Provided by: llvm-8_8-3_amd64 bug

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.

       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 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.

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|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).

              -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).

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

       2003-2019, LLVM Project