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 gener-
ates a code snippet that makes execution as serial (resp. as parallel)
as possible so that we can measure the latency (resp. inverse through-
put/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 auto-
matically 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 perfor-
mance characteristics. The clusters will be written out to /tmp/clus-
ters.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 inconsisten-
cies in the scheduling information. The output is an html file. For ex-
ample, /tmp/inconsistencies.html will contain messages like the follow-
ing : [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 re-
sult in measuring every existing opcode. See example 1 for de-
tails. 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 re-
quirements and options.
latency mode can be make use of either RDTSC or LBR. la-
tency[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-inconsisten-
cies-output-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 performance measurements. However, it is possible to
stop at some stage before measuring. Choices are: * pre-
pare-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-mea-
sured-code: Same as prepare-and-assemble-snippet. but also cre-
ates the full sequence that can be dumped to a file using
--dump-object-to-disk. * measure: Same as assemble-mea-
sured-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 op-
tion and when the mode is latency, we will use LBRs for measur-
ing. 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 in-
struction encoding which may put greater pressure on the front-
end 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 en-
abled 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 (re-
peating the snippet num-repetitions/snippet size times). loop
will, optionally, duplicate the snippet until the loop body con-
tains 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 op-
codes, 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-repeti-
tions/snippet size. Higher values lead to more accurate mea-
surements 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 po-
tentially 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 perfor-
mance 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 im-
mediate assignments lead to different performance characteris-
tics.
-benchmarks-file=</path/to/file>
File to read (analysis mode) or write (latency/uops/in-
verse_throughput modes) benchmark results. “-” uses stdin/std-
out.
-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 mem-
ory (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 cluster-
ing (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 bench-
marks may end up not being clustered into the same cluster if
the measured performance characteristics are different. by de-
fault 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 tempo-
rary 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
2003-2023, LLVM Project
15 2023-10-16 LLVM-EXEGESIS(1)
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