[llvm-exegesis] Add benchmark latency option on X86 that uses LBR for more precise measurements.
Starting with Skylake, the LBR contains the precise number of cycles between the two
consecutive branches.
Making use of this will hopefully make the measurements more precise than the
existing methods of using RDTSC.
Differential Revision: https://reviews.llvm.org/D77422
diff --git a/llvm/docs/CommandGuide/llvm-exegesis.rst b/llvm/docs/CommandGuide/llvm-exegesis.rst
index 321cdf5..8cc1a23 100644
--- a/llvm/docs/CommandGuide/llvm-exegesis.rst
+++ b/llvm/docs/CommandGuide/llvm-exegesis.rst
@@ -192,10 +192,24 @@
.. option:: -mode=[latency|uops|inverse_throughput|analysis]
- Specify the run mode. Note that if you pick `analysis` mode, you also need
- to specify at least one of the `-analysis-clusters-output-file=` and
- `-analysis-inconsistencies-output-file=`.
+ 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 this 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:: -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:: -repetition-mode=[duplicate|loop|min]
Specify the repetition mode. `duplicate` will create a large, straight line