1llvm-exegesis - LLVM Machine Instruction Benchmark 2================================================== 3 4.. program:: llvm-exegesis 5 6SYNOPSIS 7-------- 8 9:program:`llvm-exegesis` [*options*] 10 11DESCRIPTION 12----------- 13 14:program:`llvm-exegesis` is a benchmarking tool that uses information available 15in LLVM to measure host machine instruction characteristics like latency, 16throughput, or port decomposition. 17 18Given an LLVM opcode name and a benchmarking mode, :program:`llvm-exegesis` 19generates a code snippet that makes execution as serial (resp. as parallel) as 20possible so that we can measure the latency (resp. inverse throughput/uop decomposition) 21of the instruction. 22The code snippet is jitted and executed on the host subtarget. The time taken 23(resp. resource usage) is measured using hardware performance counters. The 24result is printed out as YAML to the standard output. 25 26The main goal of this tool is to automatically (in)validate the LLVM's TableDef 27scheduling models. To that end, we also provide analysis of the results. 28 29:program:`llvm-exegesis` can also benchmark arbitrary user-provided code 30snippets. 31 32EXAMPLE 1: benchmarking instructions 33------------------------------------ 34 35Assume you have an X86-64 machine. To measure the latency of a single 36instruction, run: 37 38.. code-block:: bash 39 40 $ llvm-exegesis -mode=latency -opcode-name=ADD64rr 41 42Measuring the uop decomposition or inverse throughput of an instruction works similarly: 43 44.. code-block:: bash 45 46 $ llvm-exegesis -mode=uops -opcode-name=ADD64rr 47 $ llvm-exegesis -mode=inverse_throughput -opcode-name=ADD64rr 48 49 50The output is a YAML document (the default is to write to stdout, but you can 51redirect the output to a file using `-benchmarks-file`): 52 53.. code-block:: none 54 55 --- 56 key: 57 opcode_name: ADD64rr 58 mode: latency 59 config: '' 60 cpu_name: haswell 61 llvm_triple: x86_64-unknown-linux-gnu 62 num_repetitions: 10000 63 measurements: 64 - { key: latency, value: 1.0058, debug_string: '' } 65 error: '' 66 info: 'explicit self cycles, selecting one aliasing configuration. 67 Snippet: 68 ADD64rr R8, R8, R10 69 ' 70 ... 71 72To measure the latency of all instructions for the host architecture, run: 73 74.. code-block:: bash 75 76 $ llvm-exegesis -mode=latency -opcode-index=-1 77 78 79EXAMPLE 2: benchmarking a custom code snippet 80--------------------------------------------- 81 82To measure the latency/uops of a custom piece of code, you can specify the 83`snippets-file` option (`-` reads from standard input). 84 85.. code-block:: bash 86 87 $ echo "vzeroupper" | llvm-exegesis -mode=uops -snippets-file=- 88 89Real-life code snippets typically depend on registers or memory. 90:program:`llvm-exegesis` checks the liveliness of registers (i.e. any register 91use has a corresponding def or is a "live in"). If your code depends on the 92value of some registers, you have two options: 93 94- Mark the register as requiring a definition. :program:`llvm-exegesis` will 95 automatically assign a value to the register. This can be done using the 96 directive `LLVM-EXEGESIS-DEFREG <reg name> <hex_value>`, where `<hex_value>` 97 is a bit pattern used to fill `<reg_name>`. If `<hex_value>` is smaller than 98 the register width, it will be sign-extended. 99- Mark the register as a "live in". :program:`llvm-exegesis` will benchmark 100 using whatever value was in this registers on entry. This can be done using 101 the directive `LLVM-EXEGESIS-LIVEIN <reg name>`. 102 103For example, the following code snippet depends on the values of XMM1 (which 104will be set by the tool) and the memory buffer passed in RDI (live in). 105 106.. code-block:: none 107 108 # LLVM-EXEGESIS-LIVEIN RDI 109 # LLVM-EXEGESIS-DEFREG XMM1 42 110 vmulps (%rdi), %xmm1, %xmm2 111 vhaddps %xmm2, %xmm2, %xmm3 112 addq $0x10, %rdi 113 114 115EXAMPLE 3: analysis 116------------------- 117 118Assuming you have a set of benchmarked instructions (either latency or uops) as 119YAML in file `/tmp/benchmarks.yaml`, you can analyze the results using the 120following command: 121 122.. code-block:: bash 123 124 $ llvm-exegesis -mode=analysis \ 125 -benchmarks-file=/tmp/benchmarks.yaml \ 126 -analysis-clusters-output-file=/tmp/clusters.csv \ 127 -analysis-inconsistencies-output-file=/tmp/inconsistencies.html 128 129This will group the instructions into clusters with the same performance 130characteristics. The clusters will be written out to `/tmp/clusters.csv` in the 131following format: 132 133.. code-block:: none 134 135 cluster_id,opcode_name,config,sched_class 136 ... 137 2,ADD32ri8_DB,,WriteALU,1.00 138 2,ADD32ri_DB,,WriteALU,1.01 139 2,ADD32rr,,WriteALU,1.01 140 2,ADD32rr_DB,,WriteALU,1.00 141 2,ADD32rr_REV,,WriteALU,1.00 142 2,ADD64i32,,WriteALU,1.01 143 2,ADD64ri32,,WriteALU,1.01 144 2,MOVSX64rr32,,BSWAP32r_BSWAP64r_MOVSX64rr32,1.00 145 2,VPADDQYrr,,VPADDBYrr_VPADDDYrr_VPADDQYrr_VPADDWYrr_VPSUBBYrr_VPSUBDYrr_VPSUBQYrr_VPSUBWYrr,1.02 146 2,VPSUBQYrr,,VPADDBYrr_VPADDDYrr_VPADDQYrr_VPADDWYrr_VPSUBBYrr_VPSUBDYrr_VPSUBQYrr_VPSUBWYrr,1.01 147 2,ADD64ri8,,WriteALU,1.00 148 2,SETBr,,WriteSETCC,1.01 149 ... 150 151:program:`llvm-exegesis` will also analyze the clusters to point out 152inconsistencies in the scheduling information. The output is an html file. For 153example, `/tmp/inconsistencies.html` will contain messages like the following : 154 155.. image:: llvm-exegesis-analysis.png 156 :align: center 157 158Note that the scheduling class names will be resolved only when 159:program:`llvm-exegesis` is compiled in debug mode, else only the class id will 160be shown. This does not invalidate any of the analysis results though. 161 162OPTIONS 163------- 164 165.. option:: -help 166 167 Print a summary of command line options. 168 169.. option:: -opcode-index=<LLVM opcode index> 170 171 Specify the opcode to measure, by index. Specifying `-1` will result 172 in measuring every existing opcode. See example 1 for details. 173 Either `opcode-index`, `opcode-name` or `snippets-file` must be set. 174 175.. option:: -opcode-name=<opcode name 1>,<opcode name 2>,... 176 177 Specify the opcode to measure, by name. Several opcodes can be specified as 178 a comma-separated list. See example 1 for details. 179 Either `opcode-index`, `opcode-name` or `snippets-file` must be set. 180 181.. option:: -snippets-file=<filename> 182 183 Specify the custom code snippet to measure. See example 2 for details. 184 Either `opcode-index`, `opcode-name` or `snippets-file` must be set. 185 186.. option:: -mode=[latency|uops|inverse_throughput|analysis] 187 188 Specify the run mode. Note that some modes have additional requirements and options. 189 190 `latency` mode can be make use of either RDTSC or LBR. 191 `latency[LBR]` is only available on X86 (at least `Skylake`). 192 To run in `latency` mode, a positive value must be specified 193 for `x86-lbr-sample-period` and `--repetition-mode=loop`. 194 195 In `analysis` mode, you also need to specify at least one of the 196 `-analysis-clusters-output-file=` and `-analysis-inconsistencies-output-file=`. 197 198.. option:: -x86-lbr-sample-period=<nBranches/sample> 199 200 Specify the LBR sampling period - how many branches before we take a sample. 201 When a positive value is specified for this option and when the mode is `latency`, 202 we will use LBRs for measuring. 203 On choosing the "right" sampling period, a small value is preferred, but throttling 204 could occur if the sampling is too frequent. A prime number should be used to 205 avoid consistently skipping certain blocks. 206 207.. option:: -repetition-mode=[duplicate|loop|min] 208 209 Specify the repetition mode. `duplicate` will create a large, straight line 210 basic block with `num-repetitions` instructions (repeating the snippet 211 `num-repetitions`/`snippet size` times). `loop` will, optionally, duplicate the 212 snippet until the loop body contains at least `loop-body-size` instructions, 213 and then wrap the result in a loop which will execute `num-repetitions` 214 instructions (thus, again, repeating the snippet 215 `num-repetitions`/`snippet size` times). The `loop` mode, especially with loop 216 unrolling tends to better hide the effects of the CPU frontend on architectures 217 that cache decoded instructions, but consumes a register for counting 218 iterations. If performing an analysis over many opcodes, it may be best to 219 instead use the `min` mode, which will run each other mode, 220 and produce the minimal measured result. 221 222.. option:: -num-repetitions=<Number of repetitions> 223 224 Specify the target number of executed instructions. Note that the actual 225 repetition count of the snippet will be `num-repetitions`/`snippet size`. 226 Higher values lead to more accurate measurements but lengthen the benchmark. 227 228.. option:: -loop-body-size=<Preferred loop body size> 229 230 Only effective for `-repetition-mode=[loop|min]`. 231 Instead of looping over the snippet directly, first duplicate it so that the 232 loop body contains at least this many instructions. This potentially results 233 in loop body being cached in the CPU Op Cache / Loop Cache, which allows to 234 which may have higher throughput than the CPU decoders. 235 236.. option:: -max-configs-per-opcode=<value> 237 238 Specify the maximum configurations that can be generated for each opcode. 239 By default this is `1`, meaning that we assume that a single measurement is 240 enough to characterize an opcode. This might not be true of all instructions: 241 for example, the performance characteristics of the LEA instruction on X86 242 depends on the value of assigned registers and immediates. Setting a value of 243 `-max-configs-per-opcode` larger than `1` allows `llvm-exegesis` to explore 244 more configurations to discover if some register or immediate assignments 245 lead to different performance characteristics. 246 247 248.. option:: -benchmarks-file=</path/to/file> 249 250 File to read (`analysis` mode) or write (`latency`/`uops`/`inverse_throughput` 251 modes) benchmark results. "-" uses stdin/stdout. 252 253.. option:: -analysis-clusters-output-file=</path/to/file> 254 255 If provided, write the analysis clusters as CSV to this file. "-" prints to 256 stdout. By default, this analysis is not run. 257 258.. option:: -analysis-inconsistencies-output-file=</path/to/file> 259 260 If non-empty, write inconsistencies found during analysis to this file. `-` 261 prints to stdout. By default, this analysis is not run. 262 263.. option:: -analysis-clustering=[dbscan,naive] 264 265 Specify the clustering algorithm to use. By default DBSCAN will be used. 266 Naive clustering algorithm is better for doing further work on the 267 `-analysis-inconsistencies-output-file=` output, it will create one cluster 268 per opcode, and check that the cluster is stable (all points are neighbours). 269 270.. option:: -analysis-numpoints=<dbscan numPoints parameter> 271 272 Specify the numPoints parameters to be used for DBSCAN clustering 273 (`analysis` mode, DBSCAN only). 274 275.. option:: -analysis-clustering-epsilon=<dbscan epsilon parameter> 276 277 Specify the epsilon parameter used for clustering of benchmark points 278 (`analysis` mode). 279 280.. option:: -analysis-inconsistency-epsilon=<epsilon> 281 282 Specify the epsilon parameter used for detection of when the cluster 283 is different from the LLVM schedule profile values (`analysis` mode). 284 285.. option:: -analysis-display-unstable-clusters 286 287 If there is more than one benchmark for an opcode, said benchmarks may end up 288 not being clustered into the same cluster if the measured performance 289 characteristics are different. by default all such opcodes are filtered out. 290 This flag will instead show only such unstable opcodes. 291 292.. option:: -ignore-invalid-sched-class=false 293 294 If set, ignore instructions that do not have a sched class (class idx = 0). 295 296.. option:: -mcpu=<cpu name> 297 298 If set, measure the cpu characteristics using the counters for this CPU. This 299 is useful when creating new sched models (the host CPU is unknown to LLVM). 300 301.. option:: --dump-object-to-disk=true 302 303 By default, llvm-exegesis will dump the generated code to a temporary file to 304 enable code inspection. You may disable it to speed up the execution and save 305 disk space. 306 307EXIT STATUS 308----------- 309 310:program:`llvm-exegesis` returns 0 on success. Otherwise, an error message is 311printed to standard error, and the tool returns a non 0 value. 312
