1llvm-exegesis - LLVM Machine Instruction Benchmark
2==================================================
3
4SYNOPSIS
5--------
6
7:program:`llvm-exegesis` [*options*]
8
9DESCRIPTION
10-----------
11
12:program:`llvm-exegesis` is a benchmarking tool that uses information available
13in LLVM to measure host machine instruction characteristics like latency or port
14decomposition.
15
16Given an LLVM opcode name and a benchmarking mode, :program:`llvm-exegesis`
17generates a code snippet that makes execution as serial (resp. as parallel) as
18possible so that we can measure the latency (resp. uop decomposition) of the
19instruction.
20The code snippet is jitted and executed on the host subtarget. The time taken
21(resp. resource usage) is measured using hardware performance counters. The
22result is printed out as YAML to the standard output.
23
24The main goal of this tool is to automatically (in)validate the LLVM's TableDef
25scheduling models. To that end, we also provide analysis of the results.
26
27:program:`llvm-exegesis` can also benchmark arbitrary user-provided code
28snippets.
29
30EXAMPLE 1: benchmarking instructions
31------------------------------------
32
33Assume you have an X86-64 machine. To measure the latency of a single
34instruction, run:
35
36.. code-block:: bash
37
38    $ llvm-exegesis -mode=latency -opcode-name=ADD64rr
39
40Measuring the uop decomposition of an instruction works similarly:
41
42.. code-block:: bash
43
44    $ llvm-exegesis -mode=uops -opcode-name=ADD64rr
45
46The output is a YAML document (the default is to write to stdout, but you can
47redirect the output to a file using `-benchmarks-file`):
48
49.. code-block:: none
50
51  ---
52  key:
53    opcode_name:     ADD64rr
54    mode:            latency
55    config:          ''
56  cpu_name:        haswell
57  llvm_triple:     x86_64-unknown-linux-gnu
58  num_repetitions: 10000
59  measurements:
60    - { key: latency, value: 1.0058, debug_string: '' }
61  error:           ''
62  info:            'explicit self cycles, selecting one aliasing configuration.
63  Snippet:
64  ADD64rr R8, R8, R10
65  '
66  ...
67
68To measure the latency of all instructions for the host architecture, run:
69
70.. code-block:: bash
71
72  #!/bin/bash
73  readonly INSTRUCTIONS=$(($(grep INSTRUCTION_LIST_END build/lib/Target/X86/X86GenInstrInfo.inc | cut -f2 -d=) - 1))
74  for INSTRUCTION in $(seq 1 ${INSTRUCTIONS});
75  do
76    ./build/bin/llvm-exegesis -mode=latency -opcode-index=${INSTRUCTION} | sed -n '/---/,$p'
77  done
78
79FIXME: Provide an :program:`llvm-exegesis` option to test all instructions.
80
81
82EXAMPLE 2: benchmarking a custom code snippet
83---------------------------------------------
84
85To measure the latency/uops of a custom piece of code, you can specify the
86`snippets-file` option (`-` reads from standard input).
87
88.. code-block:: bash
89
90    $ echo "vzeroupper" | llvm-exegesis -mode=uops -snippets-file=-
91
92Real-life code snippets typically depend on registers or memory.
93:program:`llvm-exegesis` checks the liveliness of registers (i.e. any register
94use has a corresponding def or is a "live in"). If your code depends on the
95value of some registers, you have two options:
96
97- Mark the register as requiring a definition. :program:`llvm-exegesis` will
98  automatically assign a value to the register. This can be done using the
99  directive `LLVM-EXEGESIS-DEFREG <reg name> <hex_value>`, where `<hex_value>`
100  is a bit pattern used to fill `<reg_name>`. If `<hex_value>` is smaller than
101  the register width, it will be sign-extended.
102- Mark the register as a "live in". :program:`llvm-exegesis` will benchmark
103  using whatever value was in this registers on entry. This can be done using
104  the directive `LLVM-EXEGESIS-LIVEIN <reg name>`.
105
106For example, the following code snippet depends on the values of XMM1 (which
107will be set by the tool) and the memory buffer passed in RDI (live in).
108
109.. code-block:: none
110
111  # LLVM-EXEGESIS-LIVEIN RDI
112  # LLVM-EXEGESIS-DEFREG XMM1 42
113  vmulps	(%rdi), %xmm1, %xmm2
114  vhaddps	%xmm2, %xmm2, %xmm3
115  addq $0x10, %rdi
116
117
118EXAMPLE 3: analysis
119-------------------
120
121Assuming you have a set of benchmarked instructions (either latency or uops) as
122YAML in file `/tmp/benchmarks.yaml`, you can analyze the results using the
123following command:
124
125.. code-block:: bash
126
127    $ llvm-exegesis -mode=analysis \
128  -benchmarks-file=/tmp/benchmarks.yaml \
129  -analysis-clusters-output-file=/tmp/clusters.csv \
130  -analysis-inconsistencies-output-file=/tmp/inconsistencies.html
131
132This will group the instructions into clusters with the same performance
133characteristics. The clusters will be written out to `/tmp/clusters.csv` in the
134following format:
135
136.. code-block:: none
137
138  cluster_id,opcode_name,config,sched_class
139  ...
140  2,ADD32ri8_DB,,WriteALU,1.00
141  2,ADD32ri_DB,,WriteALU,1.01
142  2,ADD32rr,,WriteALU,1.01
143  2,ADD32rr_DB,,WriteALU,1.00
144  2,ADD32rr_REV,,WriteALU,1.00
145  2,ADD64i32,,WriteALU,1.01
146  2,ADD64ri32,,WriteALU,1.01
147  2,MOVSX64rr32,,BSWAP32r_BSWAP64r_MOVSX64rr32,1.00
148  2,VPADDQYrr,,VPADDBYrr_VPADDDYrr_VPADDQYrr_VPADDWYrr_VPSUBBYrr_VPSUBDYrr_VPSUBQYrr_VPSUBWYrr,1.02
149  2,VPSUBQYrr,,VPADDBYrr_VPADDDYrr_VPADDQYrr_VPADDWYrr_VPSUBBYrr_VPSUBDYrr_VPSUBQYrr_VPSUBWYrr,1.01
150  2,ADD64ri8,,WriteALU,1.00
151  2,SETBr,,WriteSETCC,1.01
152  ...
153
154:program:`llvm-exegesis` will also analyze the clusters to point out
155inconsistencies in the scheduling information. The output is an html file. For
156example, `/tmp/inconsistencies.html` will contain messages like the following :
157
158.. image:: llvm-exegesis-analysis.png
159  :align: center
160
161Note that the scheduling class names will be resolved only when
162:program:`llvm-exegesis` is compiled in debug mode, else only the class id will
163be shown. This does not invalidate any of the analysis results though.
164
165
166OPTIONS
167-------
168
169.. option:: -help
170
171 Print a summary of command line options.
172
173.. option:: -opcode-index=<LLVM opcode index>
174
175 Specify the opcode to measure, by index. See example 1 for details.
176 Either `opcode-index`, `opcode-name` or `snippets-file` must be set.
177
178.. option:: -opcode-name=<opcode name 1>,<opcode name 2>,...
179
180 Specify the opcode to measure, by name. Several opcodes can be specified as
181 a comma-separated list. See example 1 for details.
182 Either `opcode-index`, `opcode-name` or `snippets-file` must be set.
183
184 .. option:: -snippets-file=<filename>
185
186  Specify the custom code snippet to measure. See example 2 for details.
187  Either `opcode-index`, `opcode-name` or `snippets-file` must be set.
188
189.. option:: -mode=[latency|uops|analysis]
190
191 Specify the run mode.
192
193.. option:: -num-repetitions=<Number of repetition>
194
195 Specify the number of repetitions of the asm snippet.
196 Higher values lead to more accurate measurements but lengthen the benchmark.
197
198.. option:: -benchmarks-file=</path/to/file>
199
200 File to read (`analysis` mode) or write (`latency`/`uops` modes) benchmark
201 results. "-" uses stdin/stdout.
202
203.. option:: -analysis-clusters-output-file=</path/to/file>
204
205 If provided, write the analysis clusters as CSV to this file. "-" prints to
206 stdout.
207
208.. option:: -analysis-inconsistencies-output-file=</path/to/file>
209
210 If non-empty, write inconsistencies found during analysis to this file. `-`
211 prints to stdout.
212
213.. option:: -analysis-numpoints=<dbscan numPoints parameter>
214
215 Specify the numPoints parameters to be used for DBSCAN clustering
216 (`analysis` mode).
217
218.. option:: -analysis-espilon=<dbscan epsilon parameter>
219
220 Specify the numPoints parameters to be used for DBSCAN clustering
221 (`analysis` mode).
222
223.. option:: -ignore-invalid-sched-class=false
224
225 If set, ignore instructions that do not have a sched class (class idx = 0).
226
227 .. option:: -mcpu=<cpu name>
228
229  If set, measure the cpu characteristics using the counters for this CPU. This
230  is useful when creating new sched models (the host CPU is unknown to LLVM).
231
232EXIT STATUS
233-----------
234
235:program:`llvm-exegesis` returns 0 on success. Otherwise, an error message is
236printed to standard error, and the tool returns a non 0 value.
237