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 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. 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.

EXAMPLES: BENCHMARKING

       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 of an instruction works similarly:

          $ llvm-exegesis -mode=uops -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.

EXAMPLES: 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.txt

       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.  Either opcode-index or  opcode-name  must
              be set.

       -opcode-name=<LLVM opcode name>
              Specify the opcode to measure, by name.  Either opcode-index or opcode-name must be
              set.

       -mode=[latency|uops|analysis]
              Specify the run mode.

       -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 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.

       -analysis-inconsistencies-output-file=</path/to/file>
              If non-empty, write inconsistencies found during analysis to this file. - prints to
              stdout.

       -analysis-numpoints=<dbscan numPoints parameter>
              Specify the numPoints parameters to be used for DBSCAN clustering (analysis mode).

       -analysis-espilon=<dbscan epsilon parameter>
              Specify the numPoints parameters to be used for DBSCAN clustering (analysis mode).

       -ignore-invalid-sched-class=false
              If set, ignore instructions that do not have a sched class (class idx = 0).

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 (http://llvm.org/).

COPYRIGHT

       2003-2020, LLVM Project