Provided by: llvm-17_17.0.2-1~exp1ubuntu2.1_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,  unless  requested not to, 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.

SUPPORTED PLATFORMS

       llvm-exegesis currently only supports X86 (64-bit only), ARM  (AArch64  only),  MIPS,  and
       PowerPC  (PowerPC64LE  only) on Linux for benchmarking. Not all benchmarking functionality
       is guaranteed to work on every platform.  llvm-exegesis also has a separate analysis  mode
       that is supported on every platform on which LLVM is.

SNIPPET ANNOTATIONS

       llvm-exegesis supports benchmarking arbitrary snippets of assembly.  However, benchmarking
       these snippets often requires some setup so that they can execute properly.  llvm-exegesis
       has  two annotations and some additional utilities to help with setup so that snippets can
       be benchmarked properly.

       • LLVM-EXEGESIS-DEFREG <register name> - Adding  this  annotation  to  the  text  assembly
         snippet  to  be  benchmarked marks the register as requiring a definition.  A value will
         automatically be provided unless a second parameter, a hex value, is passed in. This  is
         done  with the LLVM-EXEGESIS-DEFREG <register name> <hex value> format. <hex value> is a
         bit pattern used to fill the register. If it is a value smaller than the register, it is
         sign extended to match the size of the register.

       • LLVM-EXEGESIS-LIVEIN  <register name> - This annotation allows specifying registers that
         should keep their value upon starting  the  benchmark.  Values  can  be  passed  through
         registers  from  the  benchmarking  setup  in  some cases.  The registers and the values
         assigned  to  them  that  can  be  utilized  in   the   benchmarking   script   with   a
         LLVM-EXEGESIS-LIVEIN are as follows:

         • Scratch  memory register - The specific register that this value is put in is platform
           dependent (e.g., it is the RDI register on X86 Linux). Setting this register as a live
           in  ensures  that  a pointer to a block of memory (1MB) is placed within this register
           that can be used by the snippet.

       • LLVM-EXEGESIS-MEM-DEF <value name> <size> <value> - This  annotation  allows  specifying
         memory definitions that can later be mapped into the execution process of a snippet with
         the LLVM-EXEGESIS-MEM-MAP annotation.  Each  value  is  named  using  the  <value  name>
         argument  so  that  it  can  be  referenced  later  within a map annotation. The size is
         specified in bytes the the value is taken in hexadecimal. If the size of  the  value  is
         less  than  the  specified  size,  the  value will be repeated until it fills the entire
         section of memory.  Using this annotation requires using the subprocess execution mode.

       • LLVM-EXEGESIS-MEM-MAP <value name>  <address>  -  This  annotation  allows  for  mapping
         previously defined memory definitions into the execution context of a process. The value
         name refers to a previously defined memory definition  and  the  address  is  a  decimal
         number  that  specifies  the  address the memory definition should start at. Note that a
         single memory definition can be mapped multiple times. Using  this  annotation  requires
         the subprocess execution mode.

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 need to use snippet annotations
       to ensure setup is performed properly.

       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: BENCHMARKING WITH MEMORY ANNOTATIONS

       Some snippets require memory setup in specific places to execute without crashing. Setting
       up memory can be accomplished with  the  LLVM-EXEGESIS-MEM-DEF  and  LLVM-EXEGESIS-MEM-MAP
       annotations. To execute the following snippet:

          movq $8192, %rax
          movq (%rax), %rdi

       We  need  to have at least eight bytes of memory allocated starting 0x2000.  We can create
       the necessary execution environment with the following annotations added to the snippet:

          # LLVM-EXEGESIS-MEM-DEF test1 4096 2147483647
          # LLVM-EXEGESIS-MEM-MAP test1 8192

          movq $8192, %rax
          movq (%rax), %rdi

EXAMPLE 4: 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=.

       --benchmark-phase=[prepare-snippet|prepare-and-assemble-snippet|assemble-measured-code|measure]
              By default, when -mode= is specified, the generated snippet will  be  executed  and
              measured,  and  that  requires  that  we  are running on the hardware for which the
              snippet was generated, and that supports performance measurements.  However, it  is
              possible  to  stop  at some stage before measuring. Choices are: * prepare-snippet:
              Only generate the minimal instruction  sequence.   *  prepare-and-assemble-snippet:
              Same  as  prepare-snippet, but also dumps an excerpt of the sequence (hex encoded).
              * assemble-measured-code: Same as prepare-and-assemble-snippet.  but  also  creates
              the  full  sequence  that  can  be dumped to a file using --dump-object-to-disk.  *
              measure: Same as assemble-measured-code, but also runs the measurement.

       -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.

       -x86-disable-upper-sse-registers
              Using the upper xmm registers (xmm8-xmm15) forces  a  longer  instruction  encoding
              which may put greater pressure on the frontend fetch and decode stages, potentially
              reducing the rate that instructions are dispatched to the backend, particularly  on
              older  hardware.  Comparing  baseline  results  with  this  mode  enabled  can help
              determine the effects of the frontend and  can  be  used  to  improve  latency  and
              throughput estimates.

       -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-filter=[all|reg-only|mem-only]
              By  default,  all benchmark results are analysed, but sometimes it may be useful to
              only look at those that to not involve memory, or vice versa. This option allows to
              either  keep  all  benchmarks,  or  filter out (ignore) either all the ones that do
              involve memory (involve instructions that may read or  write  to  memory),  or  the
              opposite, to only keep such benchmarks.

       -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).

       -mtriple=<triple name>
              Target triple. See -version for available targets.

       -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).
              (-mcpu=help for details)

       --analysis-override-benchmark-triple-and-cpu
              By default, llvm-exegesis will analyze the benchmarks for the triple/CPU they  were
              measured for, but if you want to analyze them for some other combination (specified
              via -mtriple/-mcpu), you can pass this flag.

       --dump-object-to-disk=true
              If set,  llvm-exegesis will dump the generated code to a temporary file  to  enable
              code inspection. Disabled by default.

       --use-dummy-perf-counters
              If  set,  llvm-exegesis  will  not  read any real performance counters and return a
              dummy value instead. This can be used  to  ensure  a  snippet  doesn’t  crash  when
              hardware  performance  counters  are  unavailable  and  for debugging llvm-exegesis
              itself.

       --execution-mode=[inprocess,subprocess]
              This option specifies what execution mode to use. The inprocess execution  mode  is
              the  default.  The subprocess execution mode allows for additional features such as
              memory annotations but is currently restricted to X86-64 on Linux.

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