Re: [PATCH v1] perf tests: Fix data symbol test with LTO builds

[James Clark <james.clark@linaro.org>]

On 27/02/2025 5:42 am, Ian Rogers wrote:
> On Wed, Feb 26, 2025 at 3:01 PM Ian Rogers <irogers@google.com> wrote:
>>
>> With LTO builds, although regular builds could also see this as
>> all the code is in one file, the datasym workload can realize the
>> buf1.reserved data is never accessed. The compiler moves the
>> variable to bss and only keeps the data1 and data2 parts as
>> separate variables. This causes the symbol check to fail in the
>> test. Make the variable volatile to disable the more aggressive
>> optimization. Rename the variable to make which buf1 in perf is
>> being referred to.
>>
>> Before:
>> ```
>> $ perf test -vv "data symbol"
>> 126: Test data symbol:
>> --- start ---
>> test child forked, pid 299808
>> perf does not have symbol 'buf1'
>> perf is missing symbols - skipping test
>> ---- end(-2) ----
>> 126: Test data symbol                                                : Skip
>> $ nm perf|grep buf1
>> 0000000000a5fa40 b buf1.0
>> 0000000000a5fa48 b buf1.1
>> ```
>>
>> After:
>> ```
>> $ nm perf|grep buf1
>> 0000000000a53a00 d buf1
>> $ perf test -vv "data symbol"126: Test data symbol:
>> --- start ---
>> test child forked, pid 302166
>>   a53a00-a53a39 l buf1
>> perf does have symbol 'buf1'
>> Recording workload...
>> Waiting for "perf record has started" message
>> OK
>> Cleaning up files...
>> ---- end(0) ----
>> 126: Test data symbol                                                : Ok
>> ```
>>
>> Fixes: 3dfc01fe9d12 ("perf test: Add 'datasym' test workload")
>> Signed-off-by: Ian Rogers <irogers@google.com>
> 
> Ah, I see we're trying to force -O0 and -fno-inline in the Makefile
> (git.kernel.org is giving 403s):
> https://github.com/torvalds/linux/blob/master/tools/perf/tests/workloads/Build#L11
> Which LTO later undoes. I'm not seeing LTO breakages for brstack.c and
> the shell test "Check branch stack sampling". I think LTO is able to
> optimize this case as it is a variable/struct being optimized, so the
> "-O0" and "-fno-inline" mustn't be being made to apply. Not a wholly
> satisfactory reason to add the volatile, but I'm short on
> alternatives.
> 
> Thanks,
> Ian
> 
> 
> 

Adding -fno-lto to that file works for me (llvm-15):

   CFLAGS_datasym.o          = -fno-lto -g -O0 -fno-inline ...

In fact it looks like we should add this to everywhere we're already 
doing -g and -O0. If lto is just another form of optimisation it should 
be disabled as well.