Provided by: llvm-16_16.0.6-23ubuntu4_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.
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=.
--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.
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
15 2024-04-14 LLVM-EXEGESIS(1)