Re: [PATCH v1 0/4] Perf tool LTO support

[Arnaldo Carvalho de Melo]

Em Mon, Jul 24, 2023 at 01:12:43PM -0700, Ian Rogers escreveu:
> Add a build flag, LTO=1, so that perf is built with the -flto
> flag. Address some build errors this configuration throws up.
> 
> For me on my Debian derived OS, "CC=clang CXX=clang++ LD=ld.lld" works
> fine. With GCC LTO this fails with:
> ```
> lto-wrapper: warning: using serial compilation of 50 LTRANS jobs
> lto-wrapper: note: see the ‘-flto’ option documentation for more information
> /usr/bin/ld: /tmp/ccK8kXAu.ltrans10.ltrans.o:(.data.rel.ro+0x28): undefined reference to `memset_orig'
> /usr/bin/ld: /tmp/ccK8kXAu.ltrans10.ltrans.o:(.data.rel.ro+0x40): undefined reference to `__memset'
> /usr/bin/ld: /tmp/ccK8kXAu.ltrans10.ltrans.o:(.data.rel+0x28): undefined reference to `memcpy_orig'
> /usr/bin/ld: /tmp/ccK8kXAu.ltrans10.ltrans.o:(.data.rel+0x40): undefined reference to `__memcpy'
> /usr/bin/ld: /tmp/ccK8kXAu.ltrans44.ltrans.o: in function `test__arch_unwind_sample':
> /home/irogers/kernel.org/tools/perf/arch/x86/tests/dwarf-unwind.c:72: undefined reference to `perf_regs_load'
> collect2: error: ld returned 1 exit status
> ```
> 
> The issue is that we build multiple .o files in a directory and then
> link them into a .o with "ld -r" (cmd_ld_multi). This early link step
> appears to trigger GCC to remove the .S file definition of the symbol
> and break the later link step (the perf-in.o shows perf_regs_load, for
> example, going from the text section to being undefined at the link
> step which doesn't happen with clang or without LTO). It is possible
> to work around this by taking the final perf link command and adding
> the .o files generated from .S back into it, namely:
> arch/x86/tests/regs_load.o
> bench/mem-memset-x86-64-asm.o
> bench/mem-memcpy-x86-64-asm.o
> 
> A quick performance check and the performance improvements from LTO
> are noticeable:
> 
> Non-LTO
> ```
> $ perf bench internals synthesize
>  # Running 'internals/synthesize' benchmark:
> Computing performance of single threaded perf event synthesis by
> synthesizing events on the perf process itself:
>   Average synthesis took: 202.216 usec (+- 0.160 usec)
>   Average num. events: 51.000 (+- 0.000)
>   Average time per event 3.965 usec
>   Average data synthesis took: 230.875 usec (+- 0.285 usec)
>   Average num. events: 271.000 (+- 0.000)
>   Average time per event 0.852 usec
> ```
> 
> LTO
> ```
> $ perf bench internals synthesize
>  # Running 'internals/synthesize' benchmark:
> Computing performance of single threaded perf event synthesis by
> synthesizing events on the perf process itself:
>   Average synthesis took: 104.530 usec (+- 0.074 usec)
>   Average num. events: 51.000 (+- 0.000)
>   Average time per event 2.050 usec
>   Average data synthesis took: 112.660 usec (+- 0.114 usec)
>   Average num. events: 273.000 (+- 0.000)
>   Average time per event 0.413 usec


Cool stuff! Applied locally, test building now on the container suite.

- Arnaldo

> ```
> 
> Ian Rogers (4):
>   perf stat: Avoid uninitialized use of perf_stat_config
>   perf parse-events: Avoid use uninitialized warning
>   perf test: Avoid weak symbol for arch_tests
>   perf build: Add LTO build option
> 
>  tools/perf/Makefile.config      |  5 +++++
>  tools/perf/tests/builtin-test.c | 11 ++++++++++-
>  tools/perf/tests/stat.c         |  2 +-
>  tools/perf/util/parse-events.c  |  2 +-
>  tools/perf/util/stat.c          |  2 +-
>  5 files changed, 18 insertions(+), 4 deletions(-)
> 
> -- 
> 2.41.0.487.g6d72f3e995-goog
> 

-- 

- Arnaldo