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LLVM-EXEGESIS(1) LLVM LLVM-EXEGESIS(1)

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 possi-
ble so that we can measure the latency (resp. inverse throughput/uop decom-
position) of the instruction. The code snippet is jitted and, unless re-
quested not to, executed on the host subtarget. The time taken (resp. re-
source 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. How-
ever, benchmarking these snippets often requires some setup so that they
can execute properly. llvm-exegesis has five annotations and some addi-
tional 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 def-
inition. A value will automatically be provided unless a second parame-
ter, a hex value, is passed in. This is done with the LLVM-EXEGESIS-DE-
FREG <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. Val-
ues can be passed through registers from the benchmarking setup in some
cases. The registers and the values assigned to them that can be uti-
lized in the benchmarking script with a LLVM-EXEGESIS-LIVEIN are as fol-
lows:

* 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 al-
lows specifying memory definitions that can later be mapped into the exe-
cution 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 dec-
imal 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 re-
quires 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 defin-
ition 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 sub-
process execution mode.

* LLVM-EXEGESIS-SNIPPET-ADDRESS <address> - This annotation allows for set-
ting 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 snip-
pet also includes setup code, so the instruction exactly at the specified
address will not be the first instruction in the snippet. Using this an-
notation 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.

* LLVM-EXEGESIS-LOOP-REGISTER <register name> - This annotation specifies
the loop register to use for keeping track of the current iteration when
using the loop repetition mode. llvm-exegesis needs to keep track of the
current loop iteration within the loop repetition mode in a performant
manner (i.e., no memory accesses), and uses a register to do this. This
register has an architecture specific default (e.g., R8 on X86), but this
might conflict with some snippets. This annotation allows changing the
register to prevent interference between the loop index register and the
snippet.

EXAMPLE 1: BENCHMARKING INSTRUCTIONS
Assume you have an X86-64 machine. To measure the latency of a single in-
struction, 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-exe-
gesis checks the liveliness of registers (i.e. any register use has a cor-
responding 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 per-
formed 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-EXEGE-
SIS-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 anno-
tations 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 us-
ing 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 : [im-
age]

Note that the scheduling class names will be resolved only when llvm-exege-
sis 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. Ei-
ther 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 spec-
ified 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 de-
tails. 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 require-
ments 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-out-
put-file=.

--benchmark-phase=[prepare-snippet|prepare-and-assemble-snippet|assem-
ble-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 per-
formance measurements. However, it is possible to stop at some
stage before measuring. Choices are: * prepare-snippet: Only gener-
ate the minimal instruction sequence. * prepare-and-assemble-snip-
pet: Same as prepare-snippet, but also dumps an excerpt of the se-
quence (hex encoded). * assemble-measured-code: Same as pre-
pare-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 throt-
tling 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 instruc-
tion 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. Com-
paring 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|middle-half-duplicate|mid-
dle-half-loop]
Specify the repetition mode. duplicate will create a large, straight
line basic block with min-instructions instructions (repeating the
snippet min-instructions/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 min-instructions instructions (thus, again, re-
peating the snippet min-instructions/snippet size times). The loop
mode, especially with loop unrolling tends to better hide the ef-
fects of the CPU frontend on architectures that cache decoded in-
structions, but consumes a register for counting iterations. If per-
forming an analysis over many opcodes, it may be best to instead use
the min mode, which will run each other mode, and produce the mini-
mal measured result. The middle half repetition modes will either
duplicate or run the snippet in a loop depending upon the specific
mode. The middle half repetition modes will run two benchmarks, one
twice the length of the first one, and then subtract the difference
between them to get values without overhead.

--min-instructions=<Number of instructions>
Specify the target number of executed instructions. Note that the
actual repetition count of the snippet will be min-instruc-
tions/snippet size. Higher values lead to more accurate measure-
ments 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 characteris-
tics 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_through-
put 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 in-
structions 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 op-
codes are filtered out. This flag will instead show only such un-
stable 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 snip-
pet 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 exe-
cution mode is the default. The subprocess execution mode allows for
additional features such as memory annotations but is currently re-
stricted 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 bal-
ance run-time and noise reduction.

--validation-counter=[instructions-retired,l1d-cache-load-misses,

l1d-cache-store-misses,l1i-cache-load-misses,data-tlb-load-misses,

data-tld-store-misses,instruction-tlb-load-misses]
This option enables the use of validation counters, which measure
additional microarchitectural events like cache misses to validate
snippet execution conditions. These events are measured using the
perf subsystem in a group with the performance counter used to mea-
sure the value of interest. This flag can be specified multiple
times to measure multiple events. The maximum number of validation
counters is platform dependent.

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/).

19 2026-07-30 LLVM-EXEGESIS(1)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=llvm-exegesis&sektion=1&manpath=FreeBSD+Ports+15.1.quarterly>

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