bionic (1) llvm-exegesis-9.1.gz
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 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:
• 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. 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-output-file= and -analysis-inconsistencies-output-file=.
-num-repetitions=<Number of repetition>
Specify the number of repetitions of the asm snippet. Higher values lead to more accurate
measurements but lengthen the benchmark.
-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 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).
-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 temporary 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
2003-2020, LLVM Project