Provided by: llvm-13_13.0.1-13_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, 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:

          $ llvm-exegesis -mode=latency -opcode-index=-1

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. Specifying -1  will  result  in  measuring
              every existing opcode. 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  some  modes  have additional requirements and
              options.

              latency mode can be  make use  of  either  RDTSC  or  LBR.   latency[LBR]  is  only
              available on X86 (at least Skylake).  To run in latency mode, a positive value must
              be specified for x86-lbr-sample-period and –repetition-mode=loop.

              In  analysis  mode,  you   also   need   to   specify   at   least   one   of   the
              -analysis-clusters-output-file= and -analysis-inconsistencies-output-file=.

       -x86-lbr-sample-period=<nBranches/sample>
              Specify  the LBR sampling period - how many branches before we take a sample.  When
              a positive value is specified for this option and when the mode is latency, we will
              use  LBRs for measuring.  On choosing the “right” sampling period, a small value is
              preferred, but throttling could occur if the sampling  is  too  frequent.  A  prime
              number should be used to avoid consistently skipping certain blocks.

       -repetition-mode=[duplicate|loop|min]
              Specify  the  repetition  mode.  duplicate will create a large, straight line basic
              block    with    num-repetitions    instructions     (repeating     the     snippet
              num-repetitions/snippet  size  times). loop will, optionally, duplicate the snippet
              until the loop body contains at least loop-body-size instructions,  and  then  wrap
              the  result in a loop which will execute num-repetitions instructions (thus, again,
              repeating  the  snippet  num-repetitions/snippet  size  times).  The   loop   mode,
              especially with loop unrolling tends to better hide the effects of the CPU frontend
              on architectures that cache decoded  instructions,  but  consumes  a  register  for
              counting iterations. If performing an analysis over many opcodes, it may be best to
              instead use the min mode, which will run each other mode, and produce  the  minimal
              measured result.

       -num-repetitions=<Number of repetitions>
              Specify the target number of executed instructions. Note that the actual repetition
              count of the snippet will be num-repetitions/snippet size.  Higher values  lead  to
              more accurate measurements but lengthen the benchmark.

       -loop-body-size=<Preferred loop body size>
              Only  effective  for  -repetition-mode=[loop|min].   Instead  of  looping  over the
              snippet directly, first duplicate it so that the loop body contains at  least  this
              many instructions. This potentially results in loop body being cached in the CPU Op
              Cache / Loop Cache, which allows to which may have higher throughput than  the  CPU
              decoders.

       -max-configs-per-opcode=<value>
              Specify  the  maximum  configurations  that  can  be generated for each opcode.  By
              default this is 1, meaning that we assume that a single measurement  is  enough  to
              characterize  an  opcode.  This might not be true of all instructions: for example,
              the performance characteristics of the LEA instruction on X86 depends on the  value
              of  assigned  registers  and immediates. Setting a value of -max-configs-per-opcode
              larger than 1 allows llvm-exegesis to explore more configurations  to  discover  if
              some   register   or   immediate   assignments   lead   to   different  performance
              characteristics.

       -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

       2003-2023, LLVM Project