_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, unless requested not to, 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.
SUPPORTED PLATFORMS
llvm-exegesis currently only supports X86 (64-bit only), ARM (AArch64 only), MIPS, and
PowerPC (PowerPC64LE only) on Linux for benchmarking. Not all benchmarking functionality
is guaranteed to work on every platform. llvm-exegesis also has a separate analysis mode
that is supported on every platform that LLVM is.
SNIPPET ANNOTATIONS
llvm-exegesis supports benchmarking arbitrary snippets of assembly. However, benchmarking
these snippets often requires some setup so that they can execute properly. llvm-exegesis
has five annotations and some additional utilities to help with setup so that snippets can
be benchmarked properly.
• LLVM-EXEGESIS-DEFREG <register name> - Adding this annotation to the text assembly
snippet to be benchmarked marks the register as requiring a definition. A value will
automatically be provided unless a second parameter, a hex value, is passed in. This is
done with the LLVM-EXEGESIS-DEFREG <register name> <hex value> format. <hex value> is a
bit pattern used to fill the register. If it is a value smaller than the register, it is
sign extended to match the size of the register.
• LLVM-EXEGESIS-LIVEIN <register name> - This annotation allows specifying registers that
should keep their value upon starting the benchmark. Values can be passed through
registers from the benchmarking setup in some cases. The registers and the values
assigned to them that can be utilized in the benchmarking script with a
LLVM-EXEGESIS-LIVEIN are as follows:
• Scratch memory register - The specific register that this value is put in is platform
dependent (e.g., it is the RDI register on X86 Linux). Setting this register as a live
in ensures that a pointer to a block of memory (1MB) is placed within this register
that can be used by the snippet.
• LLVM-EXEGESIS-MEM-DEF <value name> <size> <value> - This annotation allows specifying
memory definitions that can later be mapped into the execution process of a snippet with
the LLVM-EXEGESIS-MEM-MAP annotation. Each value is named using the <value name>
argument so that it can be referenced later within a map annotation. The size is
specified in a decimal number of bytes and the value is given in hexadecimal. If the
size of the value is less than the specified size, the value will be repeated until it
fills the entire section of memory. Using this annotation requires using the subprocess
execution mode.
• LLVM-EXEGESIS-MEM-MAP <value name> <address> - This annotation allows for mapping
previously defined memory definitions into the execution context of a process. The value
name refers to a previously defined memory definition and the address is a decimal
number that specifies the address the memory definition should start at. Note that a
single memory definition can be mapped multiple times. Using this annotation requires
the subprocess execution mode.
• LLVM-EXEGESIS-SNIPPET-ADDRESS <address> - This annotation allows for setting the address
where the beginning of the snippet to be executed will be mapped in at. The address is
given in hexadecimal. Note that the snippet also includes setup code, so the instruction
exactly at the specified address will not be the first instruction in the snippet. Using
this annotation requires the subprocess execution mode. This is useful in cases where
the memory accessed by the snippet depends on the location of the snippet, like
RIP-relative addressing.
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 need to use snippet annotations
to ensure setup is performed properly.
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: BENCHMARKING WITH MEMORY ANNOTATIONS
Some snippets require memory setup in specific places to execute without crashing. Setting
up memory can be accomplished with the LLVM-EXEGESIS-MEM-DEF and LLVM-EXEGESIS-MEM-MAP
annotations. To execute the following snippet:
movq $8192, %rax
movq (%rax), %rdi
We need to have at least eight bytes of memory allocated starting 0x2000. We can create
the necessary execution environment with the following annotations added to the snippet:
# LLVM-EXEGESIS-MEM-DEF test1 4096 7fffffff
# LLVM-EXEGESIS-MEM-MAP test1 8192
movq $8192, %rax
movq (%rax), %rdi
EXAMPLE 4: 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=.
--benchmark-phase=[prepare-snippet|prepare-and-assemble-snippet|assemble-measured-code|measure]
By default, when -mode= is specified, the generated snippet will be executed and
measured, and that requires that we are running on the hardware for which the
snippet was generated, and that supports performance measurements. However, it is
possible to stop at some stage before measuring. Choices are: * prepare-snippet:
Only generate the minimal instruction sequence. * prepare-and-assemble-snippet:
Same as prepare-snippet, but also dumps an excerpt of the sequence (hex encoded).
* assemble-measured-code: Same as prepare-and-assemble-snippet. but also creates
the full sequence that can be dumped to a file using --dump-object-to-disk. *
measure: Same as assemble-measured-code, but also runs the measurement.
--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.
--x86-disable-upper-sse-registers
Using the upper xmm registers (xmm8-xmm15) forces a longer instruction encoding
which may put greater pressure on the frontend fetch and decode stages, potentially
reducing the rate that instructions are dispatched to the backend, particularly on
older hardware. Comparing baseline results with this mode enabled can help
determine the effects of the frontend and can be used to improve latency and
throughput estimates.
--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-filter=[all|reg-only|mem-only]
By default, all benchmark results are analysed, but sometimes it may be useful to
only look at those that to not involve memory, or vice versa. This option allows to
either keep all benchmarks, or filter out (ignore) either all the ones that do
involve memory (involve instructions that may read or write to memory), or the
opposite, to only keep such benchmarks.
--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).
--mtriple=<triple name>
Target triple. See -version for available targets.
--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).
(-mcpu=help for details)
--analysis-override-benchmark-triple-and-cpu
By default, llvm-exegesis will analyze the benchmarks for the triple/CPU they were
measured for, but if you want to analyze them for some other combination (specified
via -mtriple/-mcpu), you can pass this flag.
--dump-object-to-disk=true
If set, llvm-exegesis will dump the generated code to a temporary file to enable
code inspection. Disabled by default.
--use-dummy-perf-counters
If set, llvm-exegesis will not read any real performance counters and return a
dummy value instead. This can be used to ensure a snippet doesn't crash when
hardware performance counters are unavailable and for debugging llvm-exegesis
itself.
--execution-mode=[inprocess,subprocess]
This option specifies what execution mode to use. The inprocess execution mode is
the default. The subprocess execution mode allows for additional features such as
memory annotations but is currently restricted to X86-64 on Linux.
--benchmark-repeat-count=<repeat-count>
This option enables specifying the number of times to repeat the measurement when
performing latency measurements. By default, llvm-exegesis will repeat a latency
measurement enough times to balance run-time and noise reduction.
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
2003-2024, LLVM Project