Re-land r329156 "Add llvm-exegesis tool."

Fixed to depend on and initialize the native target instead of X86.

llvm-svn: 329169

1 files changed, 58 insertions, 0 deletions

diff --git a/llvm/docs/CommandGuide/llvm-exegesis.rst b/llvm/docs/CommandGuide/llvm-exegesis.rst
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+llvm-exegesis - LLVM Machine Instruction Benchmark
+==================================================
+
+SYNOPSIS
+--------
+
+:program:`llvm-exegesis` [*options*]
+
+DESCRIPTION
+-----------
+
+:program:`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, :program:`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.
+
+OPTIONS
+-------
+
+.. option:: -help
+
+ Print a summary of command line options.
+
+.. option:: -opcode-index=<LLVM opcode index>
+
+ Specify the opcode to measure, by index.
+ Either `opcode-index` or `opcode-name` must be set.
+
+.. option:: -opcode-name=<LLVM opcode name>
+
+ Specify the opcode to measure, by name.
+ Either `opcode-index` or `opcode-name` must be set.
+
+.. option:: -benchmark-mode=[Latency|Uops]
+
+ Specify which characteristic of the opcode to measure.
+
+.. option:: -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.
+
+
+EXIT STATUS
+-----------
+
+:program:`llvm-exegesis` returns 0 on success. Otherwise, an error message is
+printed to standard error, and the tool returns a non 0 value.