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Use patchable rdtsc sequence to avoid slowdowns under rr

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We (Julia) ship both support for using tracy to trace julia applications,
as well as using `rr` (https://github.com/rr-debugger/rr) for record-replay debugging.
After our most recent rebuild of tracy, users have been reporting signfificant performance
slowdowns when `rr` recording a session that happens to also load the tracy library
(even if tracing is not enabled). Upon further examination, the recompile happened
to trigger a protective heuristic that disabled rr's patching of tracy's use of
`rdtsc` because an earlier part of the same function happened to look like a
conditional branch into the patch region. See https://github.com/rr-debugger/rr/pull/3580
for details. To avoid this issue occurring again in future rebuilds of tracy,
adjust tracy's `rdtsc` sequence to be `nopl; rdtsc`, which (as of of the
linked PR) is a sequence that is guaranteed to bypass this heuristic
and not incur the additional overhead when run under rr.

This functionality is kept behind a compile-time flag `TRACY_PATCHABLE_NOPSLEDS`
in order to avoid polluting the instruction cache unnecessarily.
This commit is contained in:
Keno Fischer
2023-08-19 01:40:18 +00:00
committed by Cody Tapscott
parent 60a3a85069
commit 5417227e83
3 changed files with 20 additions and 0 deletions
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meson.build
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@@ -64,6 +64,10 @@ if get_option('tracy_no_system_tracing')
add_project_arguments('-DTRACY_NO_SYSTEM_TRACING', language : 'cpp')
endif
if get_option('tracy_no_extra_nopsleds')
add_project_arguments('-DTRACY_PATCHABLE_NOPSLEDS', language : 'cpp')
endif
if get_option('tracy_delayed_init')
add_project_arguments('-DTRACY_DELAYED_INIT', language : 'cpp')
endif