[Docs][llvm-exegesis] Use double dash long options

[aidengrossman]

Summary

Currently the llvm-exegesis docs use a mix of double dash and single
dash options with seemingly no pattern. This patch makes everything
double dash options as it has been suggested that we should be
advertising double dash long options exclusively in the documentation.

[aidengrossman]

This is based on a suggestion from @MaskRay on https://reviews.llvm.org/D155904. Definitely not urgent, but I'd like to begin making things more consistent where possible/convenient.

llvm/docs/CommandGuide/llvm-exegesis.rst

 EXAMPLE 1: benchmarking instructions
 ------------------------------------
 
 Assume you have an X86-64 machine. To measure the latency of a single
 instruction, run:
 
 .. code-block:: bash
 
-    $ llvm-exegesis -mode=latency -opcode-name=ADD64rr
+    $ llvm-exegesis --mode=latency --opcode-name=ADD64rr
 
 Measuring the uop decomposition or inverse throughput of an instruction works similarly:
 
 .. code-block:: bash
 
-    $ llvm-exegesis -mode=uops -opcode-name=ADD64rr
-    $ llvm-exegesis -mode=inverse_throughput -opcode-name=ADD64rr
+    $ 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`):
+redirect the output to a file using `--benchmarks-file`):
 
 .. code-block:: none
 
   ---
   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:
 
 .. code-block:: bash
 
-    $ llvm-exegesis -mode=latency -opcode-index=-1
+    $ 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).
 
 .. code-block:: bash
 
-    $ echo "vzeroupper" | llvm-exegesis -mode=uops -snippets-file=-
+    $ echo "vzeroupper" | llvm-exegesis --mode=uops --snippets-file=-
 
 Real-life code snippets typically depend on registers or memory.
 :program:`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

 -------------------
 
 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:
 
 .. code-block:: bash
 
-    $ llvm-exegesis -mode=analysis \
-  -benchmarks-file=/tmp/benchmarks.yaml \
-  -analysis-clusters-output-file=/tmp/clusters.csv \
-  -analysis-inconsistencies-output-file=/tmp/inconsistencies.html
+    $ 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:
 
 .. code-block:: none
 
   cluster_id,opcode_name,config,sched_class

 
 Note that the scheduling class names will be resolved only when
 :program:`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
 -------
 
-.. option:: -help
+.. option:: --help
 
  Print a summary of command line options.
 
-.. option:: -opcode-index=<LLVM opcode index>
+.. option:: --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.
 
-.. option:: -opcode-name=<opcode name 1>,<opcode name 2>,...
+.. option:: --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.
 
-.. option:: -snippets-file=<filename>
+.. option:: --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.
 
-.. option:: -mode=[latency|uops|inverse_throughput|analysis]
+.. option:: --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=`.
 
 .. option:: --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.
 
-.. option:: -x86-lbr-sample-period=<nBranches/sample>
+.. option:: --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.
 
-.. option:: -x86-disable-upper-sse-registers
+.. option:: --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.
 
-.. option:: -repetition-mode=[duplicate|loop|min]
+.. option:: --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.
 
-.. option:: -num-repetitions=<Number of repetitions>
+.. option:: --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.
 
-.. option:: -loop-body-size=<Preferred loop body size>
+.. option:: --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.
 
-.. option:: -max-configs-per-opcode=<value>
+.. option:: --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.
 
 
-.. option:: -benchmarks-file=</path/to/file>
+.. option:: --benchmarks-file=</path/to/file>
 
  File to read (`analysis` mode) or write (`latency`/`uops`/`inverse_throughput`
  modes) benchmark results. "-" uses stdin/stdout.
 
-.. option:: -analysis-clusters-output-file=</path/to/file>
+.. option:: --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.
 
-.. option:: -analysis-inconsistencies-output-file=</path/to/file>
+.. option:: --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.
 
-.. option:: -analysis-filter=[all|reg-only|mem-only]
+.. option:: --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.
 
-.. option:: -analysis-clustering=[dbscan,naive]
+.. option:: --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).
 
-.. option:: -analysis-numpoints=<dbscan numPoints parameter>
+.. option:: --analysis-numpoints=<dbscan numPoints parameter>
 
  Specify the numPoints parameters to be used for DBSCAN clustering
  (`analysis` mode, DBSCAN only).
 
-.. option:: -analysis-clustering-epsilon=<dbscan epsilon parameter>
+.. option:: --analysis-clustering-epsilon=<dbscan epsilon parameter>
 
  Specify the epsilon parameter used for clustering of benchmark points
  (`analysis` mode).
 
-.. option:: -analysis-inconsistency-epsilon=<epsilon>
+.. option:: --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).
 
-.. option:: -analysis-display-unstable-clusters
+.. option:: --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.
 
-.. option:: -ignore-invalid-sched-class=false
+.. option:: --ignore-invalid-sched-class=false
 
  If set, ignore instructions that do not have a sched class (class idx = 0).
 
-.. option:: -mtriple=<triple name>
+.. option:: --mtriple=<triple name>
 
  Target triple. See `-version` for available targets.
 
-.. option:: -mcpu=<cpu name>
+.. option:: --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)
 
 .. option:: --analysis-override-benchmark-triple-and-cpu
 
   By default, llvm-exegesis will analyze the benchmarks for the triple/CPU they