instrument-xray

The tracking issue for this feature is: #102921.

Enable generation of NOP sleds for XRay function tracing instrumentation. For more information on XRay, read LLVM documentation, and/or the XRay whitepaper.

Set the -Z instrument-xray compiler flag in order to enable XRay instrumentation.

  • -Z instrument-xray – use the default settings
  • -Z instrument-xray=skip-exit – configure a custom setting
  • -Z instrument-xray=ignore-loops,instruction-threshold=300 – multiple settings separated by commas

Supported options:

  • always – force instrumentation of all functions
  • never – do no instrument any functions
  • ignore-loops – ignore presence of loops, instrument functions based only on instruction count
  • instruction-threshold=10 – set a different instruction threshold for instrumentation
  • skip-entry – do no instrument function entry
  • skip-exit – do no instrument function exit

The default settings are:

  • instrument both entry & exit from functions
  • instrument functions with at least 200 instructions, or containing a non-trivial loop

Note that -Z instrument-xray only enables generation of NOP sleds which on their own don‘t do anything useful. In order to actually trace the functions, you will need to link a separate runtime library of your choice, such as Clang’s XRay Runtime Library.