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 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. 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 of an instruction works similarly:

$ llvm-exegesis -mode=uops -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:

#!/bin/bash
readonly INSTRUCTIONS=$(($(grep INSTRUCTION_LIST_END build/lib/Target/X86/X86GenInstrInfo.inc | cut -f2 -d=) - 1))
for INSTRUCTION in $(seq 1 ${INSTRUCTIONS});
do
  ./build/bin/llvm-exegesis -mode=latency -opcode-index=${INSTRUCTION} | sed -n '/---/,$p'
done

FIXME: Provide an llvm-exegesis option to test all instructions.

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 :

../_images/llvm-exegesis-analysis.png

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. 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|analysis]

Specify the run mode.

-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.

-benchmarks-file=</path/to/file>

File to read (analysis mode) or write (latency/uops 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.

-analysis-inconsistencies-output-file=</path/to/file>

If non-empty, write inconsistencies found during analysis to this file. - prints to stdout.

-analysis-numpoints=<dbscan numPoints parameter>

Specify the numPoints parameters to be used for DBSCAN clustering (analysis mode).

-analysis-espilon=<dbscan epsilon parameter>

Specify the numPoints parameters to be used for DBSCAN clustering (analysis mode).

-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).

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.