llvm-exegesis - LLVM Machine Instruction Benchmark
llvm-exegesis [options]
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 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:
#!/bin/bash
readonly INSTRUCTIONS=$(($(grep INSTRUCTION_LIST_END build/lib/Target/X86/X86GenInstrInfo.inc | cut -f2 -d=) - 1))
for INSTRUCTION in $(seq 1 ${INSTRUCTIONS});
do
./build/bin/llvm-exegesis -mode=latency -opcode-index=${INSTRUCTION} | sed -n '/---/,$p'
done
FIXME: Provide an llvm-exegesis option to test all instructions.
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:
o 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.
o 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
example, /tmp/inconsistencies.html will contain messages like the fol-
lowing : [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 if you pick analysis mode, you
also need to specify at least one of the -analysis-clusters-out-
put-file= and -analysis-inconsistencies-output-file=.
-repetition-mode=[duplicate|loop|min]
Specify the repetition mode. duplicate will create a large,
straight line basic block with num-repetitions copies of the
snippet. loop will wrap the snippet in a loop which will be run
num-repetitions times. The loop mode 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 number of repetitions of the asm snippet. Higher
values lead to more accurate measurements but lengthen the
benchmark.
-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
immediate assignments lead to different performance characteris-
tics.
-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 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
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 tem-
porary 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
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
+---------------+---------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+---------------------+
|Availability | developer/llvm/llvm |
+---------------+---------------------+
|Stability | Uncommitted |
+---------------+---------------------+
NOTES
Source code for open source software components in Oracle Solaris can
be found at https://www.oracle.com/downloads/opensource/solaris-source-
code-downloads.html.
This software was built from source available at
https://github.com/oracle/solaris-userland. The original community
source was downloaded from https://github.com/llvm/llvm-
project/releases/download/llvmorg-11.0.0/llvm-11.0.0.src.tar.xz.
Further information about this software can be found on the open source
community website at https://llvm.org/.
11 2022-06-28 LLVM-EXEGESIS(1)